PhytopathologyAgrobacterial Burls - Ambrosia Beetle - Anthracnose - Aphids - Apple Maggot - Ash Flower Galls - Beech Bark Disease - Butternut Canker - Chafers & Grubs - Chestnut Blight - Dutch Elm Disease - Emerald Ash Borer Bettles - Fire Blight - Gypsy Moth - Leafhoppers - Longhorned Beetles - Mountain Pine Beetles - Nutrient Deficiencies - Plantbugs - Rust Fungi - Sawflies - Scale Insects - Sudden Oak Death - Tent Caterpillars - Wood Decay Fungi
D. Andrew White M.Sc. 01/01/2015
Burl afflicted cottonwoods in Nevada
Dutch Elm Disease
Dutch elm disease, or 'elm wilt', is more famous, now, than chestnut blight, because the epidemic was nearer in time to our era, and because the effects were less drastic overall.
Dutch elm disease is caused by either of two species of fungus, Ophiostoma ulmi and Ophiostoma novo-ulmi. These fungi are members of the Plectomycetes series, and are distinguished by having cleistothecia, which is to say ascospores enclosed in a 'sac'. The Ophiostoma fungi are very yeast like in form. Toxins produced by the fungus are carried up conductive vessels, where they cause leaf wilt. The fungal mycelia also block the conducting elements, causing the wood to die. These fungi are spread by bark beetles, usually the native Hylurgopinus rufipes, or the European Scolytus multistriatus beetle. Both beetles, as larvae, chew galleries just below the bark in the outermost annual ring. Each gallery has a main chamber from which radiates a series of blind tunnels. The eggs were laid in the main chamber, and the individual larvae each chew out one of the side tunnels. The Ophiostoma fungi use the beetles as vectors, the spores growing on the inside of the tunnels rub off on the larvae, and more importantly they brush onto the adults that emerge from the pupae. Apparently spores can also be eaten by the beetles, and survive passage through their digestive tracts. The pupated adults then fly off, bearing spores which may infect the elm on which the beetles lay their eggs. The fungi are introduced to a new elm when the beetle lays its eggs in the bark of a new tree. The fungi then attack the sapwood of that new elm.
Since elms only have a few actively conducting annual rings, the fungi in this thin sapwood have a greater impact than they would if the sapwood layers were thicker. The fungal mycelia do not remain confined to the vicinity of the larval galleries. The fungus spread throughout the sapwood, including the conductive wood of the roots. Sometimes the mycelia can cross from the roots of one elm to another. Elms often have their roots fused with neighbouring elms.
Elms infested with the 'Dutch elm disease' may die in the same year as they are infected. Often they die slowly over the span of a few years. Older trees are more likely to be infected than young ones.
Most elms, except for a few Asian species, are sensitive to the Dutch elm disease fungus. However, the largest native elm, the American elm or white elm (Ulmus americana), is very susceptible to Dutch elm disease.
Dutch elm disease originated in Asia, spread to Europe and then to North America. Probably the disease arrived on a shipment of lumber. It made its first appearance in Ohio in 1930. By the 1950s thousands of white elms were dying throughout North America. By 1970 most truly large elms were dead giant hulks with slabs of bark peeling off. The disease seems to have flared up again in the 1990s, as some of the small surviving elms grew to medium size, and then caught the disease.
White elm did not go into a sharp decline, rather it was that old well formed trees became increasingly uncommon. Many white elm saplings live long enough to produce a seed crop before they are killed by the fungus. Some of the Asian elms are less susceptible, including Siberian elm which is a popular city tree. There are several wild species of elm also, each with varying ability to withstand the fungus. There are, in short, plenty of wild and domestic elms on which the fungus can perpetuate itself. The fungus is not 'burning itself out'. Nevertheless, the white elm could be taking the long slow route to extinction.
Several people, including Bernd Heinrich of the University of Vermont, have suggested that white elm could survive Dutch elm disease. It has been noted that some elms develop seed earlier than others. This is due to ordinary genetic variation. Since Dutch elm disease afflicts older trees more often than young ones, there is therefore natural selection for white elms that flower at a younger age. Perhaps, white elm is evolving from a tree into a large shrub.
There have been attempts to find resistant strains of white elm. There are systemic fungicide injections that can control the fungus. These must be repeated every year or so, and they are fairly expensive. New methods aim, instead, to stimulate and enhance the natural defences of the elm trees. The process is like 'inoculation'. Although, the 'immune system' of plants is much simpler than the immune system of animals. Plants do build-up chemical defences to irritants. Thus, one can 'trick' the plant into mounting a defensive response, with a chemical stimulus. Since the defences build up in the absence of an actual fungal attack, the elm becomes pre-prepared for a real fungus infestation.
If there are many elms to treat, systemic fungicides and inoculants may be an impractical solution to the problem. In practice, this means that white elms usually die, and must be removed by a qualified arborist. The cost of removal may exceed $1600.00 cdn, if the elm is large.
Eastman, John. 1992. The Book of Forest and Thicket. Stackpole Books. Mechanicsburg, PA.
Holliday, Paul. 1989. A Dictionary of Plant Pathology. Cambridge University Press. Cambridge.
Heinrich, Bernd. 1997. The Trees of My forest. Cliff Street Books. New York. 49-58.
Ware, George H. 1995. Little-known elms from China: landscape tree possibilities. Journal of Arboriculture. 21(6): 284-288.
In 1991 a canker disease was noticed in butternuts (Juglans cinerea). Within a few years it was to be found decimating butternuts, a tree that was never common to begin with. It is believed that the fungus is an introduced species. Though, this is not certain.
The wilt is caused by fungus called Sirococcus clavigignenti-juglandacearum. Like most canker fungi it causes wounds that heal slowly, if at all. It also causes a tell-tale slime-flux or gummosis. And like too many fungal diseases it cannot be easily eradicated once a tree is infected. Luckily, some strains and hybrids are fairly resistant to the fungus.
The situation has gotten serious enough that in Ontario butternuts are on the Endangered Species List. Though, many states in the USA have not taken this step – yet. If you suspect butternut canker in your neighbourhood, call the nearest Urban Forestry Service, or a professional arborist. They can help with diagnosis and suggestions for further action.
Holliday, Paul. 1989. A Dictionary of Plant Pathology. Cambridge University Press. Cambridge.
Kerr, Pat. 2008. Keeping the historic Butternut alive in Canada. Tree Service Canada. 2(1): 15.
