The role of flowers in the disease cycle of Colletotrichum fioriniae and other cranberry fruit rot fungi.

Floral extracts (FEs) can influence the infectivity and epidemiology of fruit infecting Colletotrichum species. In this study, C. fioriniae responded to cranberry FEs with an increased rate and magnitude of secondary conidiation and appressorium formation. Four other cranberry fruit rotting species also showed an increased rate of germination in the presence of FEs. However, increased appressorium formation was observed only in the latent pathogens Coleophoma cylindrospora, Colletotrichum fructivorum, and C. fioriniae. Two other fruit rotting species, Phyllosticta vaccinii and Allantophomopsis lycopodina did not form appressoria while secondary conidiation was only seen with the Colletotrichum spp. When conidia of C. fioriniae were inoculated in the presence of FE, the incidence of disease was greater on cranberry fruit. Conidia of this species also formed appressoria at lower than expected temperatures in the presence of FE. Dissection of the flowers revealed that the corolla (with stamens and stigma) was the most stimulatory part of the inflorescence. These observations suggest an important and ephemeral role of flowers in the epidemiology of fruit rot. Stimulatory floral signals were readily detected using a conidial germination bioassay and rainwater samples collected from the plant canopy throughout the growing season confirmed that bioactivity was highest during the bloom period, and declined as the fruit developed. The data presented show that floral signals can alter the growth patterns of a larger than previously observed range of fungi and the mobility of floral signals within the canopy implicates these phenology-specific cues in modifying the disease cycles of numerous plant pathogens.