Diseases of Deciduous Trees & Shrubs
Black Knot
Black knot is a fungus which causes large black growths on the branches of Schubert cherry, Mayday, pincherry trees and saskatoons. It continues to grow until the branch beyond the gall eventually dies.
Pruning the branch back below the gall is necessary because the gall prevents water from moving beyond it and it will die. In rainy weather black knot will become much more prevelant but causes only minor damage. There is no chemical control.
Dutch Elm Disease
Dutch elm disease is caused by a fungus, spread by the elm bark beetle, which prevents the movement of sap within an elm tree. Leaves wilt and turn yellow on one branch at a time, eventually falling off prematurely. It is a major killer of elm trees in some areas. If you suspect Dutch Elm Disease, contact the DED Hotline in your area.
Dutch elm disease There is no chemical control for Dutch Elm Disease. Watch for symtoms - wilting and yellowing leaves in one branch of an elm tree - and report any suspected trees to the DED Hotline in your area. It is illegal to bring firewood from any other province into Alberta, to prevent the insects that carry it from coming into the province. There is no Dutch Elm disease in Alberta at this time.
Shothole Disease
Shothole disease is a fungus which causes brown spots in the leaves of Shubert cherry trees immediately after they emerge in the spring. As the weather warms up, the spots fall out and leave small round holes which are often thought to be caused by insects.
There is no chemical control of shothole disease. The holes are a little disfiguring but cause no harm to the tree.
Fireblight
Fireblight is a bacterial disease that infects flowering trees. It is most common in our area on pear, apple, crabapple, and mountain ash. Bacteria enters the flowers, which turn brown prematurely. Leaves on infected shoots turn brown and remain on the tree well into winter. The closer to the tip of the shoot, the darker the leaves are - the ends are almost black (as if scorched by fire) and curled over like a shepherd's crook. Cankers form further down the branch that exude ooze which spreads the bacteria through wind and moisture. It is also spread by insects such as aphids and leafhoppers, so controlling them is also helpful.
Pruning well below the infected parts can eliminate the disease in early stages. Clean tools with one part chlorine bleach to four parts water after each cut. Copper spray can reduce the chance of infection if fireblight is in your area. Badly infected trees will have to be removed. Many trees are mis-diagnosed - get a diagnosis from a person knowledgeable about tree diseases.
Cytospora Canker
Cytospora canker can affect flowering trees particularly apple, mountain ash, pear and hawthorn. It is very common inside cotoneaster hedges, where there is very little light or air circulation. It causes longitudinal splits in the bark, with the bark peeling back. Leaves emerge in the spring but die when hot weather arrives because water cannot move along the branch beyond the damaged areas.
Cytospora canker cannot be controlled with a chemical. Pruning a branch back below the affected area will allow new growth. Pruning out dead wood in the centre of a cotoneaster hedge will eliminate an ideal place for it to grow. Clean tools with one part chlorine bleach to four parts water after each cut.
Bacterial Canker on Willow
Bacterial canker on willow attacks trees that have been stressed by heat, drought, insects, winter damage, rapid temperature changes or previous infection. It causes well-defined, elongated dark brown areas in trunks and branches. New cankers appear in smooth bark as sunken areas that may girdle the stem.
Bacterial canker in willow trees cannot be controlled with a chemical. Prune the branch back well below the affected part. Clean tools with one part chlorine bleach to four parts water after each cut.
Common Stem Canker of Roses
Common stem canker of roses is caused by a fungus which enters the stems through wounds in the bark, often from thorns whipping into other stems in storms. This creates black dead areas in the bark. Water cannot move upwards through the canker so the branch dies beyond the canker.
Common stem canker of roses can be controlled by pruning branches off well below the affected part. There is no chemical control.
Bacterial Wetwood
Bacterial wetwood (slime flux) is a common disease of poplars and elms. The infection usually occurs in crotches of trees or where water accumulates, such as bad pruning or mechanical damage. Diseased wood inside the tree is dark brown, and foul smelling liquid may ooze from cracks or wounds. Foliage wilts in diseased areas.
