P-386 - Fungicide sensitivity and pathogenicity of stem blight associated fungi from blueberries in Michigan

Stem blight poses a threat to highbush blueberries (Vaccinium corymbosum), resulting in damage of stems in otherwise healthy plants, causing losses in current and future harvests. Various fungal pathogens, including Diaporthe vaccinii (anamorph Phomopsis vaccinii), Pestalotiopsis spp., and Neopestalotiopsis spp., are known to contribute to stem blights in blueberries. A survey of blighted stems was conducted across southwest Michigan blueberry farms in 2021 and 2022 and D. vaccinii (n=42), D. eres (n=4), Pestalotiopsis spp. (n=357), and Neopestalotiopsis spp (n=22) were recovered. The efficacy of four fungicides (azoxystrobin, propiconazole, fenbuconazole, and mefentrifluconazole) was evaluated on the collected fungal isolates using fungicide amended media to determine the EC₅₀ value for the respective species. D. vaccinii and D. eres were most sensitive to all fungicides tested whereas Neopestalotiopsis spp. and Pestalotiopsis spp., were insensitive to the fungicides tested. Mefentrifluconazole was the most effective against Diaporthe spp., while propiconazole displayed higher efficacy in reducing growth across all species. Furthermore, the study explored the pathogenicity of fungal isolates on 'Blueray' cultivar plants in a controlled greenhouse environment. Notably, D. vaccinii induced shoot death within 3-4 weeks post-inoculation, while blight symptoms were not observed with Neopestalotiopsis spp. or Pestalotiopsis spp. inoculations. Understanding the predominant disease causing fungi and their sensitivity to fungicides could help management of this challenging disease.