P-288 - Biorational Fungicides for Sustainable Management of Bitter Rot in Virginia Apples: Reducing Reliance on High-Risk Synthetic Fungicides

Apple bitter rot, caused by Colletotrichum spp., threatens 20% to 90% of fruit yield in Virginia orchards. With U.S. apple production worth over $3 billion annually and Colletotrichum spp. at risk of developing resistance to widely used FRAC 11 fungicides (strobilurin or Quinone outside inhibitor), there is an urgent need for effective alternatives. In 2023 and 2024, we evaluated eight new biorational and synthetic fungicides, Nutriphite Magnum, Kendal, Theia, Howler, Brexil Calcium, Switch, Aprovia and Intuity, on ‘Idared’ (ID) and ‘Golden Delicious’ (GD) apples. Applications were made every 14 days or after 50 mm of rainfall, whichever came first (with an additional two Howler and Theia treatments using 7-day spray intervals in 2024). In 2023, Nutriphite Magnum and Kendal reduced bitter rot incidence on GD to 3% and 6%, respectively. On ID, Kendal allowed 25% of bitter rot, with Aprovia providing the lowest incidence at 5%. In 2024, reducing the spray interval to 7 days for Howler and Theia reduced incidence on ID to 20% and 21%, respectively, compared to 43% and 42% in their 14-day applications. On GD, we had a lower incidence overall, with Nutriphite Magnum, Theia (7-day), and Intuity reducing incidence to as low as 1%. New synthetic fungicides Aprovia, Switch and Intuity demonstrated consistent efficacy over two years. Our findings highlight biorational fungicides as effective, environmentally friendly tools for managing bitter rot and reducing dependence on high-risk chemistries in Mid-Atlantic apple production.