EFFICACY OF FUNGICIDE SEED TREATMENTS AND FOLIAR APPLICATIONS FOR MANAGEMENT OF BOTRYTIS ACLADA IN ONION BULB AND SEED CROPS IN WASHINGTON

Lindsey du Toit, Mike Derie, and Gary Pelter, Washington State University

To evaluate seed transmission of B. aclada (B. allii, cause of neck rot and scape/umbel blight of onion) in the semi-arid Columbia Basin of WA, seed lots of a cultivar with different incidences of seedborne infection were planted in commercial bulb crops in 2002, 2003, and 2004. Bulbs were harvested and placed in commercial storage facilities for >4 months. In 2002 and 2003, little neck rot developed regardless of the incidence of seedborne Botrytis. Bulbs from the 2004 trial will be assessed in February 2005. Insignificant Botrytis infection was detected on plants sampled in the field. Nine fungicide seed treatments were compared with nontreated seed for control of seedborne B. aclada in bulb trials in 2003 and 2004. In addition, the 2004 transmission and seed treatment trials were duplicated in NY to investigate the influence of environmental conditions on seed transmission. Seed health assays showed all 9 fungicides significantly reduced the incidence of Botrytis detected on the seed, with greatest control provided by Pristine WG, Rovral 4F, and Thiram 42-S, followed by Benlate 50WP, Maxim 4FS, and Protégé 100FS. In 2003, none of the plants sampled was infected, and the incidence of neck rot in storage was negligible for all treatments. In 2004, the incidence of plants infected with B. aclada averaged 98±4% in WA and 2.8±3.9% in NY, with no significant differences among seed treatments. Incidences of neck rot will be determined in 2005. The results suggest infected seed was not a significant source of inoculum in any year or at either site in 2004. The efficacy of fungicide applications for management of B. aclada in onion seed crops was evaluated in 2001/02, 2002/03, and 2004. Infection of umbels/seed by B. aclada was insignificant in all years, preventing detection of differences among treatments.

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