A bitter, complex problem: causal Colletotrichum species in Virginia orchards and apple fruit susceptibility

Authors: KHODADADI, FATEMEH - Virginia Tech; SANTANDER, RICARDO - Cornell University; MCHENRY, DIANA - Virginia Tech; Jurick, Wayne; ACIMOVIC, SRDJAN - Virginia Tech

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2023
Publication Date: 10/23/2023
Citation: Khodadadi, F., Santander, R., Mchenry, D., Jurick II, W.M., Acimovic, S.G. 2023. A bitter, complex problem: causal Colletotrichum species in Virginia orchards and apple fruit susceptibility. Plant Disease. 107(10):2907-3272. https://doi.org/10.1094/MPMI-10-22-0204-A.
DOI: https://doi.org/10.1094/MPMI-10-22-0204-A

Interpretive Summary: Fruit deterioration, losses and reduction in quality are attributed to fungal diseases. In the mid-Atlantic area, bitter rot is a significant pre and postharvest problem for pome fruit growers. Bitter rot is caused by a fungus that is primarily controlled by fungicides. However, fruit that survive the growing season and enter storage, often come out with bitter rot symptoms. The incidence, species and symptoms caused by this emerging disease are largely unexplored. Thus, our study provides new information on bitter rot to help the industry identify and manage this disease. We identified many different species, evaluated a variety of commercial apple cultivars for their susceptibility, and took high quality photos of the decay incited by bitter rot. We envision our data will be directly useful for designing tests to screen fruit for pathogen resistance studies, increase our understanding of the factors needed for this pathogen to cause bitter rot and enable development of novel control strategies to ensure fruit quality while reducing food loss and waste.

Technical Abstract: Bitter rot, caused by Colletotrichum species, is one of the most devastating summer rot diseases affecting apple production in the Eastern United States. Given the differences in pathogenicity and fungicide sensitivity levels between the C. acutatum species complex (CASC) and C. gloeosporioides species complex (CGSC), monitoring their diversity, geographic distribution, and frequency is essential for their successful management. In a 662-isolate collection from apple orchards in Virginia, isolates from CGSC were dominant (65.5%) in comparison to the CASC (34.5%). In a sub-sample of 82 representative isolates, using morphological and multi-locus phylogenetic analyses, we identified C. fructicola, C. chrysophilum, C. siamense and C. theobromicola from CGSC and C. fioriniae and C. nymphaeae from CASC. The dominant species were C. fructicola, followed by C. chrysophilum and C. fioriniae. C. siamense followed by C. theobromicola developed the largest and deepest rot lesions on ‘Honeycrisp’ fruit in our virulence tests. Detached fruit of 9 apple cultivars and one wild accession (Malus sylvestris) were harvested early and late developmental stages and tested in controlled conditions for their susceptibility to C. fioriniae and C. chrysophilum. All cultivars were susceptible to both representative bitter rot species, with ‘Honeycrisp’ fruit being the most susceptible and Malus sylvestris, accession PI 369855, being the most resistant. We demonstrate that the frequency and prevalence of species in Colletotrichum complexes are highly variable in the Mid-Atlantic and provide regional-specific data on apple cultivar susceptibility. Our findings are necessary for the successful management of bitter rot as an emerging and persistent problem in apple production both pre and postharvest.