Y NAKAZAWA¹, K UCHIDA¹, J INOUE², T TAKEDA¹, T AMANO¹ and N KITA²

¹ZENNOH Agricultural R&D Centre, 5-5-1 Higashiyahata, Hiratsuka, Kanagawa 254, Japan; ²Kanagawa Institute of Agricultural Science, Kamikisawa, Hiratsuka, Kanagawa 254, Japan

Background and objectives
Powdery mildew is the most damaging disease of strawberry in Japan. The causal fungus, Sphaerotheca aphanis var. aphanis Braun, is known to be parasitic to several genera of Rosaceae [1], while the isolates