An unusual and unexplained defoliation of hackberry trees has been reported around central Iowa during the past week. Reports of observations in other parts of Iowa are encouraged.
Reports of the problem have been confined to hackberry trees in an area from Newton to Carroll to Webster City. The symptoms appear to have followed this progression:
- leaves failed to develop normally so the leaves are "ragged" or uneven in appearance with small, irregular strips of foliage along the veins;
- the leaves dried out, turned brown and then dropped;
- the small, 2 to 6 inch long green stems of new growth fell as well.
Not all trees show all three symptoms. In Ames, many trees show only the first symptom of leaf deformation. In Webster City, almost every tree uniformly exhibited all three stages.
What's the cause? We wish we knew. The great consistency and uniformity of symptoms speaks against insect or disease activity. Also, no insect pests or disease agents have been recovered from samples. Artificial stresses such as air pollution or herbicide injury are contraindicated by the widespread uniformity of symptoms over a wide geographic area.
That leaves the usual "last resort" -- weather (unless you want to step up and argue for extraterrestrial or supernatural causes). At this time we do not know the cause but suspect it to be low temperature or frost injury to developing tissue sometime in April.
What to do? Not much. Insecticide or fungicide applications are not warranted as there is no evidence of insect or disease. Fertilizer has never been a magic-cure and is not recommended in this situation. Besides, most of the inspected trees have lived a long time already in good sites receiving adequate to excessive turfgrass fertilizer. There is nothing to suggest injections of any type will aid or speed recovery. At this time, appropriate action is to wait and watch. We anticipate affected trees will releaf and recover but it may take several weeks. We will keep in touch.
This article originally appeared in the May 15, 1998 issue, p. 58.
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