Fungicide sensitivity of cucurbit powdery mildew pathogen populations on Long Island, 2018

摘要

Fungicide resistance can be a major constraint to effectively managing powdery mildew in cucurbit crops. Fungicides that are most effective for managing powdery mildew (because they are mobile and thus can redistribute from the deposition site on upper leaf surfaces to the lower surfaces where powdery mildew develops best) are also more prone to the pathogen developing resistance (because they typically have a single-site mode of action). In this study a seedling bioassay was used to obtain site-specific information about resistance in cucurbit powdery mildew pathogen populations. Bioassays were conducted in commercial and research plantings during the growing season. Pumpkin seeds were sown in 48-cell trays. Seedlings at about the cotyledon stage were transferred individually to 4-in. pots. At approximately the 3-leaf stage, the growing tip with unexpanded leaves was removed, and then plants were sprayed to coverage with a fungicide dose. Applications were made with a backpack sprayer using a Twin-jet nozzle delivering 50 gal/A operated at 55 psi. The following morning the seedlings were organized into replications each with one plant of each treatment plus two untreated control plants. They were placed next to field-grown plants naturally infected by Podosphaera xanthii, with up to four replications in a location. Seedlings remained there for the rest of the day (5-8 hours) to be exposed to spores dispersed by wind, then the seedlings were returned to the greenhouse until symptoms developed. Seedlings regularly received water with 20-20-20 fertilizer applied to the top of the pot so leaves stayed dry and any new growth was removed. There was no supplemental lighting. Severity of powdery mildew was assessed as percent coverage with symptoms onthe upper surface of each leaf. The first four bioassays were conducted during Jul and Aug in commercial summer squash plantings not treated with targeted fungicides for powdery mildew and in non-treated and treated cucurbit experiments at LIHREC. Thesebioassays provided interesting information, but data was not suitable for statistical analysis because severity was low in some replications reflecting 1 day being too short field exposure where inoculum level was low or greenhouse temperature being toohigh where some replications were kept. The last two bioassays were conducted in cucurbit experiments at LIHREC. In bioassay 5, treated on 12 Sep, the six replications of plants were split into two groups, four replications were placed in an experiment that was only sprayed with organic fungicides (Location A, acorn squash) and the remaining two replications were placed in an experiment that had been sprayed with a rotation of Vivando, Procure, and Quintec (Location B, winter squashes). In bioassay 6, which was conducted with seedlings at the cotyledon stage treated on 25 Sep, the five replications were similarly split, the first three replications were placed in plots of a pumpkin experiment (PDMR 13:V119) that had not been treated with any fungicides(Location C) and the remaining two replications were placed in plots in the same experiment that had been treated with a rotation of Vivando, Procure, and Quintec (Location D). Data were analyzed with one-way analysis of variance (ANOVA) and Tukey's honest significance test (HSD) to separate means using JMP statistical software.