Ross-Davis, A., Huang, Z. McKenna, J., Ostry, M. and Woeste, K. 2008. Morphological and molecular methods to identify butternut (Juglans cinerea) and butternut hybrids: relevance to butternut conservation. Tree Physiology. 28: 1127-1133.
Sudden Oak Death
Sudden oak death (Phytophthora ramorum) is a particularly troublesome form of phytophthora. Certain strains of this oomycete fungoid have begun to seriously afflict coast live oaks in California. This epidemic may have begun when a strain of phytophthora from Europe, or Asia, was accidentally introduced to the Americas in the 1990s.
The symptoms of 'sudden oak death' are like those of other phytophthora diseases. However, in coast live oak (Quercus agrifolia) the disease tends to be fatal. In addition, cankers can appear on the lower boles of infected trees. Lesions of dead cambium under the bark can issue a dark exude though cracks in the bark. As is also too typical of phytophthora species, the fungoid can live in more than one host species. Rhododendrons, laurels, and tanoaks (Lithocarpus spp.) can also host the fungoid. Tanoak and a number of oak species are very sensitive to the fungoid.
It now seems probable that the P. ramorum fungoid came from Europe on imported rhododendrons. Furthermore, contrary to early reports, redwood trees are not very sensitive to the fungoid. However, laboratory studies indicate that the fungoid could seriously harm many eastern oak species. Luckily, it has not yet established itself in the east.
Sudden oak death is most prevalent where rhododendrons have been planted in the vicinity. Rhododendrons can act as carriers, and are themselves only mildly affected by the fungus. The nursery trade of rhododendrons has been in part responsible for the spread of this disease. It is possible that eastern oaks could become endangered by this epidemic. Diligence is required to ensure that this phytophthora strain spreads no further than it has already.
Recently a treatment for sudden oak (SOD) death has been found. This control is a pre-emptive treatment for SOD infection. It is not a very good post-infection ‘cure’ per se. So far it has been tested and proven efficacious for treating oaks and tanoaks. Applications of phosphites (PO33-) or salts of phosphonic acid can incur a degree of resistance to SOD and some other oomycetes. Injections of the solution into the active xylem (sapwood) are fairly effective. Spraying the bark with the phosphite solution is somewhat less effective - but cheaper. With spray applications, a surfactant is required to keep the solution in place until it is adsorbed into the tree’s tissues. The application should be during the active growing season, and the concentration and dose must be very carefully controlled (Garbelotto et al 2007).
Freinkel, Susan. 2002. If all the trees fall in the forest ... Discover. 23 (12) 67-73.
Hagen, Bruce W. 2001. Sudden Oak Death Part 1: symptoms, biology and potential impact. Arborist News. 10(6):29-31.
Asian Longhorned Beetle
A longhorned beetle (Anoplophora glabripennis) of Asiatic origin has recently taken up residence in Ontario. This 'Asian longhorned beetle' often feeds on maple, but it feeds also on poplar, birch, ash, elm and horse-chestnut. This longhorned beetle one of many longhorned beetle species. Most native longhorned beetles do not cause any great consternation among foresters. However, the Asian longhorned beetle has created quite a stir. Its damage tends to exceed that of the native longhorned beetles.
The adult of this Asian beetle is quite long bodied (3.5 cm). Its antennae are even longer than its body (4 cm). It is dark black with rows of white spots on its elytra (shield wings). The adult beetle lays its eggs under the bark of trees from spring to late summer. The eggs hatch into a borer larvae which tunnel into the sapwood. These tunnels can damage conductive sapwood, and can even kill a tree within one year's time. A complete generation takes from 12 to 18 months, the larval beetle over-winters inside its tunnel. When the pupa forms into an adult, the mature beetle exits through a rather large hole in the bark. The adult takes off, mates and repeats the cycle.
The Asian longhorned beetle has apparently been in Canada and the north-eastern U.S.A. since the late 1990s. In 2003 the beetle was detected in Toronto. Probably the beetle was introduced accidentally in some shipment of a wood product from China. The Asian longhorned has few natural controls in the Americas. There are very real fears that the Asian longhorned beetle shall cause extensive damage.
In theory injected insecticides could control the borer. However, such insecticides are poor at controlling wood-borers. Canadian forestry officials are hoping that the outbreak can be contained by limiting the spread of the beetle. This means that infected trees will require removal, chipping and thorough disposal (Strauss 2003).
There are three main kinds of 'tent caterpillars' in North America. They are moths which feed on leaves as larvae (caterpillars). Each species has a distinct type of tent, and the caterpillars are easy to distinguish. The caterpillars feed in colonies, and they retreat to their silken 'tents' in inclement weather. They can cause extensive defoliation. However, healthy trees usually recover from these attacks.
Eastern Tent Caterpillar
Eastern tent caterpillars (Malacostoma americanum) make their tents in the axils or forks of branches, not on twig tips. The caterpillars are hairy and have a white stripe along the centre of their back. They over-winter in the egg stage. Egg masses are arranged in bands around small twigs. They feed on many tree species, but cherry trees are the most common host.
Since the tents are made in larger forks, pruning off each tent is not always practical. Surprisingly, manually crushing the caterpillars along with their tent is a very effective means of diminishing their numbers. (Seems too simple!) Egg masses, which are visible in the winter, can also be crushed.
Forest Tent Caterpillar
Forest tent caterpillars (Malacostoma disstria) make 'tents' that are more like silken mats on large branches or on the trunk. The caterpillars are hairy and brownish. They have a row of white spots along their back, and pale blue stripes on their sides. They over-winter in the egg stage. Egg masses are quite large and are located on twigs. They feed on many tree species. Poplar, maple, oak, ash and birch are common hosts.
Crushing the silken mats is not as easy as for the eastern tent caterpillar. Egg masses, easily spotted in the winter, and can be crushed, if they are in accessible places.
Fall webworn (Hyphantria cunea) create large tents on branch tips. Webs are largest in the late summer and autumn. Caterpillars are hairy, and pale yellow or green. Manitoba maple and crab-apples are the common hosts, they infest other tree species also.
Fall webworm tents are the easiest of the tent caterpillars to prune out of trees. This pruning should be done as soon as possible after they appear.
