Multiplex Quantitative PCR Assay for Detection of Spinach and Lettuce Downy Mildews Using Spore Trapping

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Downy mildews are major constraints on spinach and lettuce production globally. Disease management can be achieved with fungicides, but routine applications are costly and can lead to pathogen resistance. Detection of airborne spores could guide sustainable fungicide application, and, considering that spinach and lettuce are often grown in the same cycles, simultaneous detection of both pathogens is practical. Here, a multiplex hydrolysis probe quantitative PCR assay was designed using single copy mitochondrial DNA targets for spinach downy mildew ( Peronospora effusa) and lettuce downy mildew ( Bremia lactucae). To quantify P. effusa and B. lactucae sporangia, a standard curve was developed for each pathogen using the multiplex qPCR to amplify DNA obtained from known dilutions of sporangia. Analysis of these curves revealed a greater sensitivity for B. lactucae, indicating that the sporangia of B. lactucae may harbor more mitochondria than those of P. effusa, providing insight into the biology of these pathogens. The multiplex qPCR assay was partnered with two different spore trap types: a cyclone spore trap and an impaction spore trap. Results from air sampling revealed that the cyclone spore traps collect significantly more sporangia compared with impaction traps. Exposure of P. effusa sporangia to desiccation was performed to assess the environmental impact on the assay, and although detection levels were reduced, they were still apparent. This detection and quantification tool will be useful in efforts to improve the accuracy of downy mildew forecasting, which in return may reduce fungicide usage or improve its efficiency. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 “No Rights Reserved” license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2024.