Urban Trees

D. Andrew White, 01, 01, 2015

watershoots on horse-chestnut

Ash Trees

watershoots on horse-chestnut

Ash, fresno or frêne (Fraxinus spp.) is a genus in the olive family (Oleaceae). Ash usually has a compound leaf with 3 to 9 leaflets Autumn colouration is yellow to golden-orange. Leaf arrangement is decussate like a maple. The twigs often branch in a tri-furcate manner, also like a maple. The greyish twigs tend to be stout with large leaf-scars. The bark on a mature tree is usually finely fissured. They vary from dioecious to monoecious forms. When monoecious the flowers are 'perfect' with male and female elements together. Flowering occurs before or during leaf flush. The small flowers are arranged in small panicles. In most species the flowers lack petals. The seed is a samara with a single paddle-like wing. Seeds occur in clusters. A female ash can be quite fecund. Many ash species grow into trees up to 40 metres tall. They were once widely planted as urban trees. However, the emerald ash borer (EAB) beetle is causing a massive die-off of our native ash trees.

Fraxinus is a genus common in temperate North America and Eurasia. There are even a few tropical representatives. Ash are one of the deciduous trees found in nearly every forested area in North America, even in the south-eastern boreal forest region! There are many west coast ash, as well as desert and scrubland species. There are about sixty-five ash species in the world, a few are utilised as ornamentals:

White ash (Fraxinus americana) is Carolinian forest ash with large terminal leaflets. Its grey neatly fissured bark is quite distinctive. It is dioecious, male and female trees are separate. It is a medium hardy ornamental.

The ISA Species Rating: 69-75%.

Green or red ash (F. pennsylvanica) is a Carolinian and prairie margin tree. It is a more northern tree than is white ash. It has thin tapered leaflets, each leaflet having a very short petiole. It is can be confused with European ash, but it has lighter coloured buds. It is fairly city hardy.

The ISA Species Rating: 63-72%.

European ash (F. excelsior) is a species native to Eurasia west of the Caucus. It has more (9-13) and thinner leaflets than white ash. Its nearly black leaf-buds are quite distinctive. It can be either monoecious or diecious. The bark is darker and less evenly fissured than white ash's. It makes a very hardy street tree.

The ISA Species Rating: 60%.

Ash trees are widely considered to have 'no exceptional single feature'. They are often infested with flower mites, and by the emerald ash borer. They do not, generally, have showy flowers, nor is their autumn colour outstanding. Nevertheless, in the summertime they have as splendid a form as any shade tree.


Amboyna, ghetto palm, stinking chun, stinking ash, paradise tree, Chinese sumac, tree-of-heaven, or Ailanthus altissima, is a tree of many names. Ailanthus is a species native to the temperate forests of China. It is a member of the Simaroubaceae family. It has long compound leaves, which are somewhat like walnut leaves. Its seeds are papery 'keys' with seeds in the middle. Superficially the seeds look like those of ash trees. They differ in that in mid-summer the dangling seed clusters often turn bright red, orange or yellow-orange. The tree is sometimes mistaken for a kind of ash. The tree grows rapidly and can reach 25 m height or more. Overall the crown form and aspect resembles an small walnut tree or a large staghorn sumac.

Ailanthus has been widely planted as an ornamental tree, for it is an extremely hardy tree in the urban environs. Aesthetically, other problems aside, it does have a handsome form. The tree grows rapidly into a good-sized luxuriant tree in a couple of decades. Very few tree species grow to such a size in so little time.

Ailanthus is no longer a popular tree. The stinky leaves are a minor nuisance. The odour is usually only pronounced when the leaves are crushed or the twigs are damaged. The waning popularity of ailanthus is due to the fact that it is too commonly encountered not as a deliberately planted tree, but as a naturalised weed. Ailanthus seedlings too often sprout-up between fence mesh, in the cracks of sidewalks, against walls, in the middle of gardens, in flower beds, inside hedges, and even in aevestroughs! Not only to they seed easily, but they can clone themselves by root-suckers as well! The ailanthus is very fecund. The movie entitled A Tree Grows in Brooklyn was named after this hardy 'ghetto palm'.

Ailanthus is really not so infernal a tree. What is required is a constant vigilance against the many seedling it produces. If one has the patience to constantly weed out these seedling, the ailanthus is a fine choice of a yard ornamental. If one does not have the time for this constant weeding, ailanthus is not a good choice.

