DIFFERENCES BETWEEN ROSE CULTIVARS IN SUSCEPTIBILITY TO INFECTION BY BOTRYTIS-CINEREA

Differences in the susceptibility of rose flowers to blossom blight caused by Botryis cinerea were investigated. Rose flowers, cvs. Supra and Royalty, were inoculated with various concentrations of B. cinerea conidia and incubated in humidified chambers at 21 C. Disease severity was quantified 2 days later as the number of lesions that had developed on each flower. The slope of the inoculum concentration/disease severity (IC/DS) regression line was used as a measure of susceptibility. Supra was more susceptible than Royalty, but the susceptibility of each cultivar and the difference between them varied among sampling dates. In experiments using isolated petal disks, there was no difference between cultivars in the number of B. cinerea conidia that germinated on the petal surfaces. On several sampling dates, significantly fewer of the germinated conidia penetrated into Royalty petals, but differences in penetration were not always related to differences in susceptibility. Scanning electron microscopic examination of cryofractured petal disks suggested that the cuticle was important in preventing penetration. Cuticles isolated from Royalty petals were significantly thicker than those from Supra, which may account for some of the difference in susceptibility between cultivars. However, since differences in susceptibility were not consistently related to differences in penetration or cuticle thickness, other, unknown mechanisms were implicated.