1996 Volume 65 Issue 2 Pages 381-385
Rose plants moderately resistant to the powdery mildew were selected from leaf callusderived regenerants. The process was facilitated by developing an efficient system for inoculation with the powdery mildew pathogen, Sphaerotheca pannosa. Fifty regenerants obtained from leaf explants of rose (Rosa hybrida cv. Carl Red) were grown and propagated in a greenhouse. The compound leaves of the fourth node of the new shoots were used to inoculate with conidiospores of S. pannosa KS-pl, using an apparatus devised for a large scale inoculation. Seven days after inoculation, disease severity of test plants was determined by a scale ranging from asymptomatic (0) to entirely pustule-engulfed leaves (3). Control leaves prepared from susceptible 'Carl Red' and resistant 'Magic' were rated 2.8 and 0, respectively, for disease severity. Some regenerants fell into the range of resistance, in particular, RHCL-203 and -263, which were rated 1.1 and 1.2, respectively. When leaves were harvested at random from these two regenerants cultivated in a greenhouse and retested for their resistance to the pathogen, their disease severities were ranked as being moderately resistant to powdery mildew. Therefore, these regenerants were selected as possible candidates for commercial use.