SELECTION OF POWDERY MILDEW-RESISTANT REG ENERANTS OBTAINED FROM LEAFEXPLANT-DERIVED CALLI OF ROSA-HYBRIDA

Rose plants moderately resistant to the powdery mildew were selected f rom leaf callus-derived regenerants. The process was facilitated by de veloping an efficient system for inoculation with the powdery mildew p athogen, Sphasrotheca pannosa. Fifty regenerants obtained from leaf ex plants of rose (Rosa hybrida cv. Carl Red) were grown and propagated i n a greenhouse. The compound leaves of the fourth node of the new shoo ts were used to inoculate with conidiospores of S. pannosa KS-pl, usin g an apparatus devised for a large scale inoculation. Seven days after inoculation, disease severity of test plants was determined by a scal e 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 wer e harvested at random from these two regenerants cultivated in a green house and retested for their resistance to the pathogen, their disease severities were ranked as being moderately resistant to powdery milde w. Therefore, these regenerants were selected as possible candidates f or commercial use.