The ISA Species Rating: 49%.

English Oak

English oak, also called truffle oak, pollardo, or common oak, chêne commun or chêne pédonculé (Quercus robur) is the white oak of Europe. It is the oak that was once dominant in many of the temperate forests of Europe. It was the picturesque oak of Romantic art, the oak of pastorale poetry. It is the oak after which Derry in Ireland gets its name (darach). It was one of the oaks held sacred by the Druid class of the Celts, and the oak of the Sherwood Forest. Most varieties are capable of becoming a fair sized tree, up to 25 metres or more in height. They can, if allowed to grow old, develop the twisted twigs, broad crown and massive branches so typical of white oaks.

Like many of the white oaks, it has edible acorns, and leaves with rounded lobes. The leaf lobes (3-7) are more evenly rounded than those of the American white oak (Quercus alba). The leaves have light coloured hairs on the underside. English oak leaves have very short stems, the lower leaf lobe almost touches the twig. The acorns are elongate, and have long stocks. They are seasonally deciduous, but the dead leaves tend to cling to the tree over winter. This retaining of dead winter leaves is called marcescence. (The tendency to have marcescent leaves is common in the beech-oak family.) All the white oaks are fairly closely related, and most can be hybridised. English oak is no exception, it has often been crossed with other white oaks.

English oak comes in several cultivated varieties. These vary in crown form from very erect, to broad, and from large to fairly small varieties. They also vary in marcescence, leaf form and leaf hue and colour. There are even varieties with oblong entire leaves, leaves un-lobed like those of live oaks. English oak varieties are difficult to propagate by grafting, varieties are often grown from seed.

The wood is considered one of the best and most beautiful of woods for furniture and cabinetry. In the Americas, English oak is most often grown as an ornamental. The English oak is more tolerant of urban pollution and compacted soil than are most native white oaks (eg. Q. alba & Q. macrocarpa). Since red oaks tend to be more hardy in urban milieux, they are planted more often than white oaks, especially in the east. Native white oaks display the very epitome of oak-ness, broad spreading crowns with picturesque contorted branches. Since the American white oaks do not thrive in cities, the English oak can take its place. (Unfortunately the erect-crowned 'Fastigiata' variety has been too widely planted.) If one overcomes a bias against non-native trees, the English oak can and should be welcomed as a valuable addition to our New World repertoire of hardy urban trees.

The ISA Species Rating: 75-83%.

American White Oaks

white oak leaf & galls

American white oaks can become spectacular trees. They can have broad spreading crowns, with picturesque gnarled branches. Their leaves have rounded lobes, if they are lobed at all. The autumn foliage is often a beautiful purplish golden-brown. They can live as long as three centuries, or even more. Native white oaks, unfortunately, are not very hardy as urban trees. Young trees right from the nursery are especially likely to succumb to urban stresses. Older established white oaks are much hardier.

Some examples of eastern American white oaks include:

Eastern white oak (Quercus alba) is a Carolinian species. This is a medium-moist (mesic) forest tree. It has a longer leaf petiole, and longer leaf lobes, than does the English oak. Leaf form is very variable. The acorns are rounded. The bark is light coloured and corky. After a few centuries the oak can grow to 24 metres tall. It is not very tolerant of urban stresses.

The ISA Species Rating: 79%.

Bur oak or burloak (Quercus macrocarpa) is a northern Carolinian species that ranges west into the Midwest's prairie frontier. In the pioneer days there were once broad ranges of burloak-savannah called "oak openings". These oak openings occurred on the eastern margins of the prairies. This short-grass savannah at onetime graded into the prairie proper. The burloak leaf usually has deep sinuses in the middle, with a broad shallow lobed 'spatula' tip. The nut's involucre is large, with bur-like fringes that half surround the acorn. The bark is light coloured and more corky than is eastern white oak's bark. In about a century the burloak can reach 29 metres in height. It is somewhat more tolerant of the urban milieu than is eastern white oak.

The ISA Species Rating: 76%.

The chestnut or chinkapin oaks are a set of white oaks with chestnut-like leaves and oblong acorns. Swamp chestnut oak (Quercus michauxii) grows down south 'in Dixie'. Chinkapin oak (Q. muehlenburgii) is native to the Carolinian zone, with outliers in Texas and Mexico. Swamp white oak (Q. bicolor) is native to the northern Carolinian swamps. Chestnut oak (Q. prinus) prefers dry uplands in the Carolinian zone. They can grow to 25 metres or more in height. Most chinkapin oaks can easily inter-hybridise with each other. Generally they are not hardy urban trees.

