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Black Rot gummosis disease on plum.

Apple Black Rot (Botryosphaeria obtusa) This disease occurs mostly on apple trees. This

is a complex disease that is virtually impossible to control. This fungus disease overwinters in bark scales, cankers (stem openings), mummi- fied fruit and twigs and stems killed by other causes. Early infections occur in leaves and petals, while later infections occur on fruit through skin cracks. The disease is rapidly inva- sive once it develops in the tree. There are a number of symptoms of disease damage which may occur on the tree,


as the development of long linear trunk and branch cracks called cankers; and purple and tan coloured ‘frog eye’ infections on the leaf surface. Fruit infection will appear first as reddish flecks that develop into purple, raised bumps on the fruit. Infection on more mature fruit will appear as black, irregularly shaped lesions (blotches) with a red halo.

Cedar Apple Rust (Gymnosporangium juniper- virginianae) Most rust diseases of this nature occur in

hawthorn trees. It is a disease on crab apple fruit. In appearance the fungi appear as a ‘beard’ (actually clusters of microscopic spores) on the fruit. Highly infected trees must be removed and placed in trash containers. Prune off infect- ed fruit on lightly infected trees. Remember to sterilize the pruners after every cut. Fungicidal sprays rarely control rust diseases.

Plum Pocket or Plum Bladder (Taphrina pruni) The disease appears as white blisters on the

fruit which can quickly cover the entire surface. The fruit enlarges to grow into a bloated grey bladder often called a ‘mummy’. At this stage the fruit is a ‘hollow bag’ filled with spores. Remove the mummies. On occasion the leaves and new shoots become affected and appear twisted or curled. This is a difficult disease to eradicate as it has a tendency to return to the tree annually. Fungicide sprays may help to control the disease, but the tree is best removed. The best way to prevent the plum pocket disease is to plant disease-resistant cultivars. i Michael Allen M.Sc.F,. RPF (ret.) is a consulting urban forester, tree diagnostician, certified arbor- ist and owner of Vibernum Tree Experts. He can be contacted at or at vibur-

Diseased apples from Black Rot and Fire Blight.

Crab apple gall rust.

The fruit has black rot disease and the leaf is infected with a combination of black rot and apple scab.

Plum Bladder and Brown Rot diseases. Summer 2015 • 13

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