Emerald Ash Borer
The emerald ash-borer (Agrilus planipennis) is a beetle of Asiatic origin that has recently begun to infest North America. The long bodied metallic beetle especially seeks out ash trees (Fraxinus americana & F. pennsylvanica). Though, the beetle does not much harm the rarer blue ash (Fraxinus quadrangulata). It also can infect other species besides ash. Between May and July the female lays her eggs on the bark. In about one week the eggs hatch and the pale white larvae bores through the bark, to feed on the cambium tissue. They tend to carve out long S-shaped galleries. If too prolific, the larval galleries can choke off the sapwood in a tree resulting in its death.
There are several native North american species of Agrilus that feed on a number of broadleaf species. For example, the bronze birch borer (A. anxius) feeds on birch, and the chestnut borer (A. bilineatus) feeds on chestnuts and oaks. These borers usually do not cause mortal injury - except in trees already stressed by other disorders.
In 2002 the emerald borer was found to have become established in Michigan. The emerald ash-borer does not have well established native enemies. The innate resistances of the non-Asian ash are insufficient to check the spread of this Asian borer. The native Asian species tolerate the borer quite well. But North American white ash and green ash are often mortally injured by the beetles. Consequently, the emerald ash borer has started to become a serious problem in the Americas. The ash borer epidemic has so far caused the death of millions of ash trees. The pest has spread into Canada, having gained entrance near Windsor Ontario.
Borers are difficult to control. To apply contact insecticides just at the right time so as to counter the egg laying adults, is nearly impossible. Once the larvae are in the sapwood, systemic insecticides tend not to be effective. The Canadian Food Inspection Agency (CFIA) had originally planned to halt the spread of the borer by creating a 'quarantine'. This planned quarantine was to consist of an ash-free corridor 30 kilometres wide between Essex and Kent counties. Unfortunately, budgetary and logistic problems prevented this project from being implemented in early 2003. By the end of the year the quarentine option was re-opened. The recommendation for home owners is that any emerald borer infected ash tree should be removed. As half-expected, the beetle did 'jump' the quarantine line. By 2007 the beetle had reached as far as the Mississauga and Toronto area.
American Sweet Chestnuts (Castanea dentata) are trees in the beech-oak family. Sweet chestnuts were once common in the Carolinian deciduous forests of eastern North America. In some forests associations they were the dominant species, often a quarter or more of the trees per stand were sweet chestnuts. Sweet chestnuts were most often associates with black oak, white oak, shagbark hickory, white ash and black cherry.
There are other species of chestnut. The European Castanea sativa, and the Asian Castanea mollissima, are similar and produce edible seeds. These species are, however, less damaged by chestnut blight than their American relative.
In 1904 a fungal disease (Cryphonectria parasitica) of Asian origin was introduced, by accident, to New York State. The fungus growing in the thin sapwood of the sweet chestnuts encountered insufficient resistance from the trees' defences. By 1925 this fungus infestation spread to almost the whole of the sweet chestnut's native range. Eventually, almost every sweet chestnut stand was infected. Isolated trees, and some coppice sprouts, are still producing seeds. The coppice sprouts are not good at re-establishing roots. And the old stumps are being slowly rotted away by the chestnut-tongue fungus (Fistulina hepatica). However, a few adult trees still exist. This author has seen blight-free chestnuts near the coast of Lake Erie. Perhaps they are there protected from blight spores by the direction of the wind. All hope is not lost.
Cryphonectria parasitica is a canker fungus. It causes lesions on branches or stems. On other tree species, such as oaks and hickories, the fungus seems to be purely saprophytic, meaning that it feeds on wood that is already dead. To the genus Castanea, however, the fungus is truly parasitic, it manages to kill living wood, and then it digests the now-dead tissue. It is an ascomycete fungus and produces ascospores, sexually produced spores. Ascospores are released from a small disc-like 'perithecia' bodies that develop on the bark. These ascospores are generally wind-dispersed. This fungus also produces conidia, which are asexual spores. These conidia are present in a sticky orange tendrils that ooze from the conidiospore producing bodies called 'pycnidia'. Conidia are most often spread by insects, birds and other mobile vectors that come in contact with the sticky ooze.
Like many ascomycete species, the blight also has deuteromycete forms. Some strains of the chestnut blight do not seem to produce ascospores, only conidiospores. These asexual anamorphs are known as Endothiella parasitica.
Forestry scientists have had success in developing blight-resistant strains, i.e. hybrids, of chestnut. Presently there is an ongoing attempt to transplant blight-resistant hybrid-chestnuts to the Carolinian forests of the Eastern USA.
Mountain Pine Beetle
The mountain pine beetle, Rocky Mountain beetle, or Black Hills beetle (Dendroctonus ponderosae), is a small beetle, less than 5 mm long, that bores into the bark of pine trees. There it lays its eggs, the hatching larvae feed in the bark, i.e. the phloem, of pine trees. It over-winters as a larva in the bark. In the northern States and Canada, the beetle usually has but one complete generation per year. The beetle infests mostly the lodgepole, limber and ponderosa pines. Western white pine, Scots pine, and other Pinus species can be affected also. Mostly these beetle infestations occur in the far west, and it is stands of lodgepole and ponderosa pine that are most often decimated. It can live in jack pine. By the 1990s it was known to occur in jack-lodgepole pine hybrids - in the eastern Rockies. Indeed, by the early 2010s the beetle was detected in pure-strain jack pines.
Outbreaks tend to be localised, but large in overall extent. It is not usually the mountain pine beetle itself that kills a pine tree. It is rather a series of ‘blue stain’ fungi that do this injury (eg. Ophiostoma clavigerum & Ophiostoma montium). These fungi start in the phloem, but the mycelia can eventually spread into the sapwood. If they grow too prolifically they can block the tree’s sap-flow - thereby killing the tree. As is often the case, various kinds of fungi live symbiotically with the beetle. The beetle larva eats the fungal mycelium of these fungi, and the adult spreads the fungal spores. But not all kinds of fungi are eaten by the beetle larva. The “green stain” fungi are eaten by the bark beetles, but they are not a primary foodstuff. Indeed the green stain fungi seem mostly to use the beetle as a vector. Besides which, the fungi are as much carried by parasitic mites as by the bark beetle itself. Though there is some evidence that the green stain fungi weaken the pine tree host, and thereby benefit the beetle.