Virginia live oak (Quercus virginiana) is a white oak. This oak has a simple, or very slightly lobed, oval shaped leaf. The acorns are oblong. Its bark is corky but finely fissured. Live oak is semi-deciduous when planted north of its normal range. In its native range it is an evergreen. The live oak grows wild from South Carolina south into Mexico along the coastal lowlands. Old trees can be gnarled and very broad crowned. It can grow to 30 metres tall. It can be festooned with the drooping 'spanish moss' (Tillandsia usueoides) bromeliad. Like many southern trees it can also harbour mistletoe (Phoradendron serotinum). Of course, this beautiful oak cannot be grown outdoors in Canada. It can tolerate only very mild winters.

There are other white oaks in the east. Overcup oak (Quercus lyrata) looks like a version of the burloak. Its acorn involucre nearly covers the nut. The post oak (Q. stellata) is a dry upland oak of the central Carolinian. The leaf is burloak-like, and the tree very gnarled. There are several west coast white oak species, such as the Oregon oak (Q. garryana). A few of these oaks have been utilised as ornamentals. Garry oak, for example, is an urban tree in the west.


cottonwood trees in Nevada

Let me ride through the wide open
Country that I love.
Don't Fence Me In.
Let me be by myself in the evening breeze,
Listen to the murmur of the cottonwood trees. …

Don’t Fence Me In, Cole Porter© & Robert Fletcher


Eastern cottonwood is one of the poplars, those fast growing trees planted as windbreaks on so many North American farms, beach resorts and trailer parks. It lacks the delicate twinkling leaves of trembling aspen, the interesting leaves of bigtoothed aspen, and the strange folklore of balm of Gilead. Nevertheless, cottonwood is a tree of the 'Wild West'.

Eastern cottonwood (Populus deltoides) may be an eastern race of the plains cottonwood (Populus sargentii or P. deltoides var. occidentalis). In some places the species/races overlap and hybridise. In fact, the cottonwood can even be hybridised other species of aspens and poplars. Even the balsam poplar (P. balsamifera) is connected to cottonwood - via a chain of intermediate hybrids. The cottonwood grows naturally along riversides, on new floodplains, in prairie gulches, and also most anywhere people choose to plant it.

Cottonwood is a poplar, a member of the willow family. Like most poplars it grows straight and tall, growing up to 25 metres in a few decades it can match much older trees in height. Most of the giant trees do not outlive human beings. Although, there are some cottonwoods that have lived more than a century. The leaves are deltoid with small teeth. They flutter in the wind, like many 'singing poplars' the fluttering has a distinct sound. The twigs are thick and the bark deeply furrowed on older trees. Cottonwood is dioecious, they have male and female flowers on separate trees. The flowers are wind pollinated. The seeds of the female are spread in late spring by the wind also. Each seed bears a cottony tuft that carry aloft the seeds, which can rain down by the thousands like snow flakes. Very often the trees clone themselves as root suckers. So a patch of cottonwood is often genetically one individual. Thus it is that one can find in natural stands a mosaic of female and male trees in separate clusters.

Cottonwood is a valuable tree in the pulp-and-paper industry. It is often harvested from the copious secondary growth that sprouts up after clear-cutting. Cottowood has been planted in school yards and parks, sometimes as windbreaks. This was not always a very wise choice, as the tree grows old rapidly and can become a hazard. Dead and dying cottonwoods are less wind-sturdy than vibrant young trees. The life of cottonwoods can be extended by pollarding, but this is a stop-gap solution. Cottonwoods are best be planted on beaches and as farm yard windbreaks, not in school yards and backyards. Better yet, leave the noble giants to the floodplains and prairies.

The ISA Species Rating: 35-45%.


Linden, 'lime' or basswood trees (Tilia spp.) are a genus of trees native to both North America and Eurasia. The American basswood (T. americana) is a linden with very large leaves. The littleleaf linden (T. cordata), which is more commonly planted as an ornamental, is of European origin. True to its name, the littleleaf linden has small leaves.

