Diseases of Trees
Rust
Rust is caused by various fungi of the gymnosporangium family. Some attack rosaeceous hosts ( saskatoon, apple, currant, hawthorn) with an evergreen (juniper, pine, cedar) as an alternate host.
Deciduous leaves usually have galls, orange pustules, or dark spots, often with a yellow edge. Fruit will become spotted, then become moldy, shrivel and die. Young deciduous leaves are infected in the spring and early summer. Galls form on the underside of leaves , and release spores, which infect evergreens in the summer. Evergreens typically form gelatinous galls (telia) which shrivel and die in the winter. Galls form in the spring, and swell during rainy weather, dry up during drought. Dry spores spread in the wind.
There are many combinations of deciduous and evergreen hosts:
- cedar/apple
- saskatoon/juniper (Scopolorum)
- hawthorn/juniper
- currant/white pine
Control:
- Cultural control - remove alternate hosts (not always practical), remove deciduous leaf litter, prune out galls on evergreens.
- Chemical control - none
Rust on Roses
Rust on roses does not need an alternate host. It produces bright orange, powdery masses on the underside of leaves and young stems. Some varieties are more susceptible than others.
- Cultural control - remove leaf litter in fall, water in the morning, water soil, not leaves.
- Chemical control - none
Western Gall Rust Fungus
Western gall rust fungus causes large galls on stems and branches of pine trees. It stunts growth, as moisture cannot move beyond gall and ends of branches die. The branches are weak and break easily in storms. Spores are released in late May, and are wind borne, growing on the new growth (candles). Western gall rust does not need an alternate host.
Cultural control - prune out galls, but not when spores are active (June).
Fireblight
Fireblight is a bacterial infection of trees of the rosacaea family, mostly apples, crabapples, pear and mountain ash. Blossoms appear water-soaked and turn brown. As it spreads to leaves, branch tips curl over like a shepherd’s crook and leaves turn brown, hang on the tree and don’t fall off. The leaves at the ends of the branches are so dark that they appear black, hence ‘scorched by fire’. Cankers are formed which are sunken and cracked, and exude bacterial ooze which contains the bacteria. Fruit rots and bacteria is attractive to insects, which can spread the infection. It can also be spread by birds, rain and tools.
Cultural control - remove all infected wood well back to healthy wood, clean tools!- Chemical control - copper spray can prevent infection from spreading in a tree or protect another tree if it is in the area, but only lasts 72 hours, so must be repeated often. If infection is caught early and infected branch pruned out, it may do well. If the bacteria infects other branches and is moving in the sap, the tree may not be able to be saved.
Cytospora Canker
Cytospora canker is often mis-diagnosed as fireblight, but the differences are quite obvious. It is a fungal infection which causes longitudinal cracks on branches, with the bark curled back, which often ooze sap in the spring. Because it invades the tissue, it prevents moisture from reaching beyond it, so young leaves develop, then wither and die during hot weather, whereas fireblight moves from the blossom end down the branch. Cytospora canker affects older branches of fruit-bearing trees and spruce, and also cotoneaster hedges. It multiplies in damp, dark places with poor air circulation - the inside of a hedge or spruce tree!
- Cultural control - pruning out diseased wood is all that is necessary. If old, dead wood inside a hedge is removed and outside, healthy branches allowed to grow, it will rejuvenate it but not prevent a recurrence as the inside wood again becomes an appropriate place for it to grow, and the fungus is in the sap.
- Chemical control - none
Nectria Canker (coral spot)
Nectria canker (coral spot) is a fungal infection which causes orange dots on dead wood (nectria - necrosis - death). It is often seen on wood killed by cytospora canker, and makes it easy to identify dead wood which should be removed. It is also mis-diagnosed as fireblight - spots are red - fire is red - fireblight!
Black canker
Black canker is a fungal infection which attacks poplar and willow trees under stress (from heat, drought, winter damage). It enters wounds, and begins as a sunken patch, which may girdle the stem, and is often noticeable where a stem joins a larger one or the trunk. Black areas are irregular in outline, spread rapidly and leaves wilt beyond the cankers.