Mountain pine beetle is native to western North America. However, pine die-off has reached ‘pandemic’ (panphytic?) proportions in the last decade. Vast swaths of pine forest have been killed so far by the beetle-fungus combo. It seems that global warming may have decreased the winter mortality of the beetle. With more beetles surviving winter, woodpeckers and other natural controls cannot always keep up. Outbreaks may also be exacerbated by the shorter dormant season for which the pines are perhaps somewhat maladapted. The beetle and its fungi mostly afflict old trees, dought stressed trees and/or over-crowded stands. There is some evidence that the pine beetle populations tend to flare up in even-aged stands. This is because it is mostly mature trees that are suitable nurseries for the beetle. Indeed some of the largest beetle outbreaks have occurred in so-called “managed” forests. Hence the forestry practices of clear-cutting and mass replanting have not always been helpful. Massive stands of prime beetle habitat mean huge beetle population booms are possible. Unfortunately this means that massive swarms of beetles can form. Once these swarms occur, even mixed-age stands downwind can succumb to the beetles.
Systemic insecticides can control the beetle. Controlled burning of moribund trees does work. Sometimes burning is the only workable option. Diligence is needed to slow the beetle's eastward trek.
Adams, Aaron S. & Six, Diana L., 2007. Temporal variation in mycophagy and prevalence of fungi associated with developmental stages of Dendroctonus ponderosae (Coleoptera: Curculionidae). Environmental Entomology. 36 (1): 64-72.
Kerr, Pat. 2011. Mountain Pine beetle jumps tree species. Tree Service Canada. 5 (1): 1 & 4.
Kim, J-J.; Kim, S.H. Lee, S. and Breuil, C. 2008. Distinguishing Ophiostoma ips and Ophiostoma montium, two bark beetle-associated sapstain fungi. FEMS Microbiology Letters 222: 187-192.
Rice, A.V and Langor, D.W. 2009. Mountain pine beetle-associated blue-stain fungi in lodgepole • jack pine hybrids near Grande Prairie, Alberta (Canada). For. Path. 39: 323–334.
Six, D.L. and Klepzig, K.D. 2004. Review article. Dendroctonus Bark Beetles as Model Systems for Studies on Symbiosis. Symbiosis, 37 (1-3): 1-26.
Solheim, Halvor and Krokene, Paal. 1998. Growth and virulence of mountain pine beetle associated blue-stain fungi, Ophiostoma clavigerum and Ophiostoma montium. Canadian Journal of Botany. 76(4): 561–566.
Beech Bark Disease
In the 1890s a European scale insect (Cryptococcus fagisuga) was accidently introduced to North America. The scale insect feeds on the phloem-sap of beech trees (Fagus spp.). It mostly feeds in tiny crevasses of the otherwise smooth beech bark. It is a rather typical scale insect. It is pale and about 0.5 to 1 mm long, the adult bug is housed in a waxy shell or 'scale'.
Unfortunately the holes bored by this foreign bug proved to be portals for the fungus Nectria coccinea var. faginata, and sometimes also Nectria galligena fungus. The nectria fungus has orange bead-like sexual fruiting bodies. The asexual fruiting bodies are pale. These bodies tend to cluster in cracks in the bark - cracks that they cause. The mycelium feeding in the bark can kill sections of the phloem (bark). Near the points of infection very persistent cankers can form. These cankers can eventually girdle and kill the beech tree. The American beech (F. grandifolia) seems to be more sensitive to the disease-combo than does the European species (F. sylvatica).
The combination of the insect and fungus proved to be dire, it causes beech bark disease, which is sometime fatal. Luckily the disease spread slowly starting on the East Coast. By 2009 the disease had crossed the Great Lakes and St. Lawrence into Quebec and Ontario. It is difficult to control either the scale insect or the fungus. One can, however, attempt to halt the westward spread of the disease. One means would be to limit the transport and planting of European beech trees in Ontario. It is advisable therefore not to transplant beech trees or use them as ornamentals.
Gypsy* moth or spongieuse (Lymantria dispar) is a serious pest. The adult moth is about two centimetres in wing-span. Males are brown and thin, and females are grey winged and plump. The female, although winged, does not seem to fly very often. In the spring the female crawls up the trunk, attracts a male, mates, and then she lays a large egg mass covered by a silken tent. Eggs over-winter, and hatch in the spring. The caterpillars climb into the crown to feed. The caterpillar is dark, 'hairy', and has blue spots along its thorax, and red spots on its abdomen. It feeds voraciously. It will consume whole leaves, not just making holes as winter moths do. The cocoon covered pupa matures into an adult moth in the same season. By June or July, the adults of the next generation are out and about.
Gypsy moth is one of the most serious pests in eastern North America. Originally it was an endemic of Eurasia. It was introduced accidentally in 1868, and has since become widespread. It feeds on a variety of trees including apple, pear, aspen, mountain-ash, oak, and willow. It can also feed on conifers, such as spruce and pine! Few insects consume such a variety of foodstuffs.
In the late 1980s a new strain of gypsy moth was found on the west coast. It has been repeatedly eradicated in spots. Apparently this strain is Siberian in origin, and differs a little from the familiar kind. In particular, in this subspecies both males and females fly. It is much more winter hardy, and it is even more likely to eat conifer needles. With diligence, this subspecies shall remain under control.
Gypsy moth caterpillars can be controlled by broad-spectrum insecticides. Spraying is most effective just after egg hatching in April. Luckily, the moth can be very well controlled with the biological control agent Bacillus thuringiensis. On very small trees the caterpillars can be controlled by shaking or plucking them off manually.
* Note: Obviously, the current English name of this moth is politically incorrect.
Sawflies are a group of thick-waisted wasps. The larvae look a lot like small bald caterpillars with dark heads. The female adult wasp has a saw-like ovipositor that cuts plant tissues, to inject the eggs. The eggs hatch and the larvae crawl out to seek fruits, leaves or whatever organ they specialise in eating. Some species feed en masse, others are more spread out. Some species skeletonise leaves. Some devour leaves entirely from tip to petiole. Some burrow into fruits, as the apple sawfly (Hoplocampa testudinea) does. Most when disturbed rear-up and regurgitate their stomach contents. Probably the issue of this foul-tasting vomit is meant to discourage birds and other insectivores.
There are many species of sawfly wasps, in several taxonomic families. Most feed on rather specific plant taxa peculiar to the sawfly species. Two species of special note are:
Pear sawfly (Caliroa cerasi) feeds on pear, apple and cotoneaster leaves. The larva feeds by eating the surface of the leaves, leaving a skeleton of veins. It over-winters in the larva stage, the pupa and adult phases occur in the spring.
