Tomato spotted wilt virus returned with a vengeance to some Georgia peanut fields this past year, following steady declines in the incidence of the disease since 1997.
“We had seen a steady decline in tomato spotted wilt virus since 1997, which was our worse year on record,” says Steve L. Brown, University of Georgia Extension entomologist. “Losses to the disease were down to about 1 percent in 2001, but they jumped to about 4 percent this past year. The disease was severe in some fields in 2002, but the virus wasn't as widespread as in 1996 and 1997. Almost every field was impacted severely in those years.”
This past year, some peanut fields were “horrible,” says Brown, while others had very little tomato spotted wilt pressure.
Georgia peanut growers have reduced their risk to tomato spotted wilt virus in recent years by following the recommendations of the University of Georgia risk index. The index, first developed in 1996, combines what is known about individual risk factors into a comprehensive but simple estimate of tomato spotted wilt risk for a given field. It assigns a relative importance to each factor so that an overall level of risk can be estimated.
“In an effort to insure that the index works in a year like 2002, we surveyed 60 fields in 17 Georgia counties. We gathered information from the field and plotted it out on the index. We're convinced the index still is working — that all components are working as we expect — but we did make adjustments for 2003,” says Brown.
One of the first changes, he says, is in planting date. “Planting dates seemed to be the dominant factor in 2002. In past years, variety has been the dominant factor in determining the severity of tomato spotted wilt. But every grower basically is planting at least a moderately resistant variety. No high-risk varieties are being planted in Georgia,” notes the entomologist.
Georgia Green continues to be the predominant variety planted in the state, with other resistant varieties such as C-99R and AT-201 also being planted to some degree.
“We still consider variety to be a key factor, but there wasn't a lot of difference in the incidence of tomato spotted wilt in the varieties we saw this past year. But there was a huge planting date effect. The later planted peanuts did much better than the earlier planted peanuts.
“We've always had a planting window — a ‘magic’ time when we thought we could plant and avoid most of the pressure from tomato spotted wilt. If you planted earlier or later than this window, you had more risk.”
Researchers have seen, in the past three years, that this window has continued to slide to later in the season. “We don't know why that has happened. But whatever the reason, the window of reduced risk is getting later, and that was very evident this past year. We never saw an increase in the disease on the tail-end of the window, as we usually do.
“In our data, the later planted the better for reducing your risk. So, we've revised the index. May 1-20 was our planting window with the lowest risk. Now, it's May 11-25. We want everyone to understand that we can't plant 1/2 million acres of peanuts in Georgia in that narrow time period. Many growers won't be able to hit that window, especially those with large acreage. We're just trying to give you an indication of what to expect. If you can't hit that window, the other factors in the index become even more important.”
Brown says he has a personal opinion as to why the planting window has moved to later in the season. “As growers started adopting the risk index, everyone started shifting their planting dates to later in the season. Our planting window appears to have shifted with us. So, we may have caused this to happen. Which leads to more speculation that the calendar date isn't necessarily so important — it's when you plant in relation to those growers around you. But that's only my speculation. We don't know for sure what's causing this.”
Every year can and will be different, says Brown, and changes are not made in the risk index based on only one year of data or experience. The planting date change, he adds, is in response to tendencies noted in the past several years.
Also new to the index this year are several new peanut varieties, he continues. “These varieties have been in small plot tests for a couple of years now, and we felt comfortable assigning them an index value.”
The variety DP-1 is in an entirely new category with the lowest possible risk, he says. Also added are Hull, GA02C, Norden and Andru II.
Another index change, he says, is in plant population. “We've always said that plant population is critical, and that skippy stands are conducive to a higher incidence of tomato spotted wilt.”
A category also was added in the 2003 index for the use of Classic herbicide, says Brown. “It appears that the use of Classic sometimes can make tomato spotted wilt worse. Our weed scientist has looked at it. He saw this effect in some tests, and he didn't see it in others. We don't think it's a big factor, and we don't think it happens always. But we've seen it enough to justify putting it in as a risk factor. It's the smallest factor in the index at five points.”
Looking to the future, Brown says resistant varieties will continue to help growers fight tomato spotted wilt virus. “There are several varieties in the pipeline, and they have some great characteristics. Our breeding programs really are paying dividends.”
There's no single answer to why tomato spotted wilt pressure was higher this past year, he says. “Even when statewide pressure has been low, we've seen tremendous pressure in our test plots where we did everything to encourage the disease. I think quite a few growers might have forgotten about the index this past year and allowed their risk levels to drift higher. We saw more April-planted peanuts in 2002 than we saw in the previous two to three years.