Question
Hi there. I live in Calgary, Alberta, Canada. I have a mountain ash tree in my south facing back yard that has some issues with its leaves. I have been here three years and I'm guessing the tree is about 10 years old. The leaves grow properly but as they grow they develop yellowish/brown spots that seem to be puffy but when you scrape them open, you just scrape open the leaf. I can't see any mites or insects on the leaves or branches. In addition, this year, my tree seems to have become afflicted with fire blight. We noticed it after a lightning storm and thought it odd that only some branches seemed affected by lightning. I have cut off the affected branches but I'm not sure if the leaves are leading to the fire blight end. Thanks so much for your help!

Answer
Two diseases that affect mountain ash trees (Sorbus americana) are fire blight and Cytospora canker. Fire blight (Erwinia amylovora) usually starts from the tips of the branches and works inward. The branch tips are blackened and crooking over. A few to many branches may be infected. Management of this disease begins by pruning out infected branches; prune at least 12 inches below the cankered area in late winter, if possible. Pruning in summer may more likely spread the disease, but may be necessary if trees are highly susceptible. Disinfect tools by dipping them in a 10 percent bleach solution between cuts. Other management strategies include: if fire blight has been a problem in the past, apply blossom sprays, and avoiding excessive fertilizing and pruning, which stimulate new growth. If the tree dies, plant a resistant variety.

Cytospora canker branches have brown sunken cankers circular or irregular in shape that enlarge and girdle the wood. Black, pimple-like fruiting structures form on infected bark. Yellow threads of spores ooze out of the fruiting structures during wet weather. Prune infected limbs well below the canker. Disinfect the pruning tools thoroughly between cuts. It is best to remove severely infected trees.


The leaf spots are a leaf disease and not related to the Fire Blight.  Leaf spot is a common descriptive term applied to a number of diseases affecting the foliage of ornamentals and shade trees. The majority of the leaf spots are caused by a variety of fungal pathogens but some are caused by bacteria.

The primary symptom of a leaf spot disease is spots on foliage. The spots will vary in size and color depending on the plant affected the specific organism involved, and the stage of development. Spots are most often brownish, but may be tan or black. Concentric rings or a dark margin around the spot may be present. Fungal bodies may appear as black dots in the center of the spots. Over time the spots may combine to enlarge to form blotches. Leaves may yellow and drop prematurely.

The organisms that cause leaf spots survive in fallen infected leaves and twigs. Some may remain in dead twigs on the tree. During wet weather, spores are released which may splashed or be windblown onto newly emerging tender leaves where they germinate in the moisture and infect the leaf. Overhead watering late in the day or during the night, heavy dews and close spacing of plants prolong wetting of the leaf surface and provide more opportunities for fungal or bacterial infections.

Live with the disease. Leaf spots are largely an aesthetic problem as few leaf spots seriously damage the host. Also control is seldom achieved after the infection has started.
Remove infected leaves and dead twigs. Raking up and disposing of infected leaves as they drop and pruning out dead twigs can help control the disease by removing spores that can reinfect the new leaves. This will not cure the problem but it can help minimize infections.