European sawfly (Neodiprion sertifer) feeds on Scots, Austrian and mugo pine. The larva's feeding leaves behind the rachis of each needle it eats. Strangely, they prefer to eat year-old needles, the young needles are generally left. It can infest en masse large tracts of Scots pine. Adults emerge in the autumn to mate and lay eggs inside pine leaves (needles).
Sawfly larvae are not caterpillars, thus they cannot be controlled with Bacillus thuringiensis. Broad-spectrum insecticides are somewhat effective if applied when the larvae first appear. Luckily, a natural virus(es?) can control sawfly outbreaks. Sometimes a sawfly infestation will suddenly end in a mass die-off caused by the virus.
The apple maggot fly (Rhagoletis pomonella) feeds on the fruit of apples, pears and hawthorns. It is a housefly-like fly with black zigzag markings on its wings. It lays its eggs on immature fruits. Its larva is a small legless 'maggot'. The maggot tunnels about inside the fruit. It pupates into an adult fly in the soil. Significant damage to cultivated fruits can be caused by these maggots.
A few centuries ago apple maggot flies fed mostly on the pomes of hawthorns (Crataegus spp.). Since then, a new set of races has evolved. These subspecies, each genetically suited to their host, feed on the relatives of the haws: the apples (Malus spp.), the pears (Pyrus spp.) and the roses (Rosa spp.). It can also feed on Prunus species. The large cultivated fruits the pioneers brought were a bonanza for the native hawthorn maggot. It evolved rapidly to fill the new niches.
In orchards the apple maggot is controlled by insecticides, such as Malathion, applied after the flower bloom. Home owners are sometimes taken by surprise when infestations afflict their backyard fruit trees. Generally these infestations are not equally severe every year. Household insecticide application in the spring can be effective if this is a recurring problem. For those who wish to use organic pesticides, Rotenone is somewhat effective.
Honeylocust plantbugs (Diaphnocoris chlorionis) are a true hemipteran 'bug'. They feed mostly on honey locust. They also occur on black locust, which they tend to harm less. These small bugs resemble leafhoppers, but the plantbug nymphs have more prominent pre-wing buds. The late nymphs and adults are about 3 millimetres long. The nymphs are like the adults, squat, with wide spaced eyes, stubby pre-wing buds, and piercing mouth parts. This plantbug has a cockroach-like flattened body. It is the feeding of these nymphs that can weaken developing leaves such that some branches may be virtually defoliated, even before they fully flush out. Often the more severely defoliated branches die in the following spring. By about mid June these nymphs moult into winged adults. The adults fly off to search for new locust trees, to lay the eggs for the next generation. Eggs are inserted into the bark in midsummer, where they over winter. There is only one generation per year.
Some combination of factors caused these sap sucking insects became a major problem in the spring of 2004, in the environs of Toronto.
Leafhoppers are bugs in the homopteran clade. There are many species of leafhoppers, which feed on a variety of hosts. One common species infests locust trees. The honeylocust leafhopper (Macropsis fumipennis) is a sap-feeding bug with folded-back wings. The small nymphs of leafhoppers are paler than the plantbug nymphs. Leafhoppers have broad heads, wide set eyes, and they have a lateral flattening and a cicada-like body. Leafhoppers often have several generations per year But the leafhoppers are not as damaging as are honeylocust plantbugs. Honeylocust plantbugs often share their habitat with honeylocust leafhoppers.
Plantbugs can be somewhat controlled with insecticidal soaps or contact insecticides. Like all sucking insects, these controls are not always fully effective. Usually, by the time the withered leaves are noticed, much of the damage has already been done. Leafhoppers and plantbugs can, in their early stages, be partly controlled by knocking them off by hosing them with water. When they are adults, blasting them off is not as effective, as the adults can fly back to their host tree.
Chafers & White Grubs
A number of beetles cause damage to plants in both their adults and larval stages. The large pale beetle larvae, with bodies bent head-to-tail, which live in the upper soil layers, are often called ‘white grubs’. These white grubs are one of the food items that skunks are seeking when they dig-up lawns. Most white grubs are members of the Scarabaeidae family. Many white grubs feed on grass roots, although broadleaf roots may also be attacked. Most white grubs live for about one year underground, pupate in the earth, and emerge briefly as breeding adults in the summer. The adults feed on leaves, often on different host plants than the larvae. Scarbaeid beetles are distinguished by their shiny metallic-looking forewings (elytra), thick rounded bodies, and ‘L’ shaped antennae with lamellate tips. They are very similar to the scarabs (dung beetles), which were a popular motif in ancient Egyptian artwork.
The Japanese beetle (Popillea japonica) is about a centimetre long, with ametallic-green head, thorax and legs. The elytra are shiny and copper-coloured. It has distinctive tufts of white bristles around the rim of its abdomen. The white grub feeds mostly on grass roots. Unlike most white grubs, the larvae can cause significant damage to turf grass. (Anything that kills turf grass can't be all bad.) Adult feeding can skeletonise both leaves and petals. The adult phase feeds on many species including: apple, birch, horsechestnut, linden, maple, oak, rose, and the prunus species. This introduced species can be very damaging to garden plants.
The rose chafer (Macrodactylus subspinsus) is less than a centimetre long, narrower than most other scarabs, and has golden-coloured elytra. The larval form feeds on a variety of grass and weed roots, causing little damage of human concern. The adult phase skeletonises many plant leaves, including: rose, apple, mountain-ash and other members of the rose family. Grape, peony and birch are sometimes targeted. The adult beetle is toxic, and birds learn to avoid it. The rose chafer is native to the Carolinian region, and has close relatives in other regions.
June beetles, May beetles or ‘Junebugs’ (Phyllophaga spp. ) are a set of scarabaeids with bodies over a centimetre long with chestnut-brown elytra. The larva forms generally feeds on grass roots. Some species, like the P. crinita of Texas, are a turf pest. In the north, the larval forms may spend up to three years underground, and do not significantly damage turf grass. The adult beetle feeds on a wide variety of plant hosts. Oak leaves are the preferred food source of many June beetle species.
White grubs often attract skunks and raccoons, which dig-up turf to catch the grubs, and in the process they ruin the lawn. Often the animal damage is worse than the grubs themselves. Mothballs can be used to repel these digging animals. Some white grubs can kill turf grass outright. Most white grubs can be reduced in number with various commercially available poisons. An application of insecticidal dusts around valued plants can reduce local white grub populations, and thus reduce the number of adult beetles. The adult beetles are of greater significance as garden pests. The feeding beetles can be knocked down with a strong contact insecticide. It is best to apply the contact insecticide when the beetles actually appear on the leaves.
