Defoliation and Digging
I had a question on the phone about peanut defoliation and digging. Generally, it takes 30 to 40% defoliation to begin reducing yield (well, this may be the case of defoliation and yield impacts that can be documented in small-plot research.) The grower this morning had peanuts in good shape late during the week of September 10 but then looked at peanuts this past Tuesday and noticed considerable defoliation due to late leaf spot. On the phone we ended up thinking it was less than 5% and was associated with spots and not general defoliation across the field. Disease generally always looks worse than it really is when we make the assessment visually (without some objective counts of incidence or severity.) He indicated that he needed to wait until early October to have peanut reach optimum maturity. He sprayed this week. In the conversation he indicated that he sprayed chlorothalonil about 2 weeks ago. The key issue was probably the spray about a month ago which was tebuconazole only. This is probably where the epidemic began as tebuconazole is weak on leaf spot due to resistance developed following many years of applying 3, 4, and 5 block sprays of Folicur. For next year, make sure you encourage growers to supplement tebuconazole with something that is effective against leaf spot. For this grower, if his “salvage” spray is ineffective he may have to dig early and this could impact yield and grade. Keeping vines protected gives us greater flexibility when deciding when to dig.
I have a data set with salvage treatments of chlorothalonil plus pyraclostribin applied about this time of year when different levels of leaf spot control were obtained. I don’t have that in front of me but it seems like the salvage treatment helped only minimally. I’ll try to find those data and send them.
Article first appeared as North Carolina Peanut Note (PNNC-2015-135)