Soil-borne diseases hitting peanuts Stem rot or white mold continues to be one of the most damaging peanut diseases in the Southeast. Losses, including the cost of control, often are more than $40 million annually in Georgia alone, says Tim Brenneman, University of Georgia plant pathologist.

"Since 1994, several effective stem rot fungicides have been labeled for use on peanuts," he says. "Growers use these fungicides extensively and get significant yield responses, but these materials are more expensive than standard peanut fungicide programs for foliar diseases."

Georgia tour Brenneman reviewed the latest research on white mold and other soil-borne peanut diseases during the 2000 Georgia Peanut Tour.

White mold, which is caused by a soil-borne fungus, rots the pods and affects the crown of the plant, often killing the entire plant, he says. Historically, white mold has been the most damaging disease of peanuts in Georgia. Although tomato spotted wilt virus recently has moved into the top position.

There are a number of cases reported each year where the degree of control for white mold is not as good as expected, says Brenneman.

"Some of these problems are thought to be related to the timing of applications, which are made on a pre-set calendar basis. It has been observed that stem rot starts much earlier in some years than in others. In 1998, for example, unusually hot temperatures in June and the first part of July resulted in an early epidemic, with many plants being infected before the first spray was applied," he explains.

One of the University of Georgia's current research projects involves the development of a spray advisory to predict when fungicides for stem rot need to be applied, notes Brenneman.

"A series of models were developed based on our previous knowledge of the disease. These models currently are being evaluated under various field conditions to determine which of them perform the best. Comprehensive environmental data also is being collected to increase our understanding of what conditions are most favorable for stem rot to occur," he says.

Preventive action An increasing number of Georgia peanut producers are planting twin-row peanuts and using higher seeding rates, says Brenneman. "These practices have been shown to result in lower levels of tomato spotted wilt virus and higher peanut yields. However, we don't know what effect these practices have on the development of stem rot. We also don't know how the differences in microclimate and plant spacing affect stem rot epidemics," says the plant pathologist.

Other studies are ongoing to develop peanut cultivars with better resistance to stem rot, he adds.

"New chemistry is being screened constantly in the search for more effective fungicides, and different application technologies are being evaluated. There are no easy answers in the fight against this disease. Hopefully, this research will result in an integrated production program that enables growers to reduce losses to this damaging pathogen."