Published on 07/02/03

Crop diseases flourishing in rainy Georgia summer

By Brad Haire
University of Georgia

Georgia's row crops are enjoying some of the best weather they've had in years. But major crop enemies dwell in those healthy plant canopies. Plant diseases, too, enjoy the wet weather.

Ample rainfall all spring and summer's rising humidity have made "super" weather for certain crop diseases, says Bob Kemerait, a plant pathologist with the University of Georgia Extension Service.

"We're seeing a lot more fungal diseases in particular," he said. Fungal diseases can take their toll on major Georgia crops, like corn, peanuts and cotton. Such damage to plants can lead to lower yields.

Corn calls

"In the past few weeks, I've had more phone calls about corn (diseases) than for cotton and peanuts combined," he said.

The number of cotton and peanut calls hasn’t dropped from other years. There’s just been an increase in corn diseases and interest in controlling them this year, he said.

Southern corn leaf blight and southern rust attack corn leaves, especially during periods of wet weather and on corn varieties with reduced resistance. These diseases can easily take away yield and cause serious damage when the crop is grown for livestock feed, Kemerait said.

Fueled by wet, humid weather, these two diseases have increasingly damaged Georgia's corn in recent years. They cost growers about $3 million in damage each year. The damage could be greater this year, he said.

Corn diseases aren't as economically damaging as cotton or peanut diseases. But all together, they cause about $10 million a year in damages.

Historically, corn farmers haven’t felt that the value of field corn justified the expense of disease control. But that way of thinking is changing, Kemerait said.

The Georgia Commodity Commission for Corn wants studies to determine if farmers can afford to fight these diseases before they get out of hand.

Funky peanuts

The wet weather may have helped Georgia's peanut crop to look a little more "funky" this year, he said. Symptoms are being called funky leaf spot.

It looks similar to early leaf spot disease, but conventional chemicals don't appear to affect it much. Farmers and researchers will continue to watch this disease, he said.

Tomato spotted wilt virus, a deadly peanut disease, may take it easy this year, Kemerait said. It hasn't been as severe on other crops. This is usually good news for the peanut crop.

Farmers also stuck to the UGA Spotted Wilt Index, a guide developed to reduce the risk of the disease, this season. In the past, Georgia farmers had most of their peanuts planted before May 15. But this year the risk index recommended not to start planting before May 10.

But planting beginning in the middle of May means that the peanut plants will likely have greater exposure to leaf spot diseases, which tend to be more severe on later planted peanuts.

Most farmers can keep leaf spot diseases at bay with regular spraying, he said.

Georgia peanut farmers spend about $65 million fighting diseases and still lose $50 million to disease damage each year.

Cotton worms

Wet weather blight affects cotton in periods of damp weather and has appeared on much of the 2003 crop. However, it has typically been only a short-lived cosmetic problem.

Nematodes, the microscopic worms that feed on cotton roots, have shown up with a vengeance in some fields early this year, Kemerait said.

The wet weather may provide a better environment for their feeding and development, he said. However, poor crop rotation is usually the reason that cotton growers have a problem in their fields.

"The best thing farmers can do right now is to keep an eye on their fields, know what's going on and be ready (to fight diseases if they appear)," Kemerait said.

But the best defense against diseases, he said, is having a healthy plant. And, so far, nature has provided the weather for that.

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