Venturia inaequalis trapped: molecular quantification of airborne inoculum using volumetric and rotating arm samplers
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Site-specific high throughput monitoring of airborne ascospores of Venturia inaequalis, the causal agent of apple scab, can improve existing warning systems. A new qPCR assay was developed to quantify ascospores collected by a simple rotating-arm spore sampler. The qPCR assay was highly specific and sensitive, with a limit of quantification of 20 ascospores per sample. The new detection system was compared to sampling with a traditional Burkard volumetric spore trap and to microscopic quantification. During controlled ascospore release experiments in a closed environment, strong correlations (ρ: 0.96 to 0.99) were observed between the two types of samplers and the two methods of quantification but significantly larger numbers of spores (log difference: 0.43 to 0.69) were obtained when using the rotating-arm sampler and when using molecular quantification. During comparisons under outdoor conditions over a three-year period, reasonable correlations between the techniques (average ρ = 0.61) were observed. When rotating-arm samplers operate continuously they can get saturated but their counts still correlated better with those from the Burkard sampler than when they only operate during rain and until two hours after. This suggests that ascospores were also captured outside of rain events. Based on these comparisons, molecular quantification of spores captured with the rotating-arm sampler appears to be a sensitive and reliable method to determine airborne ascospores of V. inaequalis and holds promise as a tool to guide targeted fungicide applications in commercial orchards as well as to increase our knowledge of the aerobiology of this pathogen.
KeywordsqPCR Rotating-arm spore trap Burkard spore trap Aerial spore sampling Apple scab
This research was funded by Flanders Innovation & Entrepreneurship (VLAIO) with co-funding from industry and growers. The authors thank Valerie Caffier for providing the isolate of Venturia asperata and Amelie Grammen for providing isolates of Colletotrichum acutatum, Neofabraea sp. and Nigrospora sp. We also thank Fran Focquet and Thomas Goedefroit for their technical help and Miriam Levenson for English language editing.
This study was funded by grant “LA Traject 135078” from Flanders Innovation & Entrepreneurship (VLAIO), which includes co-funding from industry and growers.
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Conflict of interest
The authors declare that they have no conflict of interest.
Research involving human participants and/or animals
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