Cupping leaves on soybean
click image to zoomSoybeans infected with soybean mosaic virus exhibit crinkled leaves and a yellow mosaic pattern on leaves. Soybeans across Indiana are curling and cupping and may have a mottled appearance. In some cases, these soybeans are also under drought stress, or suffering from other disorders. While it is easy to look at cupping and curling symptoms on soybean and claim injury from a growth regulator, it is much more difficult and important to determine the origin of growth regulator herbicide injury or determine if it is a viral disease that can cause a similar symptom. When looking at soybean fields with these symptoms, use the following information to determine if it is a pest or pesticide issue.
Plant Growth Regulator Herbicide Injury
When soybeans are exposed to low doses of growth regulator herbicides the plant leaves will cup, strap, and drawstring at the point of newest growth, rather than twisting and epinasty of stems that would be associated with lethal doses. The cupping and strapping can take up to two weeks to show up in an affected field. The sources of low dose growth regulator injury on soybeans are drift, volatilization, tank/boom contamination, or a combination these sources from growth regulator herbicides used in corn, pastures, fencerows, and home lawns.
click image to zoomSoybean plants with cupped and strapped leaves due to potential drift, volatilization, or tank contamination of a plant growth regulator herbicide. Distinct patterns in the field of changes in injury severity, and other susceptible plants in the field and surrounding fields can assist you in determining the source of contamination. A field that received drift will have the most severe injury closest to the source of drift. Broadleaf weeds in the source field will have the twisting, curling, and epinasty expected with a full dose application. If the drift occurrence occurred across a road or fence line, look for twisting and curling of weeds in the ditches or fence line between the fields.
Volatilization can be a little trickier to identify than a drift occurrence, especially if both occurred at the same time. Look for injury patterns that follow air current pathways and changes in elevation. Injury occurring in low lying patches or valley’s can be an indicator of growth regulator volatilization. As with drift, a source field of volatilization will need to be identified by looking at surrounding fields, pastures, and road side could have potentially received a growth regulator herbicide application.