Temporal trends of <i>Botrytis</i> inoculum, and fungicide tolerance in Oregon and Washington vineyards

Grape Botrytis bunch rot, primarily caused by Botrytis spp., poses a significant risk to grape production in Oregon during years with high disease pressure. Current management practices involve both cultural methods and applications of fungicides. However, the emergence of Botrytis populations tolerant to multiple fungicide modes of action threatens the effectiveness of fungicides. Targeted management strategies require an understanding of inoculum presence and sources in Oregon vineyards, aiming to optimize efficacy while minimizing costs and reducing the development of fungicide-tolerant populations. Over a six-year period at eight vineyards in the Willamette Valley, qPCR analysis of airborne Botrytis cinerea inoculum collected from rotating impaction air samplers revealed consistent presence of Botrytis in the growing season, with peaks typically coinciding with bloom to berry touch. Sampling surveys indicated that the most likely source of inoculum was rachis debris from the preceding year, though potential sources also include adjacent wild Rubus, orchards or other crops within the region. Fungicide tolerance assays of collected isolates demonstrated that all tested modes of action had at least one tolerant isolate. Approximately 10% of isolates exhibited tolerance to multiple modes of action. A subset of isolates was sequenced for genes associated with modes of tolerance. One or more SNPs associated with alleles previously shown to be associated with fungicide tolerance were identified in 50% of sequenced isolates. This highlights the need for diversified and targeted management approaches and the importance of continuous monitoring of inoculum and fungicide tolerance to adapt strategies in response to evolving pathogen dynamics.