Tom Glasgow: Identify the problem before applying pesticide

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Insect or disease? To spray or not to spray?

Photo by Nathaniel Glasgow/Contributed photo
Published: Friday, August 1, 2014 at 07:19 PM.


This probably sounds so obvious as to not be worth even mentioning. However, on a fairly regular basis we hear from folks who are dealing with some unknown plant problem, have already tried spraying some fungicide or insecticide, and aren’t seeing any improvement in the situation.

Here are some quick points to consider.

First, a significant majority of landscape plant problems, including decline, dieback and death, aren’t related to insects or disease in the first place. Most scenarios involve “abiotic” causes such as physiological stress, drainage, age, physical damage, soil problems, poor adaptibility to the region or planting site and so forth.

Second, a significant percentage of problems that are caused by insects or disease can’t actually be remedied with pesticides anyway. The long list of problems that can’t be cured with chemicals includes ground pearls in turf, borer damage after the fact and wood decay pathogens in shade trees.

Third, many of the insect and disease infestations we do encounter pose no threat to the health of the plant, and chemical treatments can’t be justified. Finally, without assistance from a knowledgeable professional, insect infestations are often confused with disease infestations, and vice versa; or, improper timing makes a pesticide application useless.

The example pictured with this article happens to be an infestation of the sumac gall aphid (Melaphis rhois), recently spotted on smooth sumac (Rhus glabra) in the Bristol, Tenn., area. Not being familiar with sumac insect or disease issues, I first guessed an Exobasidium fungus (as in azalea or camellia leaf gall), but realized on breaking open the galls that insects were involved. Not only would a fungicide spray been ineffective against these insects, it turns out that sumac gall aphids are not harmful to the sumacs and treatment can’t be justified.

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