Herbicide Injury in Grapes and Other Sensitive Plants
Bruce Bordelon, Professor, Department of Horticulture and Landscape Architecture, Purdue University
Many plants are sensitive to damage from plant growth regulator herbicides, including the phenoxy, benzoic, and pyridine classes of compounds. The most commonly used products are 2,4-D (phenoxy) and dicamba (benzoic), which are widely used in corn and soybean production, and on lawns, turf, and right-of-ways. Most formulations of these chemicals are volatile and prone to vaporization. They can move as vapor from the site of application in concentrations high enough to damage sensitive plants. Applications to lawns and turf near sensitive plants can also cause significant damage through root uptake.
Grapes, tomatoes and redbud trees are the most sensitive commonly grown plants in Indiana, but many other dicotyledonous plants can be damaged. Timing of exposure has a significant affect on the amount of damage that occurs. When these herbicides are applied in early spring before trees and grapes bud out, or tomatoes and other sensitive crops are transplanted, there is seldom any damage. But when applications are delayed because of a cool wet spring such as we had this year, they coincide with rapid growth of sensitive plants and damage can be severe. Damage from 2,4-D recently began showing up in commercial plantings and many home gardens.
Exposure to growth regulator herbicides usually affects only new, developing growth. Older growth will be normal and subsequent growth may be normal as well. In grapes, damage usually shows up as distortion of the youngest leaves at the shoot tips (See Picture 1). Small, fan shaped leaves with parallel veins are typical of 2,4-D damage (See Picture 2). Damage to leaves and shoot tips is not always economically important, however, when exposure occurs prior to fruit set, clusters fail to develop normally and complete loss of the fruit crop is possible, having significant economic impact to the grower (See Picture 3). Severe damage leads to very stunted vine growth and significant reduction in yield potential for the following season. Varieties differ greatly in sensitivity.
Several other crops and ornamental plants are also sensitive to 2,4-D damage including tomatoes and redbud trees. Redbuds are frequently damaged. Exposure early in the growing season can cause severe distortion and stunting of growth (See Picture 4). Exposure later in the year may cause distortion of only the most recently developed leaves (See Picture 5). Symptoms on redbuds are very similar to those on grapes (See Picture 6).
Avoiding damage from growth regulator herbicides can be difficult. However, there are a few actions that can be helpful. Homeowners can avoid application of lawn herbicides near sensitive plants, especially the “post emergent broadleaf weed killer” type products that contain 2,4-D and dicamba. These products can be safely applied under most conditions, but care must be taken to avoid unintended damage to trees, shrubs and flowers.
Commercial growers of sensitive crops need to communicate with farming neighbors about the risk of herbicide damage. A good tool for this communication is the Driftwatch web site (www.driftwatch.org). This web-based tool allows producers of sensitive crops to identify their acreage so that applicators will know there is a risk. Applicators can sign up for automatic email notification when sensitive crops are posted in their region. The program was developed at Purdue University and is now being used in several states. Grower organizations, applicators, and agricultural chemical companies are all supporting this effort.
Click image to enlarge
Picture 1. Developing shoot tip of grapes recently exposed to 2,4-D herbicide
Picture 2 caption: Close up of grape leaf showing fan shape and parallel veination typical of 2,4-D exposure.
Picture 3 caption: Grape flower cluster damaged by exposure to 2,4-D. Note the lack of fruit set.
Picture 4 caption: Redbud shoot several damaged from early season (April) exposure to 2,4-D. Note how newest leaf is developing normally.
Picture 5 caption: Redbud shoot with only the most recently developed leaf showing damage from 2,4-D.
Picture 6 caption: Close up of damaged redbud leaf showing classic parallel veination typical of 2,4-D damage.