Changes in Polyphenols in ‘Rio Red’ Grapefruit Leaves in Response to Elsinoë australis Infection

Sweet orange scab (SOS) is a fungal disease of citrus which is caused by Elsinoe australis. It affects the aesthetics of the fruit by forming wart-like protruded lesions on the fruit skin, and also affects the leaves which act as source of inoculum in the orchards. SOS is widespread in the different citrus species such as grapefruit (Citrus x paradisi Macfd.), sweet orange [C. sinensis (L.) Osb.], and mandarin (C. reticulata Blanco). In this study, we report the development of symptoms and changes in polyphenolic profile of ‘Rio Red’ grapefruit leaves in response to E. australis infection. Qualitative and quantitative analysis of polyphenols in ‘Rio Red’ grapefruit leaves inoculated with E. australis were conducted using high performance liquid chromatography (HPLC) at two different time points post inoculation. Development of SOS symptoms under light was recorded and categorized into four different morphological stages. Caffeic acid, luteolin-7-O glycoside, naringin, naringenin, apigenin-7-O glycoside and eriodictoyl were identified in healthy grapefruit leaves. The quantitative changes in the identified polyphenols were measured 9 days post-inoculation by comparing levels on the 9-day control versus the 9-day post-inoculated leaves. Total polyphenol levels decreased substantially in E. australis-infected ‘Rio Red’ grapefruit leaves 9 days post-inoculation. Levels of naringin, naringenin, apigenin glucoside and eriodictoyl decreased with E. australis infections indicating the pathogen’s ability to overcome some of plant’s defenses. Compared to 9-day controls, 9-day inoculated leaves had much less caffeic acid.