Ambrosia Beetles & Fungi
Ambrosia beetles are a peculiar group of borers. The adult beetle is small, a few millimetres long, black and elongated. It is only the female beetle which bores, as an adult, into sapwood or heartwood to form brood chambers. These chambers are several centimetres deep, with several short side galleries. The eggs inside these chambers hatch to form little grubs. The larvae feed on a particular fungus, and do not themselves bore into wood. In the spring the adults emerge, the females seeking their preferred host species.
Ambrosia beetle larvae live on a fungus. One common species is Ambrosiella brunnea. The imperfect, or asexual, form of the fungus used to be called Monilia brunnea. The fungus, as known, is mostly asexual in its reproduction. The fungus forms a dark coloured mycelium in the wood around the brood chambers. The fungus and the beetle have a mutualistic relationship, as it is mostly the beetle which spreads the conidial spores.
There are many species of ambrosia beetle, in the genera Monothrum, Trypodendron, Xyleborus, Xylosandrus and Xyleborinus. The introduced Asian ambrosia beetle (Xylosandrus crassiusculus) is a problem for many tree species, including sweetgum, hickories, elms, oaks and magnolias. The western ambrosia borer (Monothrum scutellare) is especially a problem for coast live oak. Research has shown that, as long suspected, various decay agents can penetrate into a tree along the brood chamber tunnels. Since both the beetle and its fungus garden need oxygen, the beetle clears out her tunnel of frass. The well aired tube allows fusarium, hypoxylon, inonotus and other fungi to instigate rot deeper into the tree. Even the sudden oak death oomycete can spread more rapidly along an ambrosia tunnel (Švihra & Kelly 2004).
The ambrosia beetle can be controlled with insecticides. Only if the timing of spraying is opportune is this effective. Spraying for borers has always been problematic. Controlling the beetle could reduce the rate of heartwood decay.
Beetles and fungi often have symbiotic relationships with each other. The Scolytus beetles are often said to have a symbiotic relationship with the elm wilt fungus. The beetle more easily feeds on weak elms, and it seems to spread the fungus - as if to weaken its host. But some beetles are even more obviously symbiotic than that. These beetles can spread fungi about, and the fungi in turn feed the beetles’ larvae. This kind of symbiosis occurs in the aforesaid ambrosia beetles. Symbiosis also occurs in the beetle genera: Dendroctonus and Ips. In some species, the beetle has a special pouch (mycangium) that carries the spores or mycelial fragments. The same beetles may also carry tiny mites. The mites in turn also feed on the fungi. Mites and beetles are more-or-less competitors. The mites using the beetles mostly as mere vectors.
Aphids are homopteran bugs in the Aphididae family. They have small squat bodies, often they have horn-like siphunculi on their abdomens, they can have four wings, and they have piercing mouth parts. They have a gradual metamorphosis from nymph to adult form. During the height of their feeding-season, usually summer, the aphids are often wingless. During this feeding-season they tend to reproduce via parthenogenesis or ‘virgin birth’. That is, the mother’s unfertilised egg hatches inside her. The nymphs thus ‘live-born’ are all females. Sometimes these female nymphs are born already pregnant! Cycles of parthenogenesis may buff-up aphid populations to incredible numbers. At the end of the feeding-season, male aphids are born. Then the winged adults, both male and female, take flight to seek mates.
Like most ‘true bugs’ aphids feed on plant sap. In fact, they feed so voraciously that their excreta can contain plenty of undigested sugar. This ‘honeydew’ then becomes food for ants, fungi, and other creatures. Indeed, aphids themselves are preyed upon by many other animals.
There are many species of aphids, most are rather specialised as to the host plant species they can feed upon. Aphids do not always build into outrageous populations. This is because predators keep them in check, and also because plant sap contains toxins to deter sap-feeders. Nevertheless, sometimes aphid populations become so high that plants wilt, and even die, from loss of sap. Furthermore, aphids can spread viruses, and their honeydew can instigate fungal growth.
Ecologic conditions that lead to aphid plagues are rather particular, they depend on rain, temperature, predator populations, and other factors. It is difficult to predict when aphid populations will explode. Aphids often proliferate on plants that have been over-fertilised. With excessive nutrients, plants often produce new shoot growth out of proportion to their defensive toxin production. If the plants lacks sufficient natural toxins, aphids can more easily proliferate. Luckily, aphids are one of the few insects that can be controlled by relatively mild insecticides, including insecticidal soaps. Aphids can be checked in very early spring by dormant oil application. If a few aphids appear early in the season, it is best not to over-react. Too much insecticide, applied too soon, can kill-off natural aphid predators.
Anulewicz, A., McCullough, D.G. and Cappaert, D.L. 2007. Emerald Ash Borer (Agrilus planipennis) Density and Canopy Dieback in Three North American Ash Species. Arboriculture & Urban Forestry. 33(5): 338-349.
Cranshaw, Whitney. 2004. Garden Insects of North America. Princeton University Press. Princeton.
Grierson, D. and Covey, S.N. 1988. Plant Molecular Biology. 2nd Edition. Blackie. London.
Klepzig, K.D.; Moser, J.C.; Lombardero, F.J.; Hofstetter, R.W. and Ayres, M.P. 2001. Symbiosis and Competition: complex interactions among beetles, fungi and mites. Symbiosis. 30: 83-96.
Rose, A.H. et Lindquist, O.H. 1982. Insectes des feuillus de l'est du Canada. Ministère de l'Environnement Service canadien des foréts. Ottawa.
Smith, D.C. 1988. Heritable divergence of Rhagoletis pomonella host races by seasonal asynchrony. Nature (336): 66-67.
Strauss, S. 2003. Voracious Asian beetle discovered in city maple trees, officials say. Globe & Mail. Saturday September 13. A17.
Švihra, Pavel and Kelly, Maggi. 2004. Importance of oak ambrosia beetles in predisposing coast live oak trees to wood decay. Journal of Arboriculture. 30(6): 371-376.
Ash Flower Galls
Ash trees, especially green ash ( Fraxinus excelsior ), can develop odd witches' broom type galls on their male flowers. These growths look like a cluster of miniscule deformed leaves crowded around branch tips. Theses growths are instigated by a mite (Eriophyes fraxiniflora ). In the autumn they look like fist sized balls of tuff dangling from the branches. Such galls are fairly common in urban landscape settings. Indeed, they can be so common that some people come to believe that they are normal.
