Alert: July 18, 2006

 Chemical Spray Damage

Chemical Spray Damage

I’ve had a number of questions and samples lately relating to chemical damage, mostly from home gardeners but also from commercial growers.  Herbicides, insecticides, and fungicides can damage plants.  This damage is sometimes called spray injury or phytotoxicity.  Some plants are more sensitive than others, and some chemicals are more damaging than others.  Improper dose, improper time of application, incompatible chemicals, drift, highly sensitive plants, and other factors can lead to spray damage.  Carefully reading the label will help prevent spray injury.

Dose:  Many pesticides need to be diluted before being applied to the plant.  Make sure to read the label—it is all too easy to skip over important label information about dilution, or to misread a decimal point, and then end up applying something at 10 times the recommended rate.  Anyone can make this mistake.  I know a university researcher who killed their plants by failing to properly dilute a chemical.

Timing:  Some pesticides are more harmful at particular stages in plant growth, such as when young fruit are setting.  For certain fruit crops, copper pesticides are often recommended only when plants are dormant, or when the buds have first opened.  The label will tell you about proper timing.

Incompatibility: Pesticides should not be combined without carefully checking the label for compatibility.  For example, captan and oil compounds should not be combined, and I’ve seen recommendations that for some crops they should not be applied to the same plants within 10 days of each other because they can still interact.  I have included two photos of young peach and apple fruit with severe russeting (see figures).

 Based on the background information that came with the sample, my guess is that these fruit were treated with captan and oil, and/or the dose was too high.

Drift: Anytime you apply a pesticide there is a chance that some will spread to a non-target area as vapor or small droplets carried by wind, or by run-off on the surface.  Try to do your sprays when the wind is still or very low.  (I know, in Kansas those conditions are rare!)  Early morning is usually the best time. 

Sensitive Plants:   Herbicides are the most likely group to cause spray injury, since they are designed to be toxic to plants, but other types of pesticides can be damaging, too.  Some cases are very specific, and this is another reason why it is important to read those labels.  For example, fungicides with the active ingredient azoxystrobin (such as Abound, Quadris, and Heritage) are extremely toxic to apple trees and care must be taken to avoid drift if you are applying those chemicals anywhere near apples. 

Sulfur and copper-based fungicides are toxic to

Chemical Drift Mimics:  While chemical injury is fairly common, keep in mind that other factors can cause some of the same symptoms.  For example, certain mites, aphids and thrips can cause distortion, especially on new growth.  In addition, aphids and thrips can carry viruses that cause distortion.

The Diagnostic Lab:  In the K-State Plant Disease Diagnostic Lab we do NOT test for pesticide injury.  Those tests take special instrumentation and expertise.  We might be able to say that damage resembles chemical injury, but we can’t confirm it by any means.  We can check for other possible causes of injury, and refer you to other agencies who handle pesticide injury.   One source of help is the Kansas Dept of Agriculture.  For more information you can go to the following website:  http://www.accesskansas.org/kda/Pest&Fert/pest-misuse.htm  (phone: 785-296-3786).  We can also direct you to private laboratories that do testing.  Pesticide injury issues can be time consuming and expensive to resolve, depending on how you pursue it.  Homeowners do not generally pursue extensive testing.

As I’ve already said about 5 times in this article, make sure to read and understand those pesticide labels.  You should be reading them anyway to find out about applicator safety, pre-harvest intervals, etc.  All that fine print can be annoying to read, but it is worth it to take those extra few minutes to make sure you are using the product safely and correctly. [Kennelly][photos by Kennelly, Pierzynski, Tisserat]

Disease Highlights

Arborvitae - Natural interior foliage yellowing & shedding
Ash - Verticillium wilt
Bentgrass - Take-all patch
Blackberry - Stinkbug damage to fruit
Crabapple - Apple scab, heat scorch
Corn - Drought stress, common smut
Cucumber - Bacterial wilt
Elm - Dutch elm disease (SE KS)
Euonymus & Yew - Decline from root damage resulting from too wet soils, then hot & dry; tip scorch due to hot/windy weather
Hosta - Heat scorch
Lilac - Lilac borer
Maple - Heat scorch
Peach - Scab, peach leaf curl
Pear - More heat/drought stress
Pine - Pine wilt
Plum - Brown rot
Potato - Verticillium wilt?
Rhododendron - Phytophthora root rot
Soybeans - Potash deficiency, Septoria brown spot, Bacterial blight
Tomato - Septoria leaf spot, tomato spotted wilt virus, Verticillium wilt?, Physiological leaf roll
 

Contributing Author(s):

Megan Kennelly           kennelly@ksu.edu 
Doug Jardine                jardine@ksu.edu
Judy O’Mara                jomara@ksu.edu