Clade-specific monitoring of airborne Pseudoperonospora spp. sporangia using mitochondrial DNA markers for disease management of cucurbit downy mildew

Authors: BELLO, JULIAN - Michigan State University; HIGGINS, DOUGLAS - Michigan State University; SAKALIDIS, MONIQUE - Michigan State University; QUESADA-OCAMPO, LINA - North Carolina State University; Martin, Frank; HAUSBECK, MARY - Michigan State University

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/17/2022
Publication Date: 9/26/2022
Citation: Bello, J.C., Higgins, D.S., Sakalidis, M.L., Quesada-Ocampo, L.M., Martin, F.N., Hausbeck, M.K. 2022. Clade-specific monitoring of airborne Pseudoperonospora spp. sporangia using mitochondrial DNA markers for disease management of cucurbit downy mildew. Phytopathology. 112(10):2110-2125. https://doi.org/10.1094/PHYTO-12-21-0500-R.
DOI: https://doi.org/10.1094/PHYTO-12-21-0500-R

Interpretive Summary: Cucurbit downy mildew can cause significant losses in the field if control measures are not followed. This manuscript describes the use of a molecular assay to quantify the amount of spores in the air in an effort to identify when fungicide spray programs should be initiated. The goal is to reduce the number of sprays while effectively controlling the outbreak of disease by timing sprays only when the pathogen is present.

Technical Abstract: Current management of Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew (CDM), relies on an intensive fungicide program. In Michigan, CDM occurs annually due to an influx of airborne sporangia; timely alerts of airborne inoculum can assist growers in assessing the need to initiate fungicide sprays. The main objective of our research was to improve the detection and quantification of airborne concentrations of P. cubensis sporangia by adapting two qPCR-based assays to distinguish between P. cubensis clade I and II and P. humuli in spore trap samples. We also aim to evaluate the efficiency of Burkard and impaction spore traps for the detection airborne concentrations of P. cubensis sporangia. A new qPCR assay improved the specificity of P. cubensis detection and resulted in a better linear correlation between the number of sporangia observed using light microscopy and Cq values obtained from Burkard spore traps (R2=0.6; p = 0.01). After two years of monitoring, P. cubensis clade II and P. humuli DNA were detected in air samples collected in commercial cucumber fields, while P. cubensis clade I DNA was not detected. P. cubensis clade II DNA was detected in spore trap samples >2 days before CDM symptoms were first observed in cucumber fields (August), while P. humuli DNA was only detected early in the growing season (May and June). P. cubensis clade I DNA was not detected in air samples before or after the disease onset in cucumber fields. Additionally, the probability for P. cubensis detection in Burkard spore trap samples was higher compared to impaction spore trap samples with approximately the same number of sporangia, suggesting that the efficiency of recovery of sporangia by Burkard spore traps exceeds the recovery of impaction spore traps. Our study identified an improved methodology to monitor the airborne concentrations of Pseudoperonospora spp. sporangia using spore traps coupled with qPCR. This methodology could be used as part of a CDM risk advisory system to time fungicide applications that protect cucurbit crops in Michigan.