The biggest change in the 2005 University of Georgia Tomato Spotted Virus Wilt Index for Peanuts is that it will be combined for the first time this year with the university's risk index for fungal diseases of peanuts.
“As usual, we made a couple of changes in peanut varieties, including adding varieties and re-assigning points on some varieties,” says Steve L. Brown, University of Georgia Extension entomologist. “But the major thing we're doing this year is combining the tomato spotted wilt virus index with the fungal disease index.
“We thought combining the two indexes would be a great benefit to growers. Some of the factors are the same in both indexes. The final point tally in the combined index will indicate your relative risk to tomato spotted wilt virus, white mold or leafspot, and rhizoctonia limb rot.”
The tomato spotted virus risk 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.
The fungal disease index, modeled after the tomato spotted wilt risk index, assigns points to each production variable in the categories of leafspot diseases, white mold and rhizoctonia limb rot. Growers then can evaluate the potential risk in a given field and use this information in preparing for disease in a coming season as “low,” “medium” or “high.”
As for the tomato spotted wilt portion of the combined risk index, Brown says changes for 2005 will be limited to peanut varieties. “We're moving most of the varieties formerly in the 20-point category into the 25-point group. These include Georgia Green, Southern Runner ViruGard, Gregory and VC-2,” he says.
The change, says Brown isn't an indication that the varieties have become more susceptible to tomato spotted wilt virus. “As new varieties are introduced, we're trying to move them around to reflect their relative susceptibility. We have varieties coming in that are so low that they require a 10-point category. And, in the future, we might have a five-point category.
“We're certainly not saying that Georgia Green is any more susceptible or that it is losing its resistance to the virus. It's our position that Georgia Green's resistance is just as good now as when it was first introduced. We're just re-positioning everything on the scale,” he says.
The other two peanut varieties in the 20-point category in 2004 — ANorden and Andru II will stay at 20, says Brown. Two varieties — Georgia O2C and Georgia 01R — will be moved from the 15-point category to 10 points. In addition, Georgia 03L will be added at 15 points.
Although it isn't yet official, a new variety — expected to be named “Tift Runner” — could be added to the 10-point category. Final changes to the variety list include the removal of Florunner and Georgia Hi-OL. “We felt it was no longer necessary to keep those two varieties on the list.”
The development of new, resistant varieties continues to be a major weapon in the battle against tomato spotted wilt virus, says Brown. “A variety in the 10 or 15-point category could give us a lot of flexibility in using some of these other index factors. For example, we might not have to depend on planting date so much with some of these new varieties. But, we still don't know how well they will be accepted agronomically because of the length of their growing season, yield potential, or other characteristics.”
Brown says he and the other researchers involved in revising the index saw no need to change the planting date category, which currently shows the ideal planting window at May 11-25. Prior to May 1 is assigned 30 points, May 1-10 is assigned 15 points, May 11-25 gets five points, May 26-June 5 gets 10 points, and after June 5 is assigned 20 points.
As with other categories in the risk index, the higher point totals indicate a higher risk to the virus.
“We see a trend continuing that the tail-end of the planting window has not had as much risk as we've assigned to it in the past couple of years. But we're not ready yet to change that. The planting date effects change so much every year anyway, and we want to see more on that before we make a change to the index.”
This past year was the worst for tomato spotted wilt virus in peanuts in the last three to four years, says Brown. “The virus was severe in some fields. The planting date seemed to be a strong factor in 2004. Early planted peanuts definitely had more virus than the later-planted peanuts.
“Anything planted in April or the first week in May tended to have a lot of virus, and we saw some fairly substantial yield losses in those fields. Luckily, we planted a lot of our crop after that time period.”
Brown says no one is sure why the virus was more severe in 2004 than in past years. “There are theories. Some people speculate that maybe we're settling into an every-other-year cycle because 2002 was bad while the virus was practically non-existent in 2003 and it's back again in 2004. But there's no scientific basis for that assumption.”
Researchers believe that weather may possibly have some effect on the severity of the virus, as it is transmitted by thrips from plant to plant throughout the winter season, and as it over-winters on different weed species.
“Our plant pathology department has conducted a survey in recent years in which they've looked at winter weed populations in February and March in an attempt to determine the level of tomato spotted wilt virus in some of these wild hosts. We're not sure if this always will be the case. But in the last few years, it has been a good indicator of what kind of virus year we're going to experience.
“We found of lot of the virus in the winter weeds in 2002 and 2004, and we didn't find it in 2003. We'll continue to look at this to see how well the theory holds up from year to year. Potentially, this would give us an early warning system. And, if we developed enough confidence in it, we could tell growers if the situation for tomato spotted wilt virus is going to be bad or good. Subsequently, growers would have some indication if they needed to follow the index closely that year and try to get their point total down to the lowest possible, or if they could back off and do some other things.”