Lindens are generally trees of moist (mesic) valleys and forest under-stories. The trees have cordate (heart shaped) lopsided leaves with serrate margins. The canopies are very dense, the leaves intercept most of the direct sunlight, hence they are excellent shade trees. When grown in the open, the crowns develop a pyramidal form. Some linden species can reach over 25 metres in height. In the early summer small yellow flowers form in panicles attached to elongated (strap-shaped) bracts. When the clusters of mature drupe fruits mature, the bract acts as propeller-like key which helps the seed clusters drift in the wind. In the autumn linden leaves sometimes turn a nice yellow, but often they are rather brownish. Lindens are not therefore noted for their autumn colours.

Lindens do not have hard wood. Nevertheless, they can live to a ripe old age, if allowed to do so. The well known Wolframslinde near Ried Bavaria has lived for several centuries. It became a hollow shell of a tree, with gaps wide enough for a person to crawl into its hollow centre. The old rows of lindens in Skokloster Park in Sweden are centuries old. These fantastical shaped hollow trees are host to a very diverse insect fauna. In the Americas very few basswoods are anywhere near as old as are some lindens in the gardens and parks of Europe. Although, there are some basswoods of century-plus age in the eastern USA. All of these giants occur in acultivated settings. Lindens are not ‘old growth’ species in the wild. It is in the protection of parks and gardens that lindens reach their maximum lifespan.

There are many hybrids of American and Eurasian lindens. However, the majority of lindens in American urban settings are littleleaf lindens. Littleleaf lindens have been proven as a hardy urban species. They are a little more resilient to dry soil, gypsy moth, anthracnose and vascular wilt than native basswoods, at least in the urban milieu. So common have littleleaf lindens become that there has developed a widespread belief that they are 'cheap' trees. Many urban residents feel they are being 'ripped off' if lindens are planted on their street. By all accounts, lindens are actually a good and hardy urban choice. People do well to remember that not every species thrives in the heart of a city. Since littleleaf linden does thrive in cities, it has been well chosen for that role.

The ISA Species Rating: 67-76%.


Jonsell, Mats. 2004. Old Park Trees: a highly desirable resource for both history and beetle diversity. Journal of Arboriculture. 30(4): 238- 244.

Pakenham, Thomas. 2002. Remarkable Trees of the World. Weidenfeld & Nicolson. London.

Box-Elder - Manitoba Maple

box-elder flowers

Box-elder, Manitoba maple, or ash-leaved maple (Acer negundo) is a maple with compound leaves. In Canada this feature is unusual, but in Eurasia compound leaved maples are not uncommon. The leaves often have up to seven leaflets, although small leaves are often palmate. The keys have doubled narrow seeds with paddle-like wings. Keys are too often produced in wild abundance, the tree is very fecund. It is not the tallest maple, but it can reach up to 25 metres in height. In crown form it is similar to other maples, but in leaf form it resembles an ash. The tree prefers riparian habitats, but it will grow in dry soil, if planted there.

Manitoba maple is a common tree in riparian areas in the prairies. But it is not confined to the great plains. In fact, not only does it grow in Carolinian forests, but it can occur as an alpine tree in the Sierras of Central America. It is shade-intolerant, hence it is usually found in open urban areas, prairies and floodplains.

Manitoba maple is, believe it nor not, actually planted on purpose in the Canadian prairies by urban foresters. Or rather, easterners find this unbelievable, as box-elder is widely considered a "weed species". Remarkably, the maple is also cultivated in Europe as an exotic. There are even variegated varieties! Although considered a weed in the east, in the prairies it is a valuable urban tree. It is very hardy, and can tolerate very cold winters and long dry summers.

In cities like Toronto Manitoba maples are mostly accidental, natural seedlings that have not been weeded out. Many new home owners find they have inherited a Manitoba maple growing near a fence or by wall. If the tree is big enough, it can be tolerated, even celebrated. It is, after all, a tree. Even an accidental tree is better than no tree at all.

The ISA Species Rating: 39% ('Low' value in Ontario).

Sugar Maple

Sugar maple (Acer saccharum) is the most popular of the 'hard maples'. The leaf is palmate with five main lobes. Each lobe is sharply pointed, and the larger lobes can bear pointed sub-lobes. The leaves turn yellow, and even sometimes a beautiful orange-red in the autumn. It is generally monoecious, an hermaphrodite, as are most maples. A sugar maple flower has five petals, petals are not prominent. Several flowers occur in small umbels. The flowering period precedes leafing in the spring. Each double seeded key has a wing. The wings are bent forward giving the key a overall horse-shoe shape. Occasionally, three seeded keys forms can be found. Young bark is smooth, but older bark is roughly fissured with slabs of peeling 'shagbark'. Sugar maple grows in the mixed Woods of the northern part of the Carolinian into the Great Lakes-St. Lawrence Forest Region. It can grow to more than 30 metres tall.

