Evaluation of Fungicides for the Management of Glomerella Leaf Spot and Bitter Rot on Apple in North Carolina

Glomerella leaf spot (GLS) and bitter rot caused by species in the <i>Colletotrichum gloeosporioides</i> species complex are the most economically devastating fungal diseases of apples in North Carolina. Crop losses have reached 100% in highly susceptible cultivars. In the southeastern United States, management of this disease has predominantly relied on broad-spectrum fungicide programs targeting several summer diseases of apples. To improve control of GLS and bitter rot, field experiments to evaluate the efficacy of multi- and single-site fungicides were conducted over multiple years in a 'Gala' research orchard in Mills River, NC, from 2017 to 2019. Fungicides representing different modes of action were applied in a nonrotational program from petal fall until the immediate preharvest period to assess their efficacy in reducing GLS incidence, shoot defoliation caused by GLS, and pre- and postharvest bitter rot. Of the fungicides evaluated, Captan 80WDG (active ingredient [a.i.] captan), Merivon (a.i. fluxapyroxad and pyraclostrobin), Cabrio (a.i. pyraclostrobin), and Aprovia (a.i. benzovindiflupyr) provided good to excellent control against disease caused by <i>Colletotrichum chrysophilum</i>. In these programs, the incidence of GLS, expressed as the relative area under the disease progress curve, ranged from 4.6 to 43.8 in the Captan 80WDG (2019) and the Aprovia (2017) programs, respectively. The incidence of bitter rot at harvest in these programs from 2017 to 2019 ranged from 0.0 to 45.3% in the 2018 and 2017 Captan 80WDG programs, respectively. Conversely, the highest incidences of GLS and preharvest bitter rot were consistently observed in the Inspire (a.i. difenoconazole), Sercadis (a.i. fluxapyroxad), and untreated programs. Shoot defoliation owing to GLS followed a similar trend across the evaluated fungicide programs. In addition to nonrotational fungicide programs, the effect of carrier water pH for applications of captan was evaluated for bitter rot control in Virginia and North Carolina. No differences in captan efficacy were observed at either location. The results of this study will inform the development of new management programs for GLS and bitter rot in the southeastern United States and provide a foundation for the development of new season-long fungicide programs for apple disease management.