- Cultural control - prune out small, new cankers, destroy badly infected trees clean tools
- Chemical control - none
Dutch Elm Disease
Dutch elm disease is a fungus infection carried by elm bark beetles. There have been no known cases of the disease, but beetles have been found in Calgary. It is not known if the fungus can survive the winters here. American elm is particularly susceptible; Siberian elm is resistant. Beetles found here likely came into Alberta on firewood brought illegally into the province. Beetles are dull brown or shiny reddish-brown (two types), growing to 3 mm. long. Eggs hatch in the fall, and larvae overwinter in the bark of elm trees. The beetles emerge in May, and create egg-laying galleries in dead or dying wood. The fungus carried on the beetle’s body enters the vascular system and moves within the tree. The earliest symptom is wilting of the leaves on a single branch. The leaves turn yellow and die (flagging). and it spreads rapidly to larger branches and the entire tree.. Once the tree is dead, the fungus invades the whole tree, including the roots. It can also spread by root contact with infected trees, and in overhead canopies.
- Cultural control - all wilted branches must be removed at least 30 cm. beyond visible damage. Clean tools! Any possible case of Dutch Elm disease must be reported.
- Chemical control - not at domestic level, possible to prevent spread by professionals.
Black Knot
Black knot is a fungus that affects Shubert cherry and Mayday trees. It causes swellings encasing branches - light brown at first, then olive green and velvety. In the fall they turn black and harden. The swellings cut off moisture to the ends of the branches, so new leaves the next spring soon wither and die. It leaves trees mis-shapened but doesn’t do a great deal of harm unless it grows in the trunk of a relatively small tree. Then, cutting it off below the affected part becomes more serious! Fungus is wind-blown or rain-splashed.
- Cultural control - prune off at least 10 cm. beyond galls, clean tools.
- Chemical control - not practical at domestic level
Shot Hole Disease
Shot hole disease - is a fungus that causes brown spots in the tender, new leaves of Shubert cherry trees in damp weather. As weather warms, these spots fall out, leaving holes. This is when they are noticed, as the weather becomes pleasant enough to be out in the garden. No control in necessary - you can’t make a hole go away, and it is self-limiting.
Armillaria Root Rot
Armillaria root rot is a fungus which affects conifers - mostly spruce and pine. It can cause sudden, abrupt death or a gradual decline. Needles turn orange and die over the whole tree. The fungus invades the bark and wood around the base of the tree and fungal strands or ‘shoestrings’ form on decaying wood and in the soil. A butt rot often forms a swelling at the base of the tree. Small trees are killed quickly, large ones have reduced growth but can survive for years in an unthrifty condition.
- Cultural control - plant resistant trees, remove unhealthy ones. Fungus lives for years in the soil, but it is so widespread that trees don’t get it from infected soil unless there are open wounds. Excise cankers at base or on large surface roots, don’t cover with soil until fall.
Bacterial Wetwood (slime flux)
Bacterial wetwood (slime flux) affects poplars, willows and elms, causing oozing of bacteria and sap from wounds or branch stubs.. Infection can begin from pruning wounds, frost cracks in bark, crotches where water can accumulate. The leaves wilt beyond the affected area. Insects, attracted to the ooze, spread the bacteria from tree to tree.
- Cultural control - install drains in crotches, make rounded bottoms, tipped to one side, in crotches, not V’s. Cut out infected wood to healthy wood (clean tools!), leave open, don’t paint or cover. Learn to prune correctly.
- Chemical control - none
Shoot Blight
Shoot blight on lilac is a bacterial infection causing black spots on stems and leaves, which increase in size to form large blotches. New shoots curl over and wither, and flowers turn brown and fail to develop. The bacteria overwinter in the stems and branches. It is spread by rain and watering. White flowering lilacs are more susceptible.
- Cultural control - prune out diseased branches, clean tools! Grow resistant varieties.
- Chemical control - none