Cool Temperatures and Excess Rain may Increase Crop Injury from Labeled Herbicides

The cool weather this spring means that many early-planted warm season crops have probably been stressed by cool temperatures more than usual. A number of herbicides labeled for vegetable crops include warnings not to use when the crop is or will be under stress. A stressed crop may not be able to detoxify the herbicide or outgrow its effects quickly enough. In the case of a soil applied herbicide, the crop roots may not grow into untreated soil fast enough to avoid injury.

High rainfall amounts can also lead to crop injury from soil-applied herbicides. Many herbicides move with soil water, and when there is too much water they can move where they are not wanted. Preemergence herbicides may move deep enough to injure crops with a low margin of tolerance. For instance rain may move Curbit® deep enough to injure seeded pumpkins.

If you are seeing some of the symptoms described below, stress from cool temperatures may have made the crop more susceptible to a herbicide that was applied before or soon after planting, or herbicide may have been moved into the root zone of the crop. At this point, there is not a lot one can do about it other than minimize any additional stress to the crop. It goes without saying that it is important to keep an extra close eye on the crop to determine whether harvest will be delayed, and, if necessary, make adjustments in the marketing plan.

What type of injury might you see? Active ingredients in the dinitroaniline group, including ethalfluralin (in Curbit® and Strategy®), trifluralin (Treflan®), and pendimethalin (Prowl®) cause root stunting. Roots will appear stubby. Aboveground the plant will be stunted. Chloroacetamides like s-metolachlor (Dual Magnum®) and dimethenamid-p (Outlook®) can cause leaf malformation. Often on broadleaves the midvein is shortened or puckered, making a ‘drawstring’ look, or the leaf blade is crinkled. In corn these materials can prevent normal unfurling of leaves. Sulfonylureas like halosulfuron (Sandea®) cause chlorosis, sometimes purpling, and sometimes leaf distortion or, in corn, improper leaf unfurling. Clomazone, which is in Command® and Strategy®, causes bleaching of leaves.

Even without a herbicide, chilling itself can injure sensitive crops like melons and squash. Chilled plants may wilt and develop watersoaked spots on leaves.

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