Always unpredictable, spotted wilt disease of peanut in the southeast has ebbed and flowed over the years. For many years, the disease was moderate to severe at the North Florida Research and Education Center near Mariana, Florida. Then in 2010, the incidence decreased dramatically with only the most susceptible lines showing a few symptomatic plants even under conditions which favor disease development. Although it is unclear what 2016 holds for spotted wilt pressure, it is clear that thrips pressure has been extreme this spring and for the first time in years, we are observing diseased and stunted plants early in the season- by late May to early June.
Diseased plants such as those above in Figure 1 will remain stunted throughout the season and will produce few, if any, peanuts. Early stunting from spotted wilt is the most devastating symptom of spotted wilt in terms of yield loss, especially if incidence is high. So far, the incidence is relatively low (less than 10%) in the research plots at NFREC, Marianna, but the season is young and that could change quickly.¬† Figure 2 below shows a close-up view of the spots on leaves of peanut which are characteristic of spotted wilt disease.
Figure 2. Ring spots and other leaf symptoms of spotted wilt on peanut.
Unfortunately, there are no in-season control measures for spotted wilt.¬† Everything that can be done to reduce the risk of spotted wilt is done before, or at the time of planting.¬† Major factors to consider are variety choice, date of planting, seeding density, and in-furrow insecticide.¬† Fortunately, most of today‚Äôs cultivars have better resistance than Georgia Green which should reduce the risk of severe losses.¬† Similarly, we plant a majority of the peanut crop in the southeast during the latter half of May, which reduces risk.¬† Achieving a uniform and quick plant stand of at least four plants per foot of row is important to reduce risk.
In the past, there were two common in-furrow insecticides to control thrips- Temik and Thimet, but Temik has been withdrawn from the market. Choices for in-furrow insecticides are becoming more numerous and include CruiserMaxx seed treatment, Admire¬Æ Pro liquid (or Imidacloprid generics) in-furrow, and Thimet granular in-furrow. Of these three, only Thimet has been shown to control thrips AND reduce the risk of spotted wilt.
Spotted wilt may or may not be problematic in 2016, but it is still good for growers to scout their fields so they know how much spotted wilt is out there.¬† Just because we have been in a lull for the past several years is no reason to think that the disease can‚Äôt occur.¬† It is important to maintain vigilance against spotted wilt and do everything you can to reduce the risk of disease.