Eriophytes mites over winter in bud scales. The adult feeds on the flower buds in the spring. These mites disrupts the growth of the flowers and stimulate the development of a fringe of disfigured flowers and leaves around the bud. The mites move on to new buds when the current gall dies and dries up.
Eriophytes mites are somewhat sensitive to dormant oils. If these oils are applied just prior to bud break they can be effective in controlling the mites. However, indiscriminate use of dormant oils can kill natural predators, and make the situation worse. This exacerbating of a pest problem with pesticide application is especially prevalent for mites. Natural predators of mites are often more sensitive to pesticides than are the mites themselves.
Scale insects are small Coccoidea bugs. These create the small pale clusters of scales that often infest the stems of plants. These scales are hard waxy shields that protect the insects from predators. The insects spend most of their life stationary, feeding off plant juices which they obtain by piercing the cell walls with their stylet-like mouths. Females have no wings, and adult males are winged in some species, but not in all species. Many species rely on females that reproduce parthenogenetically. In either case, the females lay eggs under their waxy shields. When a nymph hatches it is, for a short while, without its own waxy shield. In most species the younger nymphs have some ability to crawl about. This crawling is one of the main methods by which scale insects spread. When a nymph finds a new feeding local, it settles down and excretes its own scale (waxy shield).
Luckily scale insects have many natural enemies, such as ladybird-beetles. Unfortunately, the proliferation of scales can overwhelm natural controls. Large numbers of scale insects can weaken a plant. The honey dew waste of these insects can accumulate on plants, and this can become mouldy. Because of their waxy shield the adults are little affected by contact insecticides. (Systemic insecticides, that are absorbed into the plant, often become too diluted in the plant's sap to be effective.) Therefore, the best time for chemical control is when the crawlers are moving. This crawling can be confined to a short time period, a window of vulnerability. This time window varies from one species of scale insect to another. Thus, one should watch the scales with a magnifier. When crawlers are first detected, apply the insecticide. This should kill some of the new generation of bugs, and reduce their overall numbers.
Agrobacterial -Crown Galls
Crown galls are growths instigated by Agrobacterium tumefaciens, a soil bacteria. These are the burls or 'tumours' that are appear near the base of tree trunks. Sometimes the galls form higher on the trunk, or on branches. The Agrobacterium stimulates the plant host into making the gall. The uncontrolled cell division in the cambium is instigated by genetic material from the bacterium. Strangely, this genetic material becomes incorporated into the genome of the host plant's cells. (Virus are not the only lifeforms that can pass genes to other organisms.) Crown galls can be up to 100 cm wide, but mostly they are much smaller. The bacteria can live for long time periods in the soil. It is suspected that the bacteria enter the trees through small wounds. The bacters, seemingly, can be carried on air-borne dirt in the wind. Once inside a tree they insert the gall forming genes into the host, and then multiply as the gall grows.
Note: Not all burls are caused by Agrobacterium. And not all strains of Agrobacterium tumefaciens stimulate the formation of visible galls.
There is not much evidence that crown galls harm trees. Removal of infected trees is seldom necessary. In fact, removal can actually cause the disease to spread. These galls only spread from direct contact between the bacteria and living wood. Breaking up the galls with chain saws, et cetera, could releases bacteria to the air. It is also possible to transport these agrobacteria with pruning tools that have not been cleaned between uses.
There are cultivars and species that are known to be resistant to crown gall. If agrobacteria are known to be common in an area, resistant plant varieties should be selected for planting.
Fire blight (Erwinia amylovora) is a bacterial disease. The blight afflicts plants in the apple-rose family (Rosaceae). This blight is typified by a sudden death of leaves and stems. These parts die, turn black, and the woody parts develop cankers. The dead tissue are usually near branch tips. Twigs often looks like they have been scorched with fire.
Fire blight bacteria infections enter through tiny wounds. Newly developed flowers are a common entry point. The bacteria, if en masse, appear like an orange ooze. The tissue they kill, as stated before, tends to turn black. The bacters over-winter in the cankers on twigs. The bacters are then spread by rain splashing, birds or other agents that contact the bacterial ooze. The blight can cause significant damage to the plants.
Pruning out the cankered branches is an effective method of slowing the spread of the blight. Copper compounds and other bactericides can be used to kill off fire blight bacteria. Spraying is best done when the flowers are opening. Generally, this spraying is reserved for nurseries and orchards.
Cedar-Apple Rust - 'Orange Jelly' Rust
There are many kinds of Gymnosporangium rusts. These rusts are Uredinales fungi. Cedar-apple rust Gymnosporangium juniperi-virginianae is one of the most interesting. Rust diseases are fungi with alternate hosts. The cedar apple rust alternates between apple trees (Malus spp. ) and red 'cedars' ( Juniperus virginiana ).
Sexual spores from the juniper host spread via the wind to apple trees in the spring. From these sexual spores develop spermatia and receptive strains of hyphae. (Hyphae strains are somewhat analogous to the male and female sexes in higher plants.) If the two hyphae 'strains' encounter one another they undergo sexual fusion to become the aeciospore generation. The 'fruiting bodies' appear as a typical leafspot fungus. From these leaf spots protrude aecia. These aecia bear the asexual aeciospores. A rim of pale yellow, or red, area of leaf tissue may surround each cluster of spore-bodies. The asexual spores can be carried long distance in the wind, and thus infect nearby junipers.
On red cedar junipers, the rust causes the development of swollen twigs. These swellings are teliospore masses also called telial galls. Telial galls swell up to a few centimetres wide. Teliospores produce the sexual spores on a basidium. (A basidium is a spore bearing structure typical of the Basidiomycotina fungi.) These teliospore masses eventually ooze an orange gelatinous substance. This ooze projects in long translucent strands up to four centimetres long. These jellies look bizarre enough to cause concern.
Telial galls can be pruned off junipers. Rusts are harder to treat on apples, as systemic fungicides need to be applied in advance. In apple orchards this is often done on a scheduled regimen anyway. Since the cedar-apple rust does little damage to the juniper, it can be tolerated to some degree. Indeed, some people find the orange jellies interesting.
