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Frost or Anthracnose?

May 18, 2005

The cold snap experienced in Illinois at the end of April and/or beginning of May (depending on your location) caused some symptoms that could be confused with anthracnose. Look at symptoms, timing, and host to help distinguish the two. Anthracnose was discussed briefly in issues no. 3 and 4 of this newsletter.

Symptoms of anthracnose include brown to black spots, brown to black blotches, and sometimes (as with sycamore anthracnose) death of entire young leaves. Rarely does anthracnose cause all of the foliage to die. Frost damage can range from brown to black leaf tips to death of all leaves on a tree, depending on severity. Frost damage happens suddenly, usually overnight. It often turns leaves black. Try placing some fresh leaves in the freezer for an hour, then remove them, and observe the symptoms. Leaves will look water-soaked, followed by shriveling, and eventually a brown or black color. This is how freeze injury on trees and shrubs outdoors looks as well. Often, however, we don’t see symptoms until the foliage has turned completely brown or black.

Either problem could appear at this time. In fact, anthracnose has been seen in central Illinois for the last 2 weeks on sycamore, ash, maple, and ash. Anthracnose is favored by moisture and cool (but not freezing) temperatures from bud break to 2 weeks later. Frost damage requires near freezing temperatures.

The host is also a significant clue. Anthracnose may appear on many plants; but we usually see it in Illinois on ash, birch, maple, oak, sweetgum, sycamore, and walnut. If you are seeing damage on other species, anthracnose is probably not to blame. Sandy Mason, Champaign Extension Unit, reports frost injury to Kentucky coffeetree, ash, and mulberry, with death of all emerged leaves. Leaves were just “burnt up or fried.” These trees will recover but will need moisture to help produce new leaves. Weekly watering in periods of drought would be helpful.