Georgia cotton diseases add to drought losses

While drought may have been the primary culprit behind Georgia's below-average cotton crop in 2000, disease pressure also was a contributor, says Bob Kermerait, University of Georgia Extension plant pathologist.

Based on samples submitted to the disease clinic in Tifton and requests for trouble-shooting visits to cotton fields, three diseases caused major concerns this past year, he says.

"These include seedling blight, fusarium wilt and stemphylium leafspot," says Kermerait. "There also was a single report of areolate mildew, caused by Ramularia areola, reported in Decature County in extreme southwest Georgia. Areolate mildew currently isn't of economic importance in Georgia, but it is unusual."

Seedling blight is known to be more severe on cotton that is planted when weather conditions are cool and wet - conditions which were not experienced by growers in 2000, says the plant pathologist.

"However, seedling blight was very common in many areas of Georgia this year. This may be due in part to the continuous buildup of innoculum in the soil with peanut/cotton rotations. All of the seedling blight evaluated in the clinic and seen in the field was the result of Rhizoctonia solani. Pythium seedling blight was not found," says Kemerait.

In-furrow fungicides that were tested in a study conducted at the Coastal Plain Experiment Station in Tifton did not increase final yields from the plots that received no treatment at planting, he adds.

The disease causing the most concern this past year was fusarium wilt, caused by Fusarium oxysporum f.sp. vasinfectum, he notes.

"The disease was confirmed from several areas in Georgia, although losses were most severe in Jeff Davis County and surrounding areas. Fusarium wilt was often, but not always, found in association with root knot nematodes. Conversations with pathologists in other states indicate that this problem wasn't isolated to Georgia," says Kermerait.

Reasons for the increase in Fusarium wilt pressure are unclear, he says, but poor rotations are involved in some cases. "The best control of Fusarium wilt integrates crop rotation, nematode control and the use of cotton varieties with some resistance. Research being conducted this winter should help to identify varieties that will be beneficial in Georgia."

Severe outbreaks of stemphylium leaf spot were seen in several areas of Georgia and in neighboring areas of the Florida Panhandle, says Kermerait.

"The severe cases were associated with rapid defoliation across large portions of the field. The fungal pathogen Stemphylium sp., appears to infect the foliage of plants that are affected by a potassium deficiency. The best way to manage this leaf spot is to avoid potassium deficiencies.

Nematodes also continue to take a bite out of Georgia's cotton yields. The most important step in nematode management is to identify which nematode species are likely to cause damage in a given field, says Richard Davis, University of Georgia Extension nematologist.

All nematode management decisions depend on that important first step, he says. "In cotton, as in most row crops, this first step is accomplished primarily through soil sampling, and the best time to obtain the most meaningful sample is before or soon after harvest," says Davis.

The population levels of all plant-parasitic nematodes are influenced by two factors that change significantly during this time of year - the amount of food available to the nematodes and soil temperature. Both factors, says Davis, cause population levels to begin to decline in the fall, so it's critical to obtain samples before levels decline so much that fields with nematode problems cannot be reliably identified.

"When cotton plants are defoliated, the amount of food they produce for nematodes is reduced drastically," he explains. "Nematode population levels may remain steady for a short time, but they soon will begin to decline. This is true especially for root-knot nematodes.

"Nematode eggs are highly durable and will survive until the following spring. So, the potential nematode problem is not declining. Rather, the worm-like stages which are identified and counted in soil samples are declining. If the worm-like stages decline too much, we'll fail to identify potential nematode problems. The way to prevent this from happening is to sample as soon after defoliation as possible. Sampling before defoliation is even better, though this is impractical for many farmers."

The effect of declining soil temperature is less dramatic than the effect of defoliation because it happens over a longer period of time, notes Davis. The results, however, are the same.

"Nematode metabolism, like that of insects, depends on external temperatures. As soil temperatures decline, all nematode activity stops eventually. When eggs stop hatching and nematodes that already have hatched die or enter roots for protection, the apparent population extracted from soil samples declines. When apparent population levels decline below threshold levels, potential problems cannot be identified from soil samples alone."

The Extension Nematology Laboratory at the University of Georgia has begun charging fees for specific services, says Davis. Samples for problem diagnostics, usually referred to as "troubleshooting" samples, submitted through the county Extension office of sample origin will be analyzed at no charge.

Samples for purposes other than problem diagnostics, such as planning for next year's nematode management, submitted though the county Extension office of sample origin will be charged $6 per sample. All other samples, including samples submitted from out of state and samples not submitted through the Extension office of sample origin will be charged $25 per sample.

Troubleshooting samples are taken, says Davis, when a problem has been observed and the county agent is trying to help determine the cause of the problem. If nematodes could be the cause of the problem, then a troubleshooting sample is justified. Because the goal of troubleshooting samples is to identify the cause of an existing problem in the current crop, these samples should be collected prior to harvest, he says.

The method for returning nematode assay results to county Extension offices also has changed, he says. Counties now can download the results from a special, password-protected site on the Internet. Only Georgia county Extension agents can access the site.

"This reduces the time it takes to get results back to farmers because results are available to county agents as soon as the samples are processed. The results remain archived on the Internet, allowing agents to find old results if needed.

"The form printed off the Internet and returned to farmers now has a statement, based on University of Georgia nematode thresholds, about whether or not the sample indicates a potential nematode problem."

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