Characterization and biological activity of flavonoids from ripe fruit of an anthracnose-resistant blueberry cultivar

Anthracnose fruit rot, caused by Colletotrichum acutatum, is among the most important diseases of blueberries. Most cultivars are susceptible but ‘Elliott’ is resistant. Our objective was to identify possible antifungal compounds that play a role in the resistance response. Chemical fractions from freeze-dried, ripe fruit of ‘Elliott’ and a susceptible cultivar (Jersey) were extracted with methanol and ethyl acetate. Extracts were screened on solid media for suppression of microconidiation of C. acutatum. The methanolic extract was fractionated and the soluble methanolic fraction from ‘Elliott’ was the most biologically active. This fraction was dried, dissolved in water, and screened in vivo by pre-treating ripe ‘Jersey’ fruit with 0.5, 1, 2, and 4% solutions (w/v) and subsequently inoculating the fruit with C. acutatum. An 88% reduction in infection incidence was observed after 12 days with the 4% solution. Anthocyanins and other flavonoids were then quantified in fruit of the two cultivars using HPLC-MS. ‘Elliott’ fruit contained more anthocyanins (4.87 mg/g of freeze-dried tissue) than ‘Jersey’ (3.27 mg/g of freeze-dried tissue); however, the same compounds were found in both cultivars. ‘Elliott’ fruit also contained more non-anthocyanin flavonoids (0.18 mg/g of freeze-dried tissue) than ‘Jersey’ (0.12 mg/g of freeze-dried tissue), including two distinctive compounds in ‘Elliott’. The non-anthocyanin flavonoid fractions of both ‘Elliott’ and ‘Jersey’ significantly decreased the growth of C. acutatum in a liquid bioassay but the effect was more pronounced in the ‘Elliott’ fraction. The two distinctive compounds in ‘Elliott’ were further characterized by MS/MS and were identified as quercetin 3-O-rhamnoside and, putatively, syringetin rhamnoside. Additionally, we evaluated ‘Elliott’ and ‘Jersey’ methanolic extracts for their ability to inhibit lipid peroxidation, and the extract obtained from ‘Elliott’ was almost twice as active at inhibiting peroxidation. These results provide new insights into the role of antifungal compounds in the resistance response of ripe ‘Elliott’ blueberries to infection by C. acutatum.