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Title: Season-long dynamics of spinach downy mildew determined by spore trapping and disease

Author
item CHOUDHURY, ROBIN - University Of California
item KOIKE, STEVEN - University Of California - Cooperative Extension Service
item FOX, ALAN - Fox Weather, Llc
item Anchieta, Amy
item SUBBARAO, KRISHNA - University Of California
item Klosterman, Steven
item MCROBERTS, NEIL - University Of California

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/12/2016
Publication Date: 9/20/2016
Citation: Choudhury, R.A., Koike, S.T., Fox, A.D., Anchieta, A.G., Subbarao, K.V., Klosterman, S.J., McRoberts, N. 2016. Season-long dynamics of spinach downy mildew determined by spore trapping and disease. Phytopathology. 106:1311-1318.

Interpretive Summary: The spinach downy mildew pathogen, Peronospora effusa, causes spinach leaf lesions and discolorations that render the fresh product unmarketable. This study was undertaken to examine the relationship between the levels of detectable spinach downy mildew pathogen, P. effusa, and levels of actual downy mildew disease in fields to investigate whether early pathogen detection can be used to time cost-effective pesticide applications for downy mildew disease management. We detected the windborne pathogen at a low level at four sites dispersed throughout Salinas Valley in both years of the study, in 2013 and 2014. The detection was even recorded prior to the major increases in spinach planting in March. Thus there are low levels of the pathogen present typically throughout the valley that complicate the usefulness of simple presence or absence reporting. However, increased pathogen detection was associated with increased disease incidence in commercial fields examined simultaneously. Also, some predictability of the pathogen detection was associated with weather parameters, such as wind speed and temperature. That is, increased air temperatures and wind speeds were correlated with increased P. effusa detection. Taken together, the results suggest that continual pathogen monitoring over the growing season enables some disease predictability, enabling growers to better time spray applications for disease management, as well as fungicide resistance (in the pathogen) management.

Technical Abstract: Peronospora effusa is an obligate oomycete pathogen, and the cause of downy mildew of spinach. Downy mildew threatens sustainable production of fresh market organic spinach in California, and routine fungicide sprays are often necessary for conventional production. In this study, airborne P. effusa spores were collected using rotating arm impaction spore trap samplers at four sites in the Salinas Valley between late-January to early-June in 2013 and 2014. Levels of P. effusa DNA were determined by the species-specific qPCR assay. P. effusa was detected prior to and during the growing season in both years. Nonlinear time series analyses on the data suggested that the within-season dynamics of P. effusa air-borne inoculum are characterized by a mixture of chaotic, deterministic, and stochastic features, with successive data points somewhat predictable from the previous values in the series. Analyses of concentrations of air-borne P. effusa suggest both an exponential increase in concentration over the course of the season and oscillations around the increasing average value that had season-specific periodicity around 30, 45, and 75 days, values that are close to whole multiples of the combined pathogen latent and infectious periods. Each unit increase in temperature was correlated with 1.7-6% increased odds of an increase in DNA copy numbers, while each unit decrease in wind speed was correlated with 4-12.7% increased odds of an increase in DNA copy numbers. Disease incidence was correlated with airborne P. effusa levels, and a receiver operating characteristic curve analysis suggested that P. effusa DNA copy numbers determined from the spore traps nine days prior to disease rating could predict disease incidence to some extent.