Sugar maple sap is sweet, and can be concentrated to create maple sugar. Like silver maple and black maple, sugar maple is a sugar-bush tree. Indeed, the maple’s old binomial name Acer saccharophorum meant ‘sugar carrier’. Many species of maple have sugary sap in the spring. However, many species, icluding Norway maple and box-elder, have bitter compounds in their sap that make them unsuitable as sugar trees.

Sugar maple is very similar to the black maple (Acer nigrum) which grows in the south-central part of the sugar maple's range. Black maple has larger downier leaves with fewer points. It also has a less dense crown. Near London Ontario black maple is somewhat more common than north toward Toronto. Black maple can hybridise with sugar maple. Genetic analyses show that the two 'species' hybridise even more than their visible morphological traits might suggest. Almost certainly black maple is not a distinct species (Skepner & Krane 1998). Where the ranges overlap sugar maple can also hybridise with other maple 'species' such as the Florida maple. In the Appalachians it can hybridise with chalk maple.

Sugar maple has many local races or sub-species. At one time many of these races were misclassified as separate species (Sargent 1965, USDA 2008). Some examples of sugar maple subspecies include:

I Acer saccharum subsp. floridanum is the Florida maple of the Dixieland states.
II Acer saccharum subsp. grandidentatum lives in the uplands and montane forests of the Rockies.
III Acer saccharum subsp. leucoderme is a chalk maple of the Appalachian Mountains.
IV Acer saccharum subsp. nigrum is the black maple of the Carolinian bottomlands.
V Acer saccharum subsp. ozarkense is a maple of the Ozarks and forested ‘tablelands’ in the prairies.
VI Acer saccharum subsp. skutchii is the Guatemala maple of the montane forests in Central America.
VII Acer saccharum var. sinuosum is a sugar maple that lives in semi-isolated forest pockets in Texas.

Sugar maple is a fairly hardy urban tree. It is usually judged as superior in form to Norway maple. However, sugar maple cannot tolerate extreme drought, soil compaction or root damage. But it is not unusual in these regards. It is a better choice than Norway maple if the growing conditions are good.

The ISA Species Rating: 75-82%.


Press, B. and Hosking, D. 1992. Trees of Great Britain and Europe. New Holland Publishers (UK) Ltd. London. 188-192.

Sargent, C.S. 1965. Manual of the Trees of North America. Volume Two. General Publishing Company Ltd. Toronto. 681-702.

Skepner, Adam P. and Krane, Daniel E. 1998. RAPD reveals genetic similarity of Acer saccharum and Acer nigrum. Nature. 80 (4): 422-428.

USDA. 2008. Species Records of Acer. Germplasm Resources Information Network (GRIN): http://www.ars-grin.gov/cgi-bin/npgs/html/splist.pl?66. United States Department of Agriculture Agricultural Research Service. Beltsville.

Norway Maple

Norway maple (Acer platanoides) is 'hard maple' from Europe west of the Caucus Mountains. The Norway maple leaf is palmate with seven main lobes. The lobes are quite pointed. Each lobe has rather distinct pointed sub-lobes. The leaves look like sugar maple's, but they have more 'points' and they are larger than sugar maple's. The leaves turn yellow in the autumn. Leaves are often spotted with acer tar spot fungus. Like most maples it is monoecious, that is, it is hermaphroditic. The little flowers unfold in the spring before the leaves. Each flower has five prominent yellow petals. The flower is quite small, and is arranged in a umbel with a few others. The doubled seeded key has broad outward pointing wings which are at about a 160 degree angle to each other. Sometimes three seeded keys develop. Mature bark has finely fissured bark, almost like an ash-tree's. The sap is milky in colour, and not suitable for making maple sugar. This mixed forest tree can grow to 30 metres height

Norway maple is very similar to the Cappadocian maple (A. cappadocium) which lives to the east of the Caucus all the way to Korea and Japan. Like other common maple species there is significant genetic variation throughout its natural range.

Norway is almost a sugar maple look-alike, from a distance. The bark and keys are quite distinct when viewed close-up. Norway maple is somewhat more hardy as an urban tree than sugar maple. It has long been planted as a street tree in the Americas. There are several horticultural varieties. Most common are the so-called 'red maples'.

