Unmasking Ramularia mali: Phylogenetic Analysis and Degradative Impact on Apple Biopolymers.

Abstract

Ramularia mali Videira & Crous has recently emerged as the causal agent of dry lenticel rot, a postharvest disease affecting apples in Italy, Austria, and France. The disease manifests only after long-term cold storage under hypoxic conditions, resulting in distinct brown to black lesions on the peel. In contrast, infected fruit in the field remains asymptomatic. This study investigated the phylogenetic identity of R. mali in northern Italy (South Tyrol) and its ability to degrade major apple cell wall components under various environmental conditions. A multi-locus sequence typing (MLST) analysis using five gene regions was performed on 95 isolates to confirm species identity. Additionally, fungal strains were tested for their ability to utilize cellulose, pectin, and starch at different temperatures (4°C, 10°C, 25°C, 37°C), dark/light and hypoxic conditions of the storage. Phylogenetic analysis confirmed R. mali as the dominant species and primary causal agent of dry lenticel rot in South Tyrol, Italy and Styria (Austria). The pathogen showed psychrotolerant and hypoxia-tolerant traits, with slow but sustained growth under cold storage conditions. Pectin degradation was associated with symptom development, whereas no starch degradation was observed. This suggests that the pathogen preferentially colonizes riper fruit tissue. Disease severity was greater in more mature apples, suggesting a link between ripening-related tissue changes and susceptibility. These findings highlight the role of environmental factors, fruit ripeness, and R. mali psychrotolerant nature in disease progression. Future work should focus on quantifying varietal differences in peel composition to develop strategies to mitigate postharvest losses during long-term apple storage.