Take steps to control encroaching fungi disease

Anthracnose is a general term that covers several different diseases caused by different species of fungi.

Many of these diseases develop in the spring and may have similar leaf symptoms, but the common thread to all anthracnose diseases is the fact that the causal fungi produce similar, cup-like fruiting structures called acervuli. These fungi also are host-specific, meaning the fungus which causes anthracnose on maple will not cause injury to black walnut or other ornamental plants.

Anthracnose may occur on many different plants that grow in the Rogue Valley. Some of the most important trees that may be severely impacted are dogwoods, both native and Eastern varieties, sycamore trees and several maples, including Japanese maples. The fungus is not limited to shade trees, however. Turf grasses and food crops such as tomatoes and beans are also affected. Although the invading fungus may be different, the results can be quite similar.

Symptoms of anthracnose vary considerably. Certain anthracnose diseases result in branch dieback and extensive blighting of leaves, while others cause small circular lesions on the leaves and fruit. Anthracnose diseases may result in premature defoliation. Often damage to the trees appears severe; however, these diseases rarely kill trees directly. In fact, shade trees in vigorous condition may recover from anthracnose infections if cultural guidelines are followed that inhibit growth of the fungus.

Its effects are most noticeable on tall shade trees like Modesto ash in spring. Often the infected trees' first flush of leaves is destroyed, followed by a new burst of growth at the branch tips. If the weather has warmed and dried by this time, the new leaves fully expand and present a halo-effect look to the tree, with just the outermost branches bearing foliage. One can see this quite a bit in older eastside Medford neighborhoods.

Dogwoods show slightly different symptoms. Leaf symptoms develop first in the lower crown and progress up the tree. Symptoms include tan spots that develop purple rims. Leaves also may have necrotic veins and leaf margins, and large necrotic blotches. In some cases shot holes appear. Leaves may appear to be scorched, as if they had dried out. Direct infection of shoots, resulting in tiny cankers, may occur on Cornus florida during spring and fall. Girdling cankers typically develop at leaf nodes, causing twig dieback. On both hosts, twig dieback is most common in the lower crown following years of extensive spring or fall leaf blighting. As a result of twig dieback, succulent shoots proliferate on the lower trunk and main branches of affected trees. These branches are very prone to infections, which may progress into the main stem. Bracts, often mistaken as flowers, may become spotted or blighted if rainy conditions prevail during flowering.

Trees receiving good care will be better able to withstand anthracnose during years in which the disease is favored by weather conditions. Long, cool, moist periods, which are typical here, are perfect for growth and spread of these fungi. Maintain the health of plants by watering during periods of drought in summer. Mulching trees can help to reduce watering needs as well as protect trunks from mechanical injury. Avoid overhead watering to minimize the chance for leaf infections. Improve air circulation around trees to help dry foliage and reduce infection.

Effective control is possible only if the disease is detected before extensive dieback occurs. Prune and dispose of diseased twigs and branches to reduce potential sources of infection and improve tree appearance. Raking up fallen leaves may be of some benefit. Remove succulent branches as they form to prevent trunk canker formation. Avoid high nitrogen fertilizers, which stimulate succulent branching. Applying a balanced, organic fertilizer in early spring may bolster trees that are lacking in vigor.

Fungicides may be used to supplement a cultural control program on severely infected specimens. Applications of chlorothalonil or mancozeb will protect against leaf infections. Apply 3 or 4 sprays during leaf expansion in the spring, at 10- to 14-day intervals. If conditions are favorable for disease development later in the growing season, additional fungicide applications may be beneficial.

As we head into another possibly wet spring, observe your susceptible trees for early signs of anthracnose. Caught early, this disease may be limited to minimal damage to your valuable plants.

Stan Mapolski, aka The Rogue Gardener, can be heard from 9-10 a.m. Saturday mornings on KMED 1440 AM and seen in periodic gardening segments for KTVL Channel 10 News. Reach him at stanpolski@gmail.com.

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