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Leaf Spot Diseases and How to Control Them

By Laura Pottorff, Colorado State University Cooperative Extension agent, horticulture and plant pathology

Their symptons won't show up until later, but early spring is the time to begin managing leaf spot diseases.

Such diseases include rusts, leaf spots and shoot blight that can make healthy-looking foliage look as if its canvas for a splatter-paint artist.

The good news is these foliage diseases usually reduce only the aesthetic value of the affected tree. Occasionally, however, a severe outbreak causes early leaf drop and dieback of tree parts. With repeated infection, trees become more susceptible to attack by other diseases, insects and winter injury.

Correct identification is important, because leaf spot diseases easily can be confused with non-disease problems such as leaf scorch or other environmental abnormalities. If in doubt, bring a sample for identification to your county Colorado State University Cooperative Extension office.

Leaf and Shoot Blight

Leaf and shoot blight is most common on young aspen. Infected leaves become distorted and curled and develop irregular brown-to-black blotches. Stems constrict and turn dark. Eventually they turn black and brittle and curl to resemble a shepherd's crook. Shoot blight symptoms will be evident in the spring (mid-May to June) if a tree is affected. Be cautious in your diagnosis as frost damage also looks a lot like shoot blight.

Marssonina Leaf Spot  

Marssonina leaf spot is the most common of the three diseases. This disease is most easily identified by the dark brown-to-black flecks scattered over the leaf surface. The spots gradually enlarge during the summer and may coalesce to form blotches. Leaf spot symptoms will appear in mid-August if your tree is affected.

Leaf Rust

Leaf rust is a common problem, but rarely is serious. It is characterized by yellow spots on the upper leaf surface. Close examination reveals small yellow-orange bumps filled with powdery spores on the leaves. As with leaf spot, rust infestations will become apparent in mid to late August.

CONTROL

The fungi responsible for these diseases survive the winter in leaf debris on the ground. Wet spring weather stimulates spore production, and the spores are blown and splashed from the ground to developing leaves. Foliage diseases are cyclic: bad years coincide with long-term moist weather.

Sanitation is the first line of control. Rake and dispose of infected fallen leaves in autumn. The shoot-blight fungus overwinters in diseased stems and twigs, so prune diseased parts to reduce new infection. Keep leaves as dry as possible to reduce leaf spot. Avoid sprinkling water on lower branches of small trees growing near lawn areas.

Dense tree plantings are especially susceptible to disease, because air can't move within the group. Pruning selected branches may improve air circulation and dry leaves more quickly.

Chemical control usually is not necessary and often unsuccessful if the fungicide is not applied properly. Fungicide application may be warranted if an infection develops on a particularly valuable tree, or if the tree is attacked by foliar disease every year. The following fungicides are labeled for ornamental use. Check the label to be sure the plant you want to spray is listed on the label. Read and follow label directions carefully.

If you decide chemical control is necessary, SPRING is the time to spray. Fungicides prevent foliage diseases only if applied early enough. Spraying protects uninfected leaves from new infection; it will not cure infected leaves. Proper timing is critical. Fungicidal sprays should begin at bud break and be repeated two or three times during the growing season at 12-to-14-day intervals.

Photo: Judy Sedbrook

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Date last revised: 01/05/2010