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Title: Spatiotemporal patterns in the airborne dispersal of spinach downy mildew

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: 7/25/2016
Publication Date: 1/1/2017
Citation: Choudhury, R.A., Koike, S.T., Fox, A., Anchieta, A.G., Subbarao, K.V., Klosterman, S.J., McRoberts, N. 2017. Spatiotemporal patterns in the airborne dispersal of spinach downy mildew. Phytopathology. 107(1):50-58.

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 a field plot over two winter periods in the Salinas Valley, California. Early pathogen detection can be used to time cost-effective pesticide applications for downy mildew disease management. We detected the windborne pathogen at low levels prior to disease occurrence on spinach leaves in the field plot in both years of the study, in both winter seasons of 2013-2014 and 2014-2015. Thus there are low levels of the pathogen present typically throughout the valley that complicate the usefulness of simple presence or absence reporting. However, at the Salinas plot in both the 2013-2104 and 2014-2015 winter seasons, the increase in disease incidence was reflected in the increasing levels of P. effusa DNA detectable on the spore traps, providing the potential opportunity to exploit trap and DNA based detection at the field or ranch level for an early downy mildew disease warning system.

Technical Abstract: Downy mildew, caused by the biotrophic oomycete pathogen, Peronospora effusa, is the most devastating disease of spinach that threatens sustainable production. The disease results in yellow lesions that render leaves unmarketable as the high value fresh produce. In this study, the levels of DNA from airborne P. effusa spores of P. effusa were assessed near a susceptible field in Salinas, CA during the winter months of 2013/14 and 2014/15 using impaction spore trap samplers coupled with quantitative PCR (qPCR). Low levels of P. effusa DNA were detectable from December through February in both seasons, but increased during the January in both years, in correlation with observed disease incidence; sharp peaks in P. effusa DNA detection were associated with the onset of disease incidence. Disease incidence ratings in the susceptible field suggested that spinach downy mildew displays logistic dynamics but with considerable inter-season variation; the epidemic in 2014 was more severe than in 2015. Spatial analyses indicated that disease incidence is spatially dependent within an average range of 5.9 m, approximately equivalent to the width of three planting beds in a typical production field. The spatial distribution of spores captured during an active epidemic could be fit with either the power-law or exponential distributions. These findings revealed the utility of impaction spore trap samplers linked with a qPCR assay for indicating periods of high disease risk, long distance dispersal of P. effusa spores, and the spatial aggregation of disease incidence.