Today's minor cotton diseases could become tomorrow's yield-limiting problems. Just ask Georgia's peanut growers.
“Fifteen years ago, most Georgia peanut producers had never heard of tomato spotted wilt virus, and they weren't concerned,” says Bob Kemerait, University of Georgia Extension plant pathologist. “But today, it's the major disease threat to the peanut industry. Fortunately, we don't have a disease in cotton that's near that magnitude, but some diseases may become more of a problem in the future.”
Most growers are familiar with these “emerging” diseases at some level, he says. “Several of these diseases currently are not a problem, but we need to keep an eye on them. We've looked around at cotton production areas in neighboring states to see what other growers are facing and what may become important diseases in Georgia,” says Kemerait.
The first objective, he says, is to identify those diseases that may become increasingly important to Georgia cotton producers. “We then must decide the potential threat to our industry and suggest possible factors in the future emergence of these diseases,” he notes.
One disease making Kemerait's list is Fusarium wilt. This is a fungal disease, he explains, that typically becomes evident in mid-season, although it can occur at any point in the growing season.
“Fusarium wilt usually is found in association with infections by the Southern root-knot nematode, which has a synergistic effect on the disease.
Most visible symptom
“The most visible symptom of Fusarium wilt is the presence of wilting and dying cotton plants in a field. Root-knot nematodes alone can cause wilting, but the synergistic effect with the fungus usually is required to kill plants unless the soil is extremely dry for prolonged periods,” says Kemerait.
Fusarium-infected plants can wilt even if soil moisture is adequate, he adds. “A preliminary diagnosis of Fusarium wilt can be made fairly easily in the field by slicing through the plant stem at a shallow angle to expose the vascular tissue. The disease will cause a noticeable browning of the vascular tissue. If you carefully dig up the root system of wilting plants, you'll also usually see significant galling caused by root-knot nematodes.”
During the 2000 growing season, especially in counties such as Jeff Davis, there were reports of severe Fusarium wilt in isolated fields, says Kemerait. Other reports of less severe Fusarium wilt came from throughout the state, he adds.
“There's a perceived increase in the incidence of Fusarium wilt in North Carolina, South Carolina, Mississippi, Texas and possibly other states such as Tennessee. In Australia, some fields are so severely infected with Fusarium wilt that it's no longer profitable to grow cotton in those fields. This highlights the destructive potential of the disease.”
Causing even more concern is the fact that there currently are no adequate control measures for Fusarium wilt, says the plant pathologist.
“We have severely infected fields in Georgia, but producers continue to plant cotton in those fields. We want to avoid the situation seen in parts of Australia.”
One reason for the increased incidence of Fusarium could be the increased population of Fusarium and nematodes in certain fields, says Kemerait, and this could be caused by poor rotations.
“It's been less than 10 years since we had a great increase in cotton production in Georgia, going from 300,000 acres to more than one and a half million acres. Many of our growers have gotten away from good rotation practices.
“If you don't rotate away from cotton, you'll increase the population levels of Fusarium and nematodes. We may be seeing the start of a general increase in the disease due to a buildup of the populations and a lack of rotation.”
Experts from throughout the country also have speculated that the rapid release of cotton varieties also may be contributing to the increased incidence of Fusarium wilt, he says.
“The perception is that the rapid release of new cotton varieties may be done at the expense of proper screening for different pathogens, including Fusarium wilt. These varieties may have the desired agronomic characteristics, but they may not have been screened for common diseases such as Fusarium wilt.”
The University of Georgia, he says, is working to come up with a rapid screening technique for Fusarium wilt.
Another disease that bears watching is bacterial blight, says Kemerait. This disease, which starts out as angular leaf spots, can cause cotton boll rot, he says.
“This bacterium can get into the seed and be transmitted to the plant. In Texas, no bacterial blight was reported from 1995 to 1997. From 1998 to 2000, however, 80 to 90 percent of the fields in seed-producing areas were infected with this bacterial pathogen.
“Drought very much affects this pathogen. It is very dependent on rain splash and water before it can spread. It might have been found in 80 to 90 percent of the fields, but conditions have not been good for the spread of the disease.
“North Carolina found a blight outbreak in 2000, and all infected plants came from a single seed plot. As conditions improve in Texas for spreading the disease, it may become more of a problem.”
There's a perception, says Kemerait, that a change in the de-linting procedure may be allowing infected cotton seed to be passed on to growers.
Cotton stem canker — also called wet weather blight — is another disease that has caused problems in neighboring states, he says. “It starts out as small red flakes or blue spots early in the season. Many times, these symptoms are mistaken for chemical injury or pesticide burn.
“As they mature, infected leaves may begin to defoliate, and you may have stem canker along the petioles and the stem. Later in the season, severely infected plants can lodge and then break.”
This disease must have cool, wet weather to spread, he says, and it is fairly common in North Carolina. “If the problem is severe enough, you may have symptoms of damping off. In North Carolina, 40,000 acres were affected by stem canker. Much of this was associated with weather conditions at planting time.
“In 2000, North Carolina producers had to replant 20,000 acres of cotton due to wet weather blight. Most of this was in a three-county area, and most of it was in ultra-narrow-row cotton. The best control for this disease is to avoid cool, wet weather at planting.”
Growers should be aware of cotton stem canker, but it probably won't become a real concern for Georgia growers, says Kemerait.