Issue 6, May 29, 2009

Early Season Anthracnose

Anthracnose is a term used to refer to many fungal diseases that have a specific fruiting body called an acervulus. The anthracnose diseases cause leaf and stem spots, cankers, or fruit lesions. Some anthracnose fungi thrive in cool, wet conditions. We see these in the spring because cool, wet weather favors fungal growth and foliage has not yet developed a thickened cuticle to help fight infection. Some of the early season anthracnose diseases include anthracnose of sycamore, anthracnose of peony, anthracnose of corn seedlings, and the list goes on. All are caused by different species of fungi. Most of the tree anthracnose diseases are caused by species of Discula, Gloeosporium, Kabatiella, or forms of these fungi.

Recently at the Plant Clinic at the University of Illinois we have received samples of maples with anthracnose. They have spotting and blighting that is black and very noticeable. When young leaves are infected, lesions are often black and merge together. Older leaves may have more distinct brown spotting. Infection tends to be worse at the bottom of the tree where the canopy stays wet longer. Infection may occur along the veins in blotches from the leaf edges, or as random spots on the leaves. The image shows maple anthracnose from this spring. Infected ash trees may exhibit leaf spotting or they may drop leaves because of petiole infection. If you have a sycamore tree in your area, inspect it for this disease. It is very likely that the newest growth has been infected and is brown and shriveled as in the image. By now, the older leaves have emerged and escaped infection. You will see larger, green leaves 5 or 6" back from the stem tips. The trees will fill in with healthy leaves now that warmer, drier weather has occurred. The most common early season shade trees affected by anthracnose in Illinois are ash, maple, oak, sycamore, and elm.

Maple anthracnose

Although there are always exceptions, we are not usually concerned with early season anthracnose diseases of shade trees. Ash trees probably elicit the greatest concern because owners are fearful that Emerald ash borer could be involved. Ash anthracnose causes leaf drop but branches do not die and new leaves will emerge on the same stems. When submitting a suspect plant sample to the Plant Clinic, send foliage, stems, and images to obtain a more complete and accurate assessment of the situation.

Sycamore anthracnose

Some anthracnose diseases do warrant treatment. Dogwood anthracnose, which appears in mid-June, is a disease that will cause considerable plant damage if left untreated. A recent case of peony anthracnose is also a candidate for treatment. The Home, Yard, and Garden Pest Guide or the Commercial Landscape & Turfgrass Pest Mangement Handbook offer fungicides that can be used to prevent infection by anthracnose fungi. For more on anthracnose diseases, read Anthracnose Diseases of Shade Trees.--Nancy Pataky

Nancy Pataky

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