Published on 08/15/02

Georgia farmers battle crop diseases

By Brad Haire
University of Georgia

Second only to actually buying the seed for a crop, fighting diseases is the most essential thing a farmer has to do to grow a successful crop in Georgia, said Bob Kemerait, a plant pathologist with the University of Georgia Extension Service.

Georgia peanut farmers, for example, spent $65 million fighting diseases last year and still lost $50 million of their crop to disease damage.

TSWV Terrorizes Farmers

Tomato spotted wilt virus began attacking several Georgia crops, including vegetables, tobacco and peanuts, in the mid-1980s. This disease preferred an aerial assault. Carried inside tiny insects called thrips, it has swept over much of the state. It continues to terrorize farmers, especially peanut farmers.

This year, Kemerait said, TSWV has damaged as much as 70 percent of some peanut fields. "Some growers had forgotten how bad it could be," he said.

Some diseases, like an infantry invasion, prefer to attack head on. Soreshin, a cotton disease, cuts plants off at the "knees," he said.

"The fungus nibbles at the plant at the soil line," Kemerait said. "It weakens the plant. The plant tends to topple over, like the knees have been cut from beneath it."

Cotton Bolls Provide Home

Sometimes plants open themselves up to an invasion by providing fertile campgrounds. This can happen when cotton bolls, the part of the plant that produces the lint, first open. Rain can bring into the open boll bacteria and fungi that find a wonderful environment to flourish and destroy the boll, he said.

Boll rot caused $18.5 million in economic damage to Georgia cotton in 2001 alone.

Some attacks take place underground. Plant pathologists and farmers have been fighting a tiny cotton nemesis, the nematode, for years. The nematode is a flat worm that attacks the plant's root system, choking off water and nutrients.

"A very small population can build and build and build," he said. "In a good, wet year, you may not see that dramatic an effect. But a dry year, you can see if the root system is functioning well or not."

Nematodes are hard to control. The farmers' best tool is frequently rotating the crops they plant in a field. "But with less and less land and fewer crops out there to make money, rotation becomes difficult," he said.

Diseases in Disguise

Some diseases are masters of disguise. They can look like one disease but act like something else. One such disease has started popping up in Georgia peanut fields in recent years.

Funky leaf spot appears to be similar to another leaf spot disease. But conventional chemicals don't appear to affect it much. It hasn't caused much damage yet.

"But you have to track it down to see if it's important or not," Kemerait said.

Funky leaf spot acts strangely in another way, too. It seems to help another disease in its war on the peanut plant.

One Disease Helps Another

It causes the plant to drop leaves. The leaves fall to the ground and begin to decompose. And this decomposition releases chemicals that spark another fungus, the one that causes white mold, to germinate, become more active and attack the plant.

Last year, white mold cost farmers $24 million in damage and treatment costs. "In combination, (funky leaf) could make white mold worse each year," he said.

Farmers can't relax their war on diseases, he said.

"But if you take our best growers, the ones who do everything they can right, they generally have the upper hand on the diseases," he said.

However, some growers aren't able to rotate crops. Others are late applying chemicals or other preventive measures.

"Those are the ones the diseases get the best of," he said. "And once (the diseases) get ahead, it can be difficult to bring them back in."

Brad Haire is the former news editor with the University of Georgia College of Agricultural and Environmental Sciences.