Integrated approach best for peanut diseases

Peanut producers continue to make progress managing tomato spotted wilt virus and leafspot diseases through an integrated approach, using cultural practices and fungicides along with resistant varieties, says Albert Culbreath, University of Georgia plant pathologist.

“New cultivars continue to come down the line, and we’re integrating these along with some things we’ve already been doing, with a few new things,” says Culbreath.

Tomato spotted wilt virus, he says, has become an all-too-common sight in Georgia peanut fields in the past 15 years. “This virus is spread by thrips, but most everyone has learned that you really can’t do much in terms of controlling tomato spotted wilt by controlling thrips. It has taken an integrated approach, and the cornerstone of that integrated management regime has been the use of resistant cultivars,” he says.

The Georgia Green variety has been the mainstay for much of the Southeastern U.S., says Culbreath. “However, in 2002, we still saw a lot of fields with substantial damage from tomato spotted wilt virus. You can have a lot of problems even with the level of resistance shown by Georgia Green,” he says.

Several newly released peanut cultivars have significantly better resistance to tomato spotted wilt virus than Georgia Green, he adds.

“A lot of these new lines also have good resistance to leafspot. This should help us greatly in managing tomato spotted wilt virus. Integrating these cultivars into our programs should prevent disease losses and give us some new flexibility in planting dates.

“Some of these combinations also should give us better leafspot control with the fungicide regime we’re using or possibly allow us to cut back on fungicides,” says Culbreath.

The University of Georgia Tomato Spotted Wilt Virus Risk Index lists several factors for controlling the virus, he says, including planting date, conservation tillage, twin-row patterns rather than single rows, and the use of Thimet or Phorate insecticides at planting. But none of these alone will adequately control tomato spotted wilt.

“We combine these with resistance and as many other factors as possible, and that has worked very well for Georgia growers. We also try to integrate practices for leafspot control.”

This past year was not a severe one for tomato spotted wilt virus in peanuts, says Culbreath, but it was a heavy leafspot year for many areas.

“Thankfully, we had enough rain to cause leafspot. It takes moisture to get much leafspot, and it takes moisture to get yield. Most growers would rather fight leafspot than drought.”

The first thing that comes to many growers’ minds when they think about managing leafspot is using fungicides, says Culbreath. “But we don’t need to forget the importance of rotation. A good rotation does help to delay the onset of a leafspot epidemic — cultural practices can help to suppress leafspot epidemics.

“Also, with some of the new cultivars being released, we have a better level of resistance and tolerance.”

Managing tomato spotted wilt virus, he says, requires an integrated approach, with the use of resistance cultivars and several cultural practices. “Fungicides don’t have an effect on tomato spotted wilt virus, just as we’ve seen no evidence that the use of Phorate or Thimet affects leafspot. However, there are other factors that influence both of these diseases.”

Researchers currently are doing a lot of work with tillage, says Culbreath, looking at integrating strip-till peanuts along with the use of more resistant cultivars and reduced fungicide applications, to gauge the effects on leafspot and tomato spotted wilt virus.

“When comparing conventionally tilled, non-treated Georgia Green with strip-till, non-treated Georgia Green, we saw that strip-tillage delayed the tomato spotted wilt virus epidemic. Conventional-tillage with C-99R showed a little more resistance and better suppression. C-99R combined with strip-till looked even better.

“When we combine resistance with cultural practices, we’ve been able to cut back on our fungicide regimes.”

In 2002 and 2003, researchers looked at strip-tillage compared to conventional-tillage with some of the newer peanut varieties, including GA-01R, DP-1, Hull and some of the new breeding lines from the University of Georgia, University of Florida and USDA programs. These trials consisted of 14-, 21- and 28-day spray schedules, using chlorothalonil for leafspot control. Both leafspot and tomato spotted wilt virus were evaluated in the trials.

The peanut cultivars C-99R, DP-1, Hull and GA-01R all showed improvement in leafspot and spotted wilt control when compared to Georgia Green, says Culbreath. “In all of the cultivars planted in strip-till, we saw at least some improved suppression of tomato spotted wilt when compared to conventional-tillage.”

This past year, researchers looked at the effect of twin-row patterns on tomato spotted wilt virus and leafspot. “In 2003, the twin rows did not have any effect on leafspot, so we believe that the practice won’t hurt us there. We already know twin rows can help manage spotted wilt, and Tim Brenneman has found that twin rows may help suppress white mold as well.”

Researchers also looked at the effect of reduced fungicide applications on leafspot and tomato spotted wilt virus. They compared a full regime — starting with Headline, followed by four applications of Folicur, and ending with Bravo — with a reduced regime, starting the season with Headline and following that with two Folicur applications. We didn’t see any effect of fungicides on spotted wilt.

“In looking at leafspot, however, we did get a response with six sprays compared to three, which is not a big surprise with any of the cultivars. But we didn’t see nearly as big a difference in the level of disease with GA-O1R or DP-1 as with Georgia Green, AP-3 or Carver. Even in the reduced regime, GA-O1R and DP-1 held up pretty well against leafspot.”

But not all of the new cultivars will have the same resistance package, notes Culbreath. For example, AP-3 has excellent resistance to spotted wilt, some resistance to CBR and white mold, but it doesn’t have leafspot resistance in the package.

“But even AP-3 — with no appreciable leafspot resistance — may help us in terms of leafspot control by allowing us to shift our planting dates a little earlier. Some of these new lines have enough resistance to tomato spotted wilt that they give us more flexibility in terms of moving back our planting date, especially if we combine the cultivar with other factors that will suppress the virus.

Summarizing the research findings, Culbreath says conservation-tillage may help suppress both leafspot and tomato spotted wilt virus in all cultivars tested.

“The tillage effects on leafspot do appear to be dependent on rotation. We did not see those effects in peanuts following peanuts. Most of our work has been with peanuts behind cotton or corn. That doesn’t seem to be the case with tomato spotted wilt virus - it looks as though tillage effects do help, even in continuous peanuts.”

Several of the new cultivars, he adds, may provide additional suppression of tomato spotted wilt virus and help reduce losses. Some of them also may help reduce the need for fungicides for leafspot control.

“A greater resistance to spotted wilt in the new cultivars may allow us more flexibility with regards to planting date.”

A new risk index for fungal diseases of peanut developed by Extension Plant Pathologist Bob Kemerait should help in the decision process with regard to managing several of these diseases, says Culbreath. It is patterned after the spotted wilt index and should compliment it in an integrated disease management system.

e-mail: [email protected]

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