By Drs. Tim Brenneman and Bob Kemerait
We have received many questions in recent days pertaining to the quality issues facing our peanut seed supply in 2020 and relative performance of seed treatments Dynasty PD and Rancona. Dynasty has been our main seed treatment for years, but Rancona has always looked good in trials. As a quick review, Rancona and Dynasty PD both contain several fungicides; however they do not contain the same fungicides. Active ingredients and chemical classes of each are as follows:
There are many different pathogens that can attack peanut seed and seedlings. Some are seedborne, and some are soilborne. No single fungicide covers that wide spectrum, so combinations of active ingredients are needed. The Group 4’s are strictly for Oomycetes, the primary one being Pythium. While not a common problem, it can cause issues in very wet soils. The other fungicides are broader spectrum, and control the remaining pathogens such as Aspergillus, Rhizoctonia, Rhizopus, Lasiodiplodia, etc. Of these, the primary problem in recent years has been Aspergillus niger. Aspergillus species are well adapted to the hot, dry conditions often present late in the peanut season. UGA grad student Brian Jordan’s has also shown that the mutation for high level resistance to QoI’s (Group 11) can occur in A. niger from peanut seed. This supports the clearly reduced efficacy of azoxystrobin (Abound) in furrow we have seen in recent years, and to a lesser degree with Dynasty seed treatment (although fludioxinil helps moderate that effect). This year we are seeing a much higher incidence of Aspergillus flavus in seed. This is in part due to the extreme heat and drought last August and September. One result is the higher levels of aflatoxin in our stored peanuts. However, A. flavus is also a seed pathogen and is contributing to the lower germinations found in some seed lots this spring. Seedsmen noticed that in some seed lots Rancona was resulting in higher germinations than obtained with Dynasty. We have plated numerous seed lots and verified high levels of A. flavus, as well as damaging levels of A. niger and Rhizopus. All three of these are much more abundant on seed treated with Dynasty. Working with Dr. Ali in the Molecular Diagnostic Lab, we have verified even higher levels of QoI resistance in these A. flavus isolates than found previously in A. niger. This helps explain the differences seen in the seed treatments. We are conducting grow out trials in field soil in the greenhouse to see what this may mean for growers. So far the resulting plant stands are similar to the germination data, ie. with some seed lots the Rancona is clearly better than Dynasty, but with others there is little difference.The stressful conditions last year resulted in seed that is potentially weaker and is carrying a heavier pathogen load than normal. Effective seed treatment will be critical to getting good stands with this seed. The bottom line is that much of our seed will be treated with Rancona this year, but they do not have enough to treat it all. The use of in furrow fungicides will be the best way to bridge the gap and help insure good stand establishment. This will be especially important where Dynasty is used. Velum Total has been effective on Aspergillus. If it is being used for nematodes, it should provide a good level of control for seedling diseases. A cheaper option would be Proline if nematodes are not an issue. These will both give some early season control of soilborne diseases and leaf spot as well. These could also be used with Rancona-treated seed for an additional level of protection. Of course anything growers can do to provide the best possible conditions for seed to germinate and grow will help minimize seed and seedling disease issues.