<i>Colletotrichum</i> Species Causing Apple Bitter Rot in Virginia: Field Losses, Distribution, and Efficacy of Biorational and Synthetic Fungicides in Field and In Vitro

Apple bitter rot field losses in Virginia range from 20 to 90% annually. Distribution of <i>Colletotrichum</i> species in Virginia showed that the <i>Colletotrichum gloeosporioides</i> species complex dominated in the north and central regions, with 66 and 88% of isolates respectively. The <i>Colletotrichum acutatum</i> 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 2 years, we applied five to six consecutive spray applications of 15 individual active ingredients from May to August. Bitter rot incidence was evaluated twice each summer. In the first, rainy year, the top four synthetic fungicides, Omega, Aprovia, Ferbam, and a grower standard on ID and Omega, Captan, Cabrio, and Ferbam on GD, achieved 81 to 91% bitter rot incidence (BRI) control. Biorational materials showed low to no efficacy and achieved 0 to 31% BRI control on ID and 0 to 60% BRI control 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. Effective concentration of fungicide required for 50% and 25% growth inhibition assay values for synthetic fungicides aligned well with the field observations. Given the potential for significant economic losses owing 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 spray programs to reduce the risk of <i>Colletotrichum</i> resistance to quinone outside inhibitor fungicides (FRAC 11), on which commercial apple farms highly depend. Regalia mixed with Stylet oil offers an additional rotation option during drier years with lower disease pressure.