Excessive rains timed with very recent planting and pre-herbicide applications or from late-applied post treatments delayed (by rains) are always a concern in some fields. Growers and advisors are wise to familiarize themselves with the symptoms.
Most cases of herbicide injury in field crops can be traced to six herbicide classes or families: triazines, ALS-inhibitors, HPPD-inhibitors, plant growth regulators, PPO-inhibitors, and glyphosate. Below are some common symptoms associated with each of the herbicide groups.
- Triazines (Group 5): Triazines are photosynthesis-inhibiting herbicides that block the photosynthetic process, so captured light cannot be used to produce sugars. The plant slowly starves to death due to lack of energy. In broadleaved plants, early seedling growth appears normal, but shortly after emergence (when energy reserves in cotyledons are depleted), leaves become mottled, turn yellow to brown, and die. In most cases, the oldest leaves turn yellow on the leaf margins first, the veins remain green, and eventually the plant turns brown and dies. Herbicides such as atrazine, simazine, metribuzin, and Velpar could cause this kind of injury.
- ALS-inhibitors (Group 2): These herbicides work by interfering with one or more key enzymes that catalyze the production of specific amino acids in the plant. When a key amino acid is not produced, the plant’s metabolic processes begin to shut down eventually causing plant death. Plants that are sensitive to these herbicides stop growth almost immediately after foliar treatment; seedlings die in three to seven days, established perennials in two to four weeks. Symptoms include: stunted, yellow, purple veins, dead growing point, roots malformed (bottle-brush).These herbicides have systemic activity throughout the plant and young leaves are affected first. Grass plants may be stunted, with interveinal yellowing (chlorosis) or purpling. Corn plants may be stunted and show symptoms of root inhibition such as pruning of lateral roots. Leaves emerging from the corn whorl may not unfurl properly and may be yellow to translucent in appearance. Broadleaf plants may be stunted and chlorotic or purple. Soybean injury can range from stunting to death of the terminal growing point. Soybean leaves may be yellow in appearance and leaf veination may appear red or purple in color. Herbicides such as Classic, Resolve, Cimarron, Permit, Pursuit, Scepter, and Python can cause this kind of injury.
- HPPD-inhibitors (Group 27): These products are referred to as “bleachers” since they interfere with normal chlorophyll formation. Symptoms are very evident and easy to identify. Effected plants turn white or show bleached leaves, and eventually die if concentration of herbicide is high enough. Herbicides that contain HPPD active ingredients include: Lumax, Lexar, Balance, Corvus, Callisto, Impact, and Laudis. Clomazone (Command, component of Strategy) is not an HPPD-herbicide but is also considered a bleaching herbicide since the symptoms are similar.
- Plant growth regulators (Group 4): In most cases PGR damage occurs not from carryover but from herbicides that are sprayed in adjacent fields and then drifts onto the susceptible crop. PGRs upset normal growth as follows: cells of leaf veins rapidly divide and elongate, while cells between veins cease to divide. This results in long, narrow, strap-like young leaves. Water content increases, making treated plants brittle and easily broken. Cell division and respiration rates increase, and photosynthesis decreases.Food supply of treated plants is nearly exhausted at their death. Roots of treated plants lose their ability to take up soil nutrients, and stem tissues fail to move food effectively through the plant. The killing action of growth-regulating chemicals is not caused by any single factor, but results from the effects of multiple disturbances in the treated plant. Symptoms include: broadleaf plant leaves become cupped, crinkled, puckered, strap-shaped, stunted, and malformed; leaf veins appear parallel rather than netted, and stems become bent, twisted, and brittle, with shortened internodes. Typical herbicides in the PGR family that could cause problems if they drift to adjacent vegetable crops are 2,4-D and dicamba (Banvel, Clarity.
- PPO-inhibitors (Group 14): These herbicides are referred to as contact herbicides and they kill weeds by destroying cell membranes. They appear to burn plant tissues within hours or days of application. Good coverage of the plant tissue and bright sunlight are necessary for maximum activity. The activity of these herbicides is delayed in the absence of light. Injury symptoms: all contact herbicides cause cellular breakdown by destroying cell membranes, allowing cell sap to leak out.Effected plants initially have a “watersoaked” appearance, followed by rapid wilting and “burning,” or leaf speckling and browning. Plant death occurs within a few days. However, some of the PPO-inhibitors have longer soil residual activity so potential carryover is a concern. Products like Reflex, Flexstar, Authority, Spartan, and Valor can cause injury to certain crops if recropping restrictions are not followed.
- Glyphosate (Group 9): Plant foliage, especially new growth, will first yellow and then turn brown and die within a week or so after herbicide application. Sometimes new leaves on sensitive plants exhibit a bright yellow or even white appearance which can be confused with injury from other herbicide groups.