Identifying Species of Pathogens Causing Bitter Rot of Apples in Illinois and Efficacy of Fungicides for Managing the Disease

Outbreaks of bitter rot disease occurred in Illinois apple (Malus ×domestica) orchards during 2010–20. This study was conducted to assess the incidence of bitter rot in commercial apple orchards in Illinois, identify pathogen species that cause bitter rot, and evaluate the efficacy of fungicides for managing the disease. Orchard surveys conducted during 2019–21 showed that fruits with bitter rot were present in most of the orchards in southern and central Illinois, whereas only a few orchards in the northern part of the state had symptomatic fruits. A total of 270 isolates of the pathogens were collected from symptomatic fruits of 14 cultivars, and pathogen species were identified based on the morphological and molecular characteristics of the isolates. GAPDH gene sequence analyses identified species of the pathogens as Colletotrichum fioriniae, C. siamense, and C. chrysophilum. Laboratory and orchards studies were conducted to evaluate the effectiveness of fungicides for managing bitter rot disease. Laboratory studies showed averages of 10.3, 9.6, and 0.24 mg⋅L−1 for the 50% effective concentrations (EC50) of benzovindiflupyr, captan, and fluxapyroxad + pyraclostrobin fungicides, respectively. Orchard experiments involving ‘Honeycrisp apples’ were conducted in 2019, 2020, and 2021. Benzovindiflupyr, captan, and fluxapyroxad + pyraclostrobin prevented bitter rot development in the treated plots.