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Poster: Epidemiology: Risk Assessment


Seedborne inoculum thresholds of Pseudomonas syringae pv. aptata, causal agent of bacterial leaf spot, in ‘baby leaf’ Swiss chard crops
L. DU TOIT (1), M. Derie (1), B. Holmes (1), I. Safni (2), C. Bull (2) (1) Washington State University, U.S.A.; (2) Pennsylvania State University, U.S.A.

Most US table beet and Swiss chard seed production (~90%) occurs in western Washington and Oregon. Pseudomonas syringae pv. aptata (Psa), the cause of bacterial leaf spot, has been identified in beet and chard seed from this region, but seedborne inoculum thresholds have not been established. Chard seed naturally infested with Psa at 8.6 x 103 CFU/g was mixed with seed of the same cultivar infested at <10 CFU/g to generate a log10 series of infestation. Five replicate plots/infestation level were planted in a field in Mount Vernon, WA at ~10.7 x 106 seeds/ha in May 2015 in a randomized complete block design. Bacterial leaf spot incidence was 52 ± 18% in plots planted with seed infested at 8.6 x 103 CFU/g vs. 15 to 21% in plots sown with seed infested at <103 CFU/g. Seed infestation level did not affect plant stand or amount of Psa recovered from leaves onto KBC medium. The trial was repeated in the fall, when moist, cool conditions resulted in a leaf spot incidence of 93 ± 2% across all plots, and Psa was recovered from every plot (9.97 x 1011 ± 1.82 x 1011 CFU/g dry leaf tissue), with no differences among seed infestation levels. Plots sown with seed infested at 8.6 x 103 CFU/g had 33% fewer plants, 29% less leaf dry weight, and 126% more severe leaf spot than plots planted with seed infested at <103 CFU/g. Psa was transmitted from infested chard seed, even at <10 CFU/g. Environmental conditions in the two trials affected seed transmission and leaf spot severity.