Preemergence weed control in cucumbers, pumpkins and squash

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Growers have planted cucumbers, pumpkins and winter squash during the past two weeks, and most preemergence herbicides have been applied. Heavy rains fell this week in many areas of the state, raising concerns about the potential for crop injury from herbicide movement into the root zone.

Grower concerns about herbicide injury to cucurbits is well-founded. Curbit (ethalfluralin), Command (clomazone), and Sandea (halosulfuron) all can cause injury to various cucurbit plants under adverse conditions. Normally, timing and placement of the herbicides on the surface provide sufficient selectivity for safe use of these herbicides.

Curbit normally is very safe on all of the cucurbits. However, it can cause stunting and chlorosis (yellowing) of leaves during cool conditions and after heavy rains. Curbit is very insoluble, so normally it stays on the surface well away from germinating seeds. It can be washed down into the soil by heavy rains. Crop injury appears as slow growth and yellow plants. Roots of young plants are stunted. In extreme conditions, stands can be reduced. Cucumbers are more susceptible to this injury than larger-seeded pumpkins and squash. Normally, the plants grow out of the injury as soon as the soil warms up.

Command always has the propensity to turn plants white. The cucurbits normally grow out of the early bleaching and there is no adverse effect on yield. Under extreme conditions, the bleaching may last longer, and there may be some stand thinning. Cucumbers are the least susceptible of the cucurbits. Bleaching will be most noticeable in bright orange and bright pink types of squash which always are more susceptible to bleaching.

Sandea may cause injury under normal growing conditions if not used properly. Cucumbers are the most tolerant of Sandea. Pumpkins and squash may be stunted or injured, or the stand may be thinned under cool, wet conditions. Growers may use Sandea preemergence to control yellow nutsedge, but often it is more useful for postemergence applications when plants are 5 to 8 inches tall. To avoid crop injury, apply Sandea as a directed spray to the areas between the rows. If stands have been thinned after the recent wet, cool weather, do not reapply Sandea on the same fields this year.

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