In this section:
Bacterial Canker
Cause
Bacteria Pseudomonas syringae
Affected
Apricots and plums especially, but all stone fruit are susceptible.
Symptoms
The bacteria can invade all the above ground parts of a plant. Symptoms vary widely between hosts and different climatic conditions, but are more commonly initiated on green foliage.
Although bacterial canker may affect trees of any age it is more often associated death of trees in the first or second year after planting. The tree flowers and begins its growth normally but limbs or whole trees collapse and die within a matter of weeks in spring. Leaves often remain attached to the tree. Death usually occurs in the same season and young trees seldom if ever struggle on into the next season. Once the top dies, many shoots arise from just below ground level because the disease does not invade the roots.
In mature trees, under the right climatic conditions, infection can spread quickly, killing large branches in a matter of weeks.
Areas of infection, particularly under bark of mature branches are usually sunken. These cankers often exude large amounts of gum and often give off a characteristic vinegar (sour) smell. Slicing bark off the affected area reveals a brown or tan discolouration in the bark (orange in the case of apricots). The outer layers of wood are often stained a brown colour in contrast to the normal white, but the inner wood (xylem) is not discoloured except in the more severe cases.
Symptoms on leaves vary greatly, but generally first appear as water soaked spots. These spots eventually become dry and brittle and fall out of the leaf, giving it a shothole appearance.
Infected fruit often develop depressed spots with dark centres and sometimes have underlying gum pockets.
Conditions favouring
Bacterial canker enters the tree through cracks, wounds, leaf scars and bud scales during cool wet weather in autumn and winter, though the symptoms will not be obvious till the next spring.
Control
Firstly, when purchasing trees from nurseries ensure they are free from bacterial canker.
Badly infected trees are a source of infection for adjacent trees and are generally stunted and often die. Remove all diseased trees in young (less than 4 years old) plantings and replace with healthy trees. The bacteria will not infect a tree via its root system, so you can replant in the same spot.
In older plantings careful pruning is an alternative. Limbs with developing cankers can be cut off and the wounds sealed to prevent further damage; but make sure the limb is cut back to clean wood. Do this only after leaf growth has started and while the tree is actively growing. Dip all pruning tools in an effective disinfectant (eg methylated spirits) after each cut and paint all wounds with a copper based paint or bordeaux paste.
In difficult situations where the disease is hard to control and particularly in young trees, a cover spray program involving two spring, two autumn and one winter copper spray is suggested. The application times are early budswell, 7 - 10 days later, 25 - 50 % leaf fall, 90 - 100 % leaf fall and mid-winter. Copper oxychloride, plus oil to act as a sticker is an acceptable spray to use. Bordeaux mixture is an alternative.
Apricots beyond the pink bud stage are particularly susceptible to copper sprays. Applications of copper based fungicides cause shotholing of the leaves and can damage fruit. For this reason, copper sprays should not be applied between pink bud and the commencement of leaf fall. Where the severity of disease is such that a copper spray at 25-50 % leaf fall is warranted, a half strength spray should be used at this timing.
Under normal circumstances a minimum program of at least one early budswell and one autumn leaf fall spray should be sufficient
