Assessment of field resistance to pyraclostrobin- and mefenoxam-based fungicides for managing downy mildew in cucumber, 2007

摘要

The main goal of this study was to determine efficacy of FRAC code 4 and 11 fungicides for downy mildew control because of concern that resistance had developed in the pathogen to these chemistries. Efficacy was determined by applying Ridomil Gold ECandCabrio alone weekly, and comparing these two treatments with a nontreated control and a currently recommended fungicide program. Cucumber was direct-seeded on 11 Jul in a field with Haven loam soil at the Long Island Horticultural Research and ExtensionCenter inRiverhead. A late planting date was used to increase the likelihood of downy mildew developing during the experiment. Inoculum often has arrived late in the summer growing season in this area, especially before 2004 when the current dominant pathogen strain was not known. Weeds were controlled by applying Strategy (2 pt/A) over the seeded rows on 11 Jul, and by cultivating and hand weeding as needed. Cucumber beetles were managed with insecticides, applying Admire 2 F (20 fl oz/A) over the planted rowswith Strategy on 11Jul and Asana XL (9.6 fl oz/A) as a foliar treatment on 17 Aug. Plots consisted of three 15-ft rows spaced 68 in apart with plants at approximately 24-in spacing. The plots were 15 ft apart in the row. A randomized complete block design with four replications was used.Fungicides were applied weekly for 7 weeks beginning on 9 Aug, before symptoms were seen, using a tractor-mounted boom sprayer equipped with D5-25 hollow cone nozzles spaced 17 in. apart that delivered 86 gal/A at 100 psi. Downy mildew severity was assessedweekly beginning on 30 Aug by rating severity on 12 leaves. Leaves with downy mildew symptoms from plants in an adjacent planting that had not been treated with fungicides active against downy mildew were collected on 11 Sep and sent to North Carolina StateUniversity for the pathogen to be tested for fungicide resistance. Marketable fruit and culls were harvested weekly from 30 Aug to 27 Sep. Average monthly high and low temperatures (oF) were 82/66 in Jul, 82/65 in Aug, and 77/60 in Sep. Rainfall (in.) was 3.63, 2.60, and 1.51 for these months, respectively. Overhead irrigation was used as needed to supplement rainfall.