Improving drainage (e.g. angling wounds downwards and using drains) and cutting out damaged wood can preserve the tree. Clean tools with one part chlorine bleach to four parts water after each cut. There is no chemical control.
Bacterial Blight of Lilac
Bacterial blight of lilac causes black, shrivelled tips, especially on white-flowering ones. Flowers blacken and immature leaves die quickly. It rarely affects older growth.
Bacterial blight on lilac cannot be controlled with a chemical. Prune back well below the affected part. Clean tools with one part chlorine bleach to four parts water after each cut.
Aspen Blight
Aspen and poplar twig blight is caused by a fungus. Young shoots blacken and wilt, the trees are disfigured and new growth is severely affected, but it does not signficantly affect the health of the tree. Infection on older trees is rare.
Aspen and poplar twig blight cannot be controlled with a chemical. Prune the branches back well below the affected part. New growth will quickly replace affected areas.
Clematis Wilt
Clematis wilt is caused by a fungus. One branch of a clematis will suddenly turn dark brown and wilt, while others are unaffected. The roots are not damaged.
Clematis wilt can be controlled if the affected branch is cut back to ground level, and a systemic fungicide is applied to the soil at that time and again in two weeks. New growth will occur very quickly.
Black Leaf of Saskatoon
Black leaf of Saskatoon is caused by a fungus which covers the undersides of infected leaves with an olive-brown fungal mat. Fruit production is severely reduced. It over-winters on dead, infected leaves.
Infected leaves should be removed and disposed of in the fall. Pruning out infected branches will reduce the spread of the disease, but removing the shrub may be the best answer. There is no chemical control.
Silverleaf
Silverleaf is a fungus which enters the tree or shrub through wounds and infect the sapwood. It spreads rapidly throughout the tree, causing a leaden or silvery sheen on the leaves. Later symptoms are browning of the leaf rib and margins. It is usually confined to several branches a year but each year more are affected, until the tree dies.
There is no chemical control - pruning out infected branches well below the affected parts and protecting trees from physical wounds may help control it. Removing the shrub is possibly be the best answer.
Septoria Leaf Spot on Poplar
Septoria leaf spot on poplar is a fungal infection that causes numerous black spots on poplar leaves. New, tender leaves are the first infected, and small spots enlarge under wet conditions. Trees may also develop trunk cankers from the same fungus. Sunken black areas may girdle the tree and cause death. This is more likely in young trees with soft bark. Native poplars are more susceptible than newer cultivars.
Septoria leaf spot is a disease of native poplars and doesn't usually affect domestic trees. There is no chemical control. Cankers can be cleaned out to healthy wood - leave them open and exposed to air to heal naturally.
Rust
Rust on hawthorn and saskatoon is caused by a fungus which needs two alternate hosts. The alternate host in this case is a juniper, where the disease overwinters. In the summer, yellow spots appear on the underside of leaves, and spots on the top surface have orange centres. Later, fruiting bodies that look like thorns appear on the underside of the leaves. Fruit can also have thready growths on them. It is much more prevalent in rainy weather.
Rust on hawthorn and saskatoon cannot be controlled with a chemical. The only control is to remove all the junipers within a two mile radius in every direction!
Iron Chlorosis
Iron chlorosis is a yellowing of leaves between the veins, with the vein remaining green. It is caused by an inability of the plant to absorb iron from alkali soil. Our water, and therefore our soil, is very alkali in Calgary.
Sulphur added to the soil can make the soil less alkali - read the label carefully before use. When iron is added it can be absorbed by the plants. Compost (your own or purchased) can help somewhat. Do not add lime to soil in our area.
Heat Scorch
Heat scorch causes dark, irregular blotches or dark edges on leaves of susceptible plants. It is not a disease or insect but a reaction of the plant to extreme heat. Often, the plant's roots cannot absorb as much water as the leaves are losing from evaporation, so leaf tissue dies. It is common in hot weather in newly planted trees, particularly mountain ash, which have inefficient roots and cannot tolerate soggy soil.
Heat scorch cannot be controlled, as we cannot modify the weather. Giving the plant extra water does not help - it depends on whether the roots can absorb it fast enough. Too much water in the soil will do more harm than good. It all depends on the tree's ability to survive.