Detection of latent Peronospora effusa infections in spinach

Authors: SUBBARAO, CHAITRA - University Of Southern California; Anchieta, Amy; Ochoa, Lorena; DHAR, NIKHILESH - University Of California; KUNJETI, SRIDHARA - University Of California; SUBBARAO, KRISHNA - University Of California; Klosterman, Steven

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2018
Publication Date: 4/12/2018
Citation: Subbarao, C.S., Anchieta, A.G., Ochoa, L., Dhar, N., Kunjeti, S.G., Subbarao, K.V., Klosterman, S.J. 2018. Detection of latent Peronospora effusa infections in spinach. Plant Disease. https://doi.org/10.1094/PDIS-12-17-1956-RE.
DOI: https://doi.org/10.1094/PDIS-12-17-1956-RE

Interpretive Summary: Downy mildew disease of spinach, caused by the plant pathogenic microorganism Peronospora effusa, is a major disease constraint on spinach worldwide, and in the U.S. This microorganism produces asexual spores that are airborne, land on spinach leaves and infect the plant. The period between plant infection and symptom development is known as the latent period. This research focused on detection of the pathogen DNA before symptoms are visible. DNA was detected about one week before symptoms in three field plot experiments. Early detection of the pathogen in the leaves in the field will help to more effectively target fungicide applications prior to symptom development and thereby help to prevent downy mildew epidemics.

Technical Abstract: Downy mildew, caused by Peronospora effusa, is the most serious disease of spinach in central coastal California. The disease is managed in conventional production fields by a combination of host resistance and calendar-based fungicide applications, in which fungicides are applied to prevent downy mildew epidemics regardless of the infection status of spinach crops. A more streamlined approach would be to develop methods to target latent infections for fungicide application, before symptom appearance, to ensure judicious application when the pathogen is present, and prior to secondary inoculum spread throughout the remaining and nearby fields. In this study, conventional polymerase chain reaction (PCR) was applied to detect P. effusa in symptomless spinach leaves in three spatially and temporally separated field plots, each containing four 2 m beds by 35 m length, Spinach leaves were sampled at 3 m intervals at 48 locations throughout each plot. The plots were examined for disease incidence weekly, and weekly leaf samplings were continued for five to six. Initial samples were asymptomatic and yet PCR enabled detection of P. effusa in DNA extracted from spinach leaves as early as seven days prior to symptom development. Detection of latent downy mildew infection in spinach leaves was confirmed by PCR at three to four weeks after planting. Timely applications of fungicides or biopesticides on asymptomatic but infected spinach plants may prevent downy mildew epidemics in both conventional and organic production systems and thereby reduce the overall inoculum load in central coastal California.