Purdue Extension Service

Vanderburgh County, Indiana

Leaf Spots Spotted

By Larry Caplan, Extension Horticulture Educator, Vanderburgh County, IN

For the Evansville Courier and Press, May 17, 2009

With all of the rain we've had this spring, I'm already seeing some leaf spots and blotches on trees, and I expect to see more on other plants as the season progresses. Some of these leaf spots are very noticeable, but not very harmful; others need to be addressed as soon as you see them if you want a high quality plant.

Anthracnose is a fungus-caused leaf spot that typically appears in cool, wet spring weather. It's most commonly seen on sycamore, ash, white oak, and maple. Anthracnose leaf spots are usually appear alongside the veins of the leaf, and then eventually spread outwards to the leaf edge. The spots are a uniform dark brown to nearly-black and are irregular in shape.

On ash and maple, the leaves usually curl up and become distorted, before eventually falling off the tree. Sycamore leaves often wilt completely and turn brown, sometimes being confused with a late freeze. An infected sycamore usually looks more dead than alive during the early to mid spring, but by early summer will usually put on a second crop of leaves.

It's not worth trying to treat large trees, partly because of the difficulty in reaching the top of the tree with the fungicide, and partly because the tree is not in any real danger. Young trees can be protected by treating with chlorothalonil (Daconil) when the buds first open, and then twice more at 10 day intervals. Rake up and destroy fallen leaves to reduce overwintering spores.

Apple scab will soon be visible on susceptible varieties of apple and crabapple. Brown to olive-green, roughly circular, fuzzy spots often originate along the veins of the leaves. In time, spots become black and velvety and develop fringed margins. Finally, leaves yellow and drop. In a “bad” year it is not unusual for trees to suffer near complete defoliation by the end of June.

The best control is to get rid of susceptible varieties, and replace them with those that are "scab-free", or resistant. These trees may still get a few spots, but will not suffer anywhere near the amount of damage as the older susceptible varieties. If replacement is not an option, then you need to resign yourself to spraying your trees three or four times, starting as the blooms begin to open. The fungicides Daconil and captan do a good job in protecting the trees.

Black spot of rose is fairly similar to scab. As the name suggests, black, circular spots form on leaves during spring and summer. Infected leaves eventually yellow and drop prematurely. Severe infection may cause some canes to become completely defoliated, thereby making them more prone to winter injury. Good sanitation and air circulation are important for reducing this disease. Regular fungicide treatments with Daconil will reduce the disease; triforine (Funginex) not only does an excellent job against black spot, but also against powdery mildew, which normally crops up later in the summer.

For more information on springtime fungal diseases, contact the Purdue Extension Service at (812) 435-5287.