Efficacy and Economic Analysis of Fungicide Applications for the Management of Apple Bitter Rot and Glomerella Leaf Spot

Apple bitter rot (ABR) and Glomerella leaf spot (GLS) caused by Colletotrichum species are highly damaging diseases on apple in the Mid-Atlantic. Current management strategies rely on the application of synthetic single-site and multi-site fungicides that are under review by the Environmental Protection Agency. New options are needed to supplement our current arsenal of fungicides to continue to mitigate resistance and provide effective control. We analyzed the in-field and in vitro efficacy of 5 synthetic materials and 4 biorational or biocontrol options. We found that Luna Sensation was effective at reducing ABR incidence on ‘Idared’ (62%–70%). Kenja provided weak ABR control on fruit (32%), but fair control of GLS on leaves in preliminary studies on ‘Idared’ and ‘Golden Delicious’ cultivars (31%–65%). A return on investment (ROI) model was also developed based on the in-field efficacy against ABR for more than 30 fungicides tested since 2022. Generally, ROI tended to be lower for ABR tolerant cultivar ‘Golden Delicious’ and higher for ABR susceptible cultivar ‘Idared.’ Synthetic products usually offered a higher ROI than biorational or biological products, but often biological and biorational products had ROI values greater than zero suggesting profitable outcomes. In-field ABR disease suppression did not necessarily correlate with ROI values. Kinoprol, for instance, had variable ABR suppression in the field but consistently had high ROI (98 on ‘Golden Delicious’ and 30 on ‘Idared’). Overall, our study provides actionable information for growers and scientists. This study demonstrates that there is unlikely to be a unified approach to ABR and GLS management, and we highlight the gap in efficiency between synthetic and biological products from an economic perspective.