Don't assume symptoms you see on tomatoes, peppers, melons, or other crops are necessarily caused by herbicide drift.

Tom Lanini, Extension weed ecologist at the University of California, Davis, says it pays to take a much closer look, perhaps take photos and collect samples, and consult with an expert before blaming a wind-carried weed killer.

“At least half the calls I get about crop damage from potential herbicide drift turn out to be a nutrient imbalance, mechanical damage, temperature, or some other problem,” he said.

Lanini described some quick ways to distinguish typical symptoms associated with various classes of herbicides in a talk before a recent vegetable production meeting at the UC West Side Research and Education Center at Five Points.

He has compiled signs of herbicide damage in trials where he applied one-twentieth of the normal use rate of an herbicide over the crop.

The class of amino acid synthesis inhibitors includes Roundup, Raptor, Pursuit, Londax, Matrix, and several others. “Reduced leaf size is a common symptom, and you always see shortened internodes. New leaves may turn yellow or have chlorotic spots. You see a little yield loss, even though the crop may grow out of it, and when a plant is stressed, it's more susceptible to disease and other pests,” he said.

Among the growth regulator herbicides, such as 2,4-D, MCPA, Banvel, Garlon and others, petiole twisting, stem twisting and malformed leaves are common crop damage. Symptoms are most often seen on new growth but may occur on mature growth. Stem cracking and node enlargement may also appear.

Check nearby weeds

Lanini recommended inspecting any nearby weed growth for similar effects because if they appear only on the crop and not on weeds, the cause is likely disease.

Goal, Chateau, Gramaxone, Shark, and Clincher are in the cell membrane disruptor class and show symptoms of spotting on either new or old leaf growth with normal new leaf growth. “New tissue is not affected, and these chemicals are not translocated in the plant.”

This class of chemicals, however, will affect fruit with spotting or gummosis, and if exposure is early enough in fruit development, the fruit or grape berries will fall.

“One effect that Goal has is a girdling of plants. If Goal has been applied and can't be incorporated before planting because of wet conditions, this occurs. Then wind or cultivation will break off many of the plants,” he said.

Another among this group, Shark, can produce a halo effect on foliage, but if it does not contact the fruit, there is no damage.

Soil line swelling

Dinitroaniline class compounds, such as Treflan, Surflan, Devrinol, Kerb, and Visor, inhibit cell division and seedling growth. “These cause an overall reduction in growth, and you may not see anything in particular until you examine transplants right at the soil line. There's a swelling that becomes brittle and may cause the plant to break off.”

Lanini added that the dinitroanilines can also enlarge root tips and cause reduced lateral rooting. “When you put tomatoes, peppers, or other vegetable transplants in the ground, you must make sure the roots are below the zone treated with these chemicals. If they are in wet, cool conditions and cannot get out of the zone, you can have plant losses.”

The pigment synthesis inhibitor class, including Solicam and Zorial, blocks carotenoid biosynthesis, produces veinal whitening that may become necrotic, and bleaches the foliage white.

Princep, Sencor, Karmex, Spike and others in the photosynthesis inhibitor class, at low rates, produce symptoms on mature leaves. In the case of Princep and Sencor, chlorosis of leaves is the most common symptom, and veinal chlorosis is common with Karmex, Lorox, and Spike.

The grass herbicides Post and Fusillade are in the lipid biosynthesis inhibitor class and their damage shows as stunting and death of the gravity point of plants and purpling that leads to necrosis.

Symptoms of shortened midveins and failure of leaves to unfold properly are associated with seedling growth inhibitors, such as Dual Magnum and Eptam.

“Dual Magnum is registered for tomatoes and we are looking at it for melons,” Lanini said. In one melon trial, he was unable to treat the crop because of windy conditions and when the treatment was made the melons had cracked and some injury occurred. However, when he applied Dual Magnum right after planting and sprinkled it in, there was no crop damage.

Other indicators

Symptoms may be caused by several other things, including nutrient shortages or excesses, water excess or drought, wind driven sand or soil particles, high or low temperatures, mechanical damage, or other pesticides.

Lanini said photos to document effects of a problem early and later, perhaps after the crop grows out of it, can help to determine if losses occur.

He said herbicide drift can be prevented or reduced by simply applying when there is no wind. “But the problem is, particularly in a wet year when everything is pushed into a short time frame and you must make an application, you often make it when conditions are less than ideal.”

Continuing studies to learn the cause of tomato vine decline in the lower Sacramento Valley have been inconclusive, according to another presenter, Gene Miyao, farm advisor for Yolo, Solano, and Sacramento counties.

The disorder reduces vine canopy and exposes fruit to high temperatures, resulting in quality and yield losses.

One of the fronts in finding answers has been selection of varieties having more robust canopies. Another has been evaluation of soil pathogens and different applications of Vapam fumigant and foliar applications of urea.

Supplemental nitrogen treatments were not successful in improving plant vigor, although the research indicated that no low-level foliar disease appears to be involved. No benefit was seen from use of mustard cover crops to reduce soil pathogens, although deeper shanking of Vapam did reduce verticillium wilt by half and improved yield.

Miyao is investigating the problem again this year using treatments of calcium and micronutrients.