May 18, 2005 - Several reports of injury in fields treated with either Authority (sulfentrazone) or Valor (flumioxazin) have been received this week. Injury from these herbicides is most common when heavy rains occur near the time of emergence. When these events occur, the soybean is exposed to high concentrations of the herbicide and the plant's defense system may be overwhelmed. The prolonged cool temperatures experienced across the state would enhance the potential for injury by decreasing the vigor of the soybean. The severity of the injury will be affected by herbicide rate, soil type, rainfall amount, stage of soybean development at timing of rain, and varietal tolerance.

In most fields with damage this year the soybean seedlings had not emerged at the timing of the heavy rain, thus the injury was limited to the hypocotyl and cotyledons. The left photo shows necrotic lesions on soybean in the crook stage. Lesions on the cotyledon normally will have little effect on future development of the plant. The hypocotyls in the center photo were completely girdled by the herbicide and these plants would die. It is possible to have discoloration of the hypocotyl by the herbicide that doesn't result in the magnitude of cell destruction shown in this photo. The injury in the first two photos is typical when heavy rain occurs just as the soybeans are breaking the soil surface. Injury often is more severe in poorly drained areas of the field.

The lower photos illustrate damage to the soybean growing point due to splashing of the herbicide with heavy rain following cotyledon expansion. If herbicide contacts the growing plant developing tissue will be disrupted. The growing point on the plants in the left photo were completely destroyed and the plants would not progress from this state. The meristems on the plant in right photo were not completely killed and leaves are beginning to develop that will eventually allow recovery. In many situations the damage will not be as severe as pictured and only leaf malformations will be observed.

Timing of the first rainfall event following application is the critical factor influencing injury from these herbicides. Damage typically occurs when the first significant rains after herbicide application occur at the time of emergence. I think many fields will have escaped severe injury this year due to the soybeans being deep enough in the soil to avoid high concentrations of the herbicide. Once the herbicide has been moved into the soil profile by rain the risk of injury is greatly diminished.