Anthracnose of Shade Trees or Wind Injury: Look Alike Symptoms Can Be Perplexing​

The cool, wet, weather experienced periodically this spring has been ideal for the development of anthracnose on shade trees.

 

Anthracnose is the common name for a type of leaf spot and canker disease caused by certain kinds of fungi. Anthracnose diseases affect many trees, but are particularly prevalent on ash, maple, sycamore, white oak, walnut and dogwood. Each species of tree is infected by a different species of fungus, thus the fungus does not spread from oak to maple or maple to ash or ash to sycamore. A different fungal species is also responsible for dogwood anthracnose.

 Symptoms will vary depending on the type of tree and the stage of plant development at the time of infection: leaf spots or blotches; twig dieback and wilting; and browning or death of emerging leaves are all possible. Premature leaf drop often occurs with anthracnose diseases, however, most of the trees infected with anthracnose usually show good resilience, and are not permanently damaged by this early season leaf blight.

 Sycamore anthracnose causes severe blighting of newly emerging leaves and shoots, and eventually causes twig and branch cankers which distort growth. Twig and branch cankers, shoot blight, and leaf blight are all symptoms of the fungus that causes sycamore anthracnose. Leaflet drop, as well as dead tissue along leaf veins or at the leaf edges is a symptom for ash anthracnose. Although defoliation may be so great that anthracnose-infected leaflets practically carpet the walks and lawns nearby, the tree is not dying, it simply puts out a new set of leaves. Anthracnose symptoms on maple and oak range from leaf spots to enlarged blighted dead areas along veins and sometimes to shoot blight.

 Abiotic (noninfectious) injury from environmental factors such as excessive winds or late frost/cold damage have also occurred this spring and may also cause foliar symptoms similar to anthracnose on various deciduous trees.

The most appropriate course of action for established trees suffering from severe effects of anthracnose is to rake and remove fallen leaves from beneath the tree; stimulate vigorous new growth with a balanced fertilizer after the leaves open and the spring rains have stopped; water regularly during extended dry periods this summer; avoid irrigation systems that wet leaves. 

​Leaf Diseases BP-143-W (pdf file)​​​

Gail Ruhl, Senior Plant Disease Diagnostician - Purdue Plant and Pest Diagnostic Laboratory

Shade Trees and Scale Insects in Hamilton County Landscapes

High populations of scale insects are drawing attention in Hamilton County yards this spring. The insects, commonly called lecanium scales, feed on the sap of a variety of trees including maple, oak and many others. This week alone we have had two samples come into our Educational Center for problem diagnosis. One sample was found on oak while the other was a mature maple.

The first signs of scale infestation are often noticed before one sees the insect itself. Most noticeable is the sticky, wet substance on under-story plants, yard furniture, or automobiles. This material, known as honeydew, is excess fluid that is excreted as the insects feed. Under-story leaves and other surfaces may be lightly speckled or heavily coated with the shiny drops of honeydew. Eventually a sooty mold fungus grows on this substrate, blackening the foliage, twigs, and bark, and other objects beneath scale-infested trees. Dieback of twigs and branches and premature leaf drop may result as heavily-infested trees compete with scale insects for necessary moisture.

Lecanium scales can vary in size and color but are generally oval in shape. Females molt several times before reaching adulthood. With each molt, the female’s body grows bigger, while her legs and antennae become proportionately smaller. By the time of the second molt, the female scale, with her hardened, brownish, hemispherical body, is incapable of moving from the spot on which she has become fixed. Male scales are delicate, flat and nearly transparent.

Normally, scale populations are kept in check by environmental factors and natural enemies such as ladybird beetles. Heavily-infested trees may show some dieback of twigs and branches and there may be some early leaf drop. Landowners can reduce other stress on trees to help them recover. For landscape trees be sure to water in times of drought and avoid disturbances such pruning and root damage. When populations are heavy on specimen trees, horticultural oils and other insecticides can be applied.  For more information, click here for a great publication from Michigan State University Extension.

Excerpts were taken from Vermont Dept. of Forestry Lecanium scale fact-sheet.

Tar Spot on Maple

This week I have had numerous calls about black tar-like spots on maple trees throughout Hamilton County. The spots are not truly tar, they are in fact fungal leaf spots that are becoming noticeable that likely started forming after the rainy start we had earlier this summer.

Tar spots on maples are caused by fungi in the genus Rhytisma. Although tar spot disease may cause premature defoliation of leaves, seldom are they detrimental to the overall health of infected trees. Tar spot diseases are best managed by raking and destroying fallen leaves since the fungi overwinter on leaves.

There are a few other common foliage leaf infections found on maples, click here to read more about these diseases.