Results of the 2010 On-Farm Cucurbit Powdery Mildew Fungicide Resistance Trial

Bob Mulrooney, Extension Plant Pathologist;

In 2010 a multi-state project including DE, PA, MD, VA, NY and NJ was undertaken to determine the extent of resistance to fungicides for powdery mildew control in cucurbits. Plants of a susceptible pumpkin variety were grown to the 2-3 leaf stage, sprayed with different fungicides at varying rates and placed in commercial cucurbit fields with powdery mildew for 1-2 days. The plants were collected and allowed to continue to grow and develop symptoms in a greenhouse, then evaluated for the amount of powdery mildew on the leaves compared to the untreated plants exposed at the same time. The results from the two fields in DE showed that at least for these two fields that there is resistance in the powdery mildew populations to Topsin M, Flint, Endura, and Rally. The resistance to Rally at the high rate was very low indicating that some control would be expected at the highest label rate of Rally. No resistance was detected to Quintec, either in DE or any other state. I did not include Inspire or Folicur. No resistance was found to Inspire, but NY and PA did see some resistance to tebuconazole (Folicur). Cucurbit growers in DE and MD should not expect to see control of powdery mildew from thiophanate-methyl (TopsinM), or a stand-alone strobilurin fungicide like Flint, Quadris or Cabrio. The results of this trial indicate that the high rate of Pristine (Endura plus Cabrio) may still provide some control but there is resistance to the boscalid (Endura) component occurring in the region.