Ah, springtime in Iowa. The crabapples have just finished regaling us with a bonanza of blooms, pagoda dogwoods and late-flowering lilacs are ready to take center stage, liquid precipitation is an every other day occurrence, and green ash in many parts of the state are raining leaflets as if it were October. So, what's new. Springtime defoliation on green ash happens every year. And the reason? Experience tells us the most likely culprit is ash anthracnose, a common disease of green ash caused by a fungal pathogen in the genus Discula. You've all seen the symptoms. Irregular, water-soaked or blackish-green (eventually black or brown) blotches develop from the leaf margin inward, usually stopping at the midvein. But a new wrinkle has confounded the issue this year. Leaflets are falling, but those familiar anthracnose symptoms are absent! What's this? Are green ash experiencing an early Y2K problem? Who or what is to blame? Ash yellows? Herbicide drift?
Our best guess is, one or several low temperature events early in the growing season might have a hand in this latest green ash mystery. Several regions in Iowa experienced temperatures in the mid 20 F range in late April, and again in early May. As you might expect, spring frosts could be quiet damaging to new leaves as they emerge, and could be responsible for leaflet drop. Of course, trees in frost pockets or low-lying areas would be worse off than those on higher ground or where air movement is better. Sometimes frost injury will not be uniformly expressed over an entire tree (lower leaves would be more prone to this type of injury than those higher on the tree). The bottom line is, regardless of the cause of defoliation (low temperature or anthracnose), trees should rapidly refoliate and regain their normal appearance by early summer.
This explanation, however, might not be good enough for some of your clients. Those hardest to convince might have listened to a distant relative, neighbor, or landscape maintenance "wanna-be" operating at the fringes of respectability, and now feel some kind of treatment is absolutely essential. No, no, a thousand times no! Trees will recover from both anthracnose and low temperature injury without any human intervention. Fertilization? Only warranted if a mineral element deficiency has been identified. What about crown thinning? Absolutely the last thing you should do to a mature tree trying to recover from defoliation. Branches that are bare but still living, will give rise to new leaves needed to manufacture energy for the tree. Prune (remove) only those branches and limbs that are dead, structurally unsound, and/or those contributing to poor form.
This article originally appeared in the May 28, 1999 issue, p. 68.
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