Colletotrichum Species Causing Apple Bitter Rot in Virginia: Crop Losses, Distribution and Efficacy of Biorational and Synthetic Fungicides in Field and in vitro

Apple bitter rot losses in Virginia range from 20 to 90% annually. Distribution of Colletotrichum species in Virginia showed that the Colletotrichum gloeosporioides species complex dominated in the north and central regions, with 66% and 88% of isolates respectively. Colletotrichum acutatum species complex was more prevalent in the south with a 58% dominance shift in mountainous regions. To determine the efficacy of fungicides against apple bitter rot, we assessed 18 treatments of natural origin biorational materials and synthetic fungicides on 'Idared' (ID) and 'Golden Delicious' (GD) apple trees. Over two years, we applied 5 to 6 consecutive spray applications of 15 individual active ingredients from May to August. Bitter rot incidence was evaluated twice each summer. In the first, very rainy year, the top four synthetic fungicides, Omega, Aprovia, ferbam and a grower standard (GS) on ID, and Omega, Captan, Cabrio and Ferbam on GD, achieved 81% to 91% bitter rot control. Biorational materials showed low to no efficacy, allowing 37% to 89% BRI on ID and 21% to 48% BRI on GD. In the second, much drier year, synthetic fungicides achieved 86% to 100% BRI control on GD and 77% to 96% on ID. Among the biorational materials, only Regalia mixed with JMS Stylet oil and only in the second year provided 58% BRI control. EC50 and EC25 assay values for synthetic fungicides aligned well with the field observations. Given the potential for significant economic losses due to bitter rot, reliable fungicides such as Omega (FRAC 29), Aprovia (FRAC 7), Ferbam, Ziram (FRAC M3), and Captan (FRAC M4) are essential management tools for growers. These fungicides should be incorporated into the spray programs to reduce the risk of Colletotrichum resistance to quinone outside inhibitor fungicides, from which commercial apple farms highly depend now. Regalia mixed with Stylet oil offers an additional rotation option during drier years with lower disease pressure.