The spread of tomato spotted wilt virus throughout the peanut producing areas of the United States has been a frustrating and devastating problem. The incidence of the virus in North Carolina reached serious levels in 2000 and was at almost-epidemic levels in 2001.
In 2002, we saw the disease in virtually every peanut field, with levels of 30 percent to 40 percent of the plants infected not uncommon. Thankfully, due to the efforts of researchers in Texas, Alabama and Georgia, the peanut farmers of the V-C region have a bit of an advantage.
We already have a reasonable idea of practices that might help growers minimize the impact of the virus in their fields. Practices such as variety selection, planting date, insecticide use, seeding rate and planting pattern, as well as tillage have all been demonstrated to be useful in reducing the level of virus in peanuts.
Over the past two years, extensive studies in North Carolina have looked at the impact of a number of cultural practices to help us respond to the increasing threat of this disease.
The next task is for us to assemble into a complete package, with appropriate values for how much each particular practice will reduce the level of the virus.
Our trials investigated the different seeding rates of two, four and five plants per row, single- and twin-row production, the use of Thimet (phorate) or Temik (aldicarb), and planting date. We have looked at variety selection for two years and the effect of tillage during one season.
In 2001, we saw thrips populations move into the field late, but still saw a lot of plants with typical virus symptoms. Then late in the season, we observed a lot of plants that suffered from yellowing and a general decline in the plant health. Sometimes the plant appeared to die late in the season. From analysis of the tap roots, we documented that 85 percent of these plants had the virus present in them despite showing no early symptoms.
During 2002, the thrips came in quite early and we saw a much higher number of plants with typical virus symptoms. The drought conditions made it very difficult to grow a good crop in 2002.
Late rains saved some crops, but unlike 2001, we saw very few plants turn yellow late in the season due to the virus.
In both years, plant population gave us consistent results. Twin-row production and higher seeding rates in single row production always had a lower incidence of virus. The number of plants in our plots with higher plant populations was always less, but when we looked at the actual percentage of plants infected, the difference was even greater.
Our studies on the use of Thimet (phorate) and Temik (aldicarb) indicated a trend for less virus in those peanuts receiving an at-plant application of phorate as compared to aldicarb. This difference was usually fairly consistent, but it was often quite small.
We compared three varieties: Perry, NC-V 11 and Gregory. We typically saw less virus in the Gregory and NC-V 11 and often the Gregory came out on top. The performance of the varieties did vary somewhat based upon the production system and the intensity of the virus. However, we can fairly confidently say that Gregory appears to be the most tolerant.
Our planting date studies have led to variable results. This is due primarily to the fact that thrips movement in the spring of 2001 was quite late and the movement in 2002 was quite early. The thrips movement in 2002 is more typical and I think we can safely assume that the earliest-planted peanuts are going to be at the greatest risk. The latest-planted peanuts may also be a little higher risk as well.
Thus far, we have only critically evaluated tillage approaches for one year. The trends we observed are exactly what we expected: The reduced-tillage production had less virus. This may be due to the fact that reduced-till peanuts are usually infested with fewer thrips.
The next task is to bring this information together into a complete package and assign appropriate values for how much each particular practice will reduce the level of virus.
When used in combination with other practices, one approach may cause a different response than when used with other practices. Therefore, we must consider each approach as part of a package. We hope to have a publication available this winter that will help provide guidance on which approach is best for you. This will be based not only on the work conducted in North Carolina, but comparing our results with the excellent studies and advice that has come from the program in Georgia.
We have every reason to believe that TSWV will be with us again this coming year. That means this winter you need to start developing a plan. Don't start thinking about TSWV management after you've planted the 2003 crop. Then, it will be too late.