Novel infection strategies of Colletotrichum acutatum on ripe blueberry fruit

The infection and colonization process of Colletotrichum acutatum on ripe blueberry fruit from two cultivars with different susceptibility to anthracnose were examined using light and confocal laser scanning microscopy. Ripe fruit from susceptible cv. Jersey and resistant cv. Elliott were drop‐inoculated with a conidial suspension of C. acutatum, and epidermal peels were evaluated at selected times after inoculation and incubation. Results from pre‐penetration studies demonstrated that there were significant differences in the rate of formation of melanized appressoria between the two cultivars, with the rate of formation being faster in the susceptible one. In both cultivars, penetration by the pathogen occurred via appressoria 48 h post‐inoculation (hpi). However, in the susceptible cv. Jersey, C. acutatum then adopted an intracellular hemibiotrophic‐like infection strategy, whereas in the resistant cv. Elliott subcuticular intramural‐like infection occurred. In cv. Jersey by 108 hpi, intracellular growth of the pathogen led to the formation of numerous acervuli, with orange conidial masses. By 120 hpi, the conidial masses had coalesced covering the entire inoculated area. In cv. Elliott, acervuli were not seen until 144 hpi and contained few conidia. These results demonstrate for the first time the ability of C. acutatum to adopt a different infection and colonization strategy depending on the susceptibility of the host tissue being colonized.