Changes in the secondary compounds of persimmon leaves as a defense against circular leaf spot caused by Plurivorosphaerella nawae

Circular leaf spot, caused by the ascomycetous fungus Plurivorosphaerella nawae (= Mycosphaerella nawae), is the most problematic fungal disease of persimmon worldwide. In Korea, persimmon exposed to P. nawae inoculum (ascospores) from May to August shows visible circular leaf spot disease symptoms from the end of August to early September. It is important to identify factors affecting this long latent period. The objective of this study was to elucidate the relation between the development of symptom of circular leaf spot and the content of phenolics compounds and vitamin C as well as the antioxidant activities in leaves. Healthy leaves (both young and old) and infected leaves of circular leaf spot-susceptible persimmon cultivar were harvested in 2016. The content of phenolics (total phenols, flavonoids, and tannins) and vitamin C, and their antioxidant activities were analyzed in all types of leaves. Compared with the asymptomatic leaves (old) and the asymptomatic parts of the infected leaves, the symptomatic parts of the infected leaves, symptomatic leaves, and asymptomatic young leaves showed significantly higher content of phenolics and vitamin C, and higher antioxidant activities. Disease incidence and severity were estimated for older leaves (emerged in early May) and younger leaves (emerged at the end of June) in 2017 and 2018. The AUDPC was higher in old leaves than younger leaves. The disease progression was much faster and severe in the older than in the younger leaves. Similar results were found in field experiments. Higher content of phenolics and antioxidant activities in the younger leaves may contribute to circular leaf spot resistance in persimmon. Furthermore, accumulation of phenolics and antioxidant activity in the infected leaves is a post-infection response and the first stage of the defense mechanism.