The Crimson King has dark reddish-purple leaves. Seeds from the variety usually produce offspring of intermediate leaf colour.

While the Goldsworth Purple has a more pleasing purple leaf colour. Usually it is a cultivar of distinct heterozygous gene mix.

Norway maple was a popular urban tree. However, it is not quite as handsome as the native sugar maple. Norway maple was widely planted as a replacement for elm from the 1940s to the 1970s. In the 1990s and 2000s an increase in crown dieback started occurring in southern Ontario. This decline has been blamed on a combination of drought stress, verticillium wilt and root girdling. The maple is not quite adapted to Ontario's weather extremes. There is also a desire on the part of many urban foresters to prevent the Norway maple from naturalising any more than it already has. Feral trees are fairly common in the Maritime provinces and parts of New England. They are also present in Toronto's urban ravines.

Allelopathy: There is a widespread belief that turf grass and Norway maple ‘do not mix’. This is sort of true. The maple produces chemicals that inhibit smaller plants from growing under them. Norway maple is more allelopathic than, for example, sugar maple. But it is all relative, many tree species are allelopathic to some extent.

The ISA Species Rating: 68%.

Eastern White Pine

white pine - wolf tree

Eastern white pine (Pinus strobus ) is native to Ontario's Great Lakes St. Lawrence region. It is also known as the Weymouth pine. In 1605 Captain George Weymouth took some seeds back to England. These were planted at the Longleat estate of Thomas Viscount Weymouth. The tree has not thrived in Britain, but it has remained common in the Great Lakes - St. Lawrence region.

Eastern white pine has long thin soft leaves (needles) in bundles of five. The leaves are retained for a little over a year, leaves are seldom older than two years. The seed cones are over a decimetre long. Originally, wildfires helped to open the cones and to remove balsam fir competition. Pollen cones are and female cones are on the same tree, but the male elements tend to be lower in the crown. The pine can grow to 50 metres in height, often with very strait boles, but also often with large irregularly spaced scaffold branches. Its picturesque structure is one of the reasons that it is highly valued as an ornamental, its beauty improves with age. (Ancient contorted white pines are a favourite subject for landscape painters.)

Eastern white pine can grow in the dry soils of the uplands. But it grows best in moist sandy and loamy soils. At one time it was a dominant tree in much of the St. Lawrence watershed. Enormous pines grew well spaced over an open under-storey, almost like a pine savannah. The pine occurred in association with red oak (Quercus rubra), and hemlock (Tsuga canadensis). Eastern white pine is closely related to the western white pine (P. monticola), which grows in the Rocky Mountains.

White pine wood is light and very soft, but not atypical of pine wood. It was the largest pine in eastern North America. In the eighteenth century white pine was highly valued by the British. The boles of ancient trees were ideal for the construction of ship masts. The British Crown reserved all rights to all white pines in its colonies. This decree was not popular with most of the rural colonists. The pine proclamation was one of the many grievances that led to the American Revolution. After the American War of Independence. Britain sought the great pine in its Canadian colonies. By the mid nineteenth century most of the really large pines were logged out.

White pine is fairly hardy as an urban tree within its natural range, or within equivalent climates. While it is subject to a wide range of diseases, very few of these seriously affect its health. It also takes well to transplanting, and is available in most tree nurseries in the Great Lakes region. One mistake commonly made is for people to plant these pines too close together or in too confined spaces. They would do well to remember that the pine can eventually have a trunk in excess of a metre wide, and a crown that could cover most urban backyards. Basically, eastern white pine is potentially a giant, give it lots of space.

The ISA Species Rating: 75%.

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. (The rust fungi tend to have multiple hosts.) 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. Do not plant blackcurrents near white pine.


Barlow, Connie. 2000. The Ghosts of Evolution - nonsensical fruit, missing partners, and other ecological anachronisms. Basic Books. New York. 135-147.

Morency, Pierre. 1989, L'Oeil Américain - histoires naturelles du Nouveau Monde. Boréal / Seuil. Québec. 276-288.

Peattie, Donald Culross. 1991. Trees of Eastern and Central North America. Houghton Mifflin Company. Boston.

Spiller, Mary. 1980. Growing Fruit. Allen Lane. London.

Sternberg, Guy and Wilson, Jim. 1995. Landscaping with Native Trees. Chapters Publishing Ltd. Shelburne, Vermont.

The ISA Species Rating: 67-81%.

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