Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
We have recently identified growth regulator herbicide damage in a commercial vineyard.
Late April and May is often the time that we see severe damage to grapes from growth regulator herbicide drift, most commonly 2,4-D, which is used in herbicide burndown programs for no-till field crops (soybeans). Dicamba can also cause severe damage and is often applied to turf, lawns, pastures, and hayfields. Vineyards next to areas where these growth regulator herbicides will be applied will be at high risk of injury from off-target movement (volatilization and drift). Grapes are very sensitive to these compounds and injury can occur at levels 100 times lower that labeled rated.
Grapes are most sensitive when new shoot growth is occurring prior to flowering. When exposed to 2,4-D or dicamba at this critical growth stage, the new growth will show severe leaf distortion and stunted shoots. In the most severe cases shoots will die. Grapes will have poor fruit set and low yield when exposed prior to and during flowering. When grapes are exposed later in the season, while leaf and shoot symptoms may be present, there is usually minimal yield loss. Grapes may eventually grow out of 2,4-D damage and produce normal leaves; however dicamba damage may cause abnormal growth throughout the season.
Grape growers and vineyard managers should work closely with neighboring farmers or property managers and educate about the need to avoid growth regulator herbicide applications in late April and May near the vineyard. Planted windbreaks and screens can help reduce movement but will not replace distance or vineyard isolation as a management tool.
2,4-D damage to grape shoots. Photo from Virginia Tech Learning Resources Center, Virginia Polytechnic Institute and State University, Bugwood.org