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Poster: Biology & Disease Mgmt: Chemical Control


Podosphaera xanthii strains with resistance and reduced sensitivity to several fungicides may challenge control of cucurbit powdery mildew
M. McGrath (1) (1) Cornell University, U.S.A.

Fungicides at risk for resistance development are important for effectively managing powdery mildew (PM) in cucurbit crops because they can move to lower leaf surfaces where the pathogen develops best. Isolates were collected from research and commercial plantings on Long Island, NY, on 16 Sep to 1 Oct 2015, which was after several fungicide applications for PM. A leaf disk bioassay was used to determine fungicide sensitivity. All but 1 of the 57 isolates tested were determined to be resistant to QoI fungicides (FRAC 11) and 79% of the isolates were resistant to boscalid, an SDHI fungicide (FRAC 7). Many grew on leaf disks treated with 500 ppm boscalid as well as on untreated disks. Isolates with very limited or no growth on 500 ppm boscalid disks tended to also be slightly tolerant or sensitive to 40 ppm myclobutanil (FRAC 3) and 40 ppm quinoxyfen (FRAC 13); most of these were collected from research plants at LIHREC not treated with any at-risk fungicides (organically-managed and non-treated plots in a fungicide evaluation). Many boscalid-resistant isolates also grew well on disks with 40 ppm myclobutanil and on disks with 40 ppm quinoxyfen. Some of these exhibit tolerance to higher doses. These results document strains of Podosphaera xanthii with resistance and reduced sensitivity to multiple fungicide chemistries can be selected during one growing season, including where not exposed to all 3 chemistries. Such strains could foil alternation fungicide programs.