MICROSCOPIC STUDIES OF THE INFECTION OF GERBERA FLOWERS BY BOTRYTIS-CINEREA

The formation of lesions on ray florets of gerbera flowers caused by single conidia of Botrytis cinerea was studied in two cultivars infected by two isolates of the pathogen. No differences in reaction after inoculation with conidia of either isolate were seen on either cultivar. The conidia produced usually one germ tube not longer than 10 mu m, but conidia with five germ tubes were also seen. Direct penetration of germ tubes through the upper cuticle of ray florets was observed. No appressoria or other specialised structures were observed before penetration, and degradation of the cuticle did not occur. Germination of conidia and subsequent flower infection was dependent on the availability of free water, but not on the addition of external nutrients. Between 18 to 25 degrees C, fungal development usually stopped after cuticle penetration, two to four cells around the site of penetration becoming necrotic. This number did not increase when inoculated flowers were subsequently placed at 4 degrees C, conditions conducive for the formation of spreading lesions. When flowers were incubated constantly at 4 degrees C, lesions became visible 3 days after inoculation as a group of 10 to 14 cells. Initially from a vesicle-like structure, mycelium spread subcuticularly or in the lumen of epidermal cells resulting in the death of 40 to 50 cells at 18 days after inoculation. Ungerminated conidia and conidial germlings which has not yet penetrated the cuticle did not cause any visible symptoms in underlying epidermal cells.