Cedar-apple rust can be avoided by planting juniper and apple trees fairly far apart. Since the spores can travel for kilometres in the wind, the distance between host species needs to be great. This is not always practical, as some one else may own the other host tree(s).
Pear Trellis Rust
Pear trellis rust Gymnosporangium sabinae is similar to apple-cedar rust, but it infects pears (Pyrus spp. ) instead! This rust fungus even uses juniper as its alternate host! The orange jellies on the junipers are somewhat similar. But, the orange spots on the pear leaves are a little more colourful.
The rust fungus was originally native to Europe. It spread to western North America in the 1960s. From British Columbia it then spread by the late 2000s to Ontario! However, the comestible pear is a native to Eurasia anyway. So in a sense the fungus simply has found its old host in a new land.
Hawthorn rust (G. globosum) and quince rust (G. clavipes) are yet other related rusts. They afflict hawthorns, and the later fungus attacks quince as well. All of these fungi are very similar in that juniper is the alternate host. If anything, hawthorns are even more likely to suffer from rust than are pears or apples. As with these other fruits, these rusts seldom kill the trees outright. They do however make the fruit unappetising and unsightly.
Hawthorn rust and pear trellis rust fungi are controlled in nearly the same manner as apple-cedar rust. Like other rusts if the life cycle can be checked at any point, it reduces the contagion in the following year. This includes all of the standard hygienic measures such as disposing of old leaves and minimising the number of junipers in the neighbourhood.
White Pine Blister Rust
Cankers on white pine stems are often a sign of Cronartium ribicola or 'white pine blister rust'. Blister rust is a fungus with a complex life cycle. A fungus growing on blackcurrents, and other Ribes species, releases teliospores during the growing season. These spore infect white pine, where they grow as fungal mycelia in the cambium of the pine host. Later in the year yellowing blisters, and cankers, develop on the white pine stems. These blisters release asciospores which re-infect blackcurrents. Blister rust can be fatal in white pine, but not always.
If possible do not plant blackcurrents near white pines, or vice versa.
Anthracnose is a general name for a number of fungi, including: Discula spp. , Glomerella spp. , Gnomonia spp. , Kabatiella spp. , Marssoniella spp. , Marssonia spp. , and Monostichella spp. Although caused by diverse types of fungi, all anthracnose diseases are typified by dying leaf margins, that look like frost damage. From the leaf margins, the zones of necrosis can expand to the petiole, and even onto the twig, causing cankers on the bark.
Avoiding moist conditions under, and around, a plant can decrease the spread of the disease. For leaf infecting anthracnose produces spores on fallen dead leaves. Wet weather tends to favour the development of spores, and increase their chance of taking hold. The case is generally different for those anthracnose fungi that cause cankers on twigs. These species do not generally grow on fallen leaves. However, these species also spread more rapidly in rainy weather. The spores, produced in the bark cankers, can be splashed around by rain drops.
Dogwood anthracnose is best controlled by cutting off the branches which have cankers and leaves that exhibit dying margins. Anthracnose can be diminished by applying fungicides in the spring just before the buds break. Another application should be applied when the leaves are half open, about two weeks later. Like most fungicide treatments, one must presuppose that a given plant is susceptible in advance of the actual outbreak. Fungicide application after the fact is too late.
Wood Rot Fungi
Armillaria mellea or honey mushroom.
Toadstools are often visible under trees. Some toadstools are mycorrhizae fungi, which are in fact beneficial, as they live symbiotically with tree roots. The Amanita toadstools are important mycorrhizae fungi of birch, pine and fir. Boletus toadstools often form mycorrhizae in deciduous forests. Very few soil toadstools are harmful to trees in any way. Most micro-fungi are saprobes which are beneficial as decay agents. A number of micro-fungi do cause some problems for plants. Micro-fungal parasites cause needle cast and a host of other plant diseases. A few toadstools and bracket fungi (fomes) also grow upon plant tissues as parasites.
A fungus does not engulf food as an animal does. Rather, it is as if a fungus wears its stomach on its outside. Each feeding hypha releases digestive enzymes from its surface. These enzymes digest food in the surrounding milieu, and then the nutrients are absorbed back into the hypha. A wood rot fungus tends to gain access to wood by insinuating its hyphae into the conductive cells of the plant. Once inside a tracheid, or a vessel, a hypha releases enzymes which breaks down the cell's wall. In this manner they devour wood from the inside out.
Most of these wood rot fungi have both asexual and sexual sporocarps. The asexual anamorphs can differ greatly from sexual sporocarps in form and colour. The chlamydospore and conidiospore bodies are often small and hidden inside the wood or in hollows. The asci or basidia bearing bodies are larger and more exposed.
If the 'fruiting bodies' of wood-rotting fungi appear on trees, this means two possible things: (1) fungi are digesting dead tissue in the tree, or (2) fungi are killing and then digesting living tissues in the tree. The dead tissue is usually heartwood, and the living tissue is usually sapwood.
The vast majority of wood-feeding fungi live off dead wood. These can still be a problem, if they make the tree structurally less sound. Hollow trees are often hollow because fungi have digested away the dead heartwood in the core. Such hollow trees are prone to breaking in windstorms, ice storms and other stresses.
For more information on fungi go to the Mycology Page.
Buszacki, Stefan and Harris, Keith. 1998. Pests, Diseases & Disorders of Garden Plants. Harper Collins Publishers. London.
Soil Nutrient Deficiencies
The elements nitrogen (N), phosphorus (P) and potassium (K) are the mineral nutrients most commonly lacking in soils. Generally this is not because they are totally absent, but rather because they are in a form unusable by plants. Nutrient deficiencies often have tell-tale symptoms in plants. Most of the essential nutrient elements are in the 'upper' part of the Periodic Table. Some elements are essential, and are necessary for the production of proteins, carbohydrates, lipids and nucleic acids. In particular, hydrogen (H), carbon (C), nitrogen (N), oxygen (O) and phosphorus (P) appear to be necessary for all known life forms.
Biological Periodic Table
In the tables below, the 'Symptom' boxes refer to a plant's symptoms when the element is lacking or less than optimal. The 'Physio' boxes indicate what physiological role the nutrient has for plants.
Plants require some of the transition metals as components of their enzymes, co-enzymes, proteins and other important metal-organic compounds. Cadmium, palladium, silver, mercury and even tungsten are used by some organisms. Higher plants do not seem to require most of these metals. Nevertheless, some transition metals are essential for plant metallo-enzymes. Some of the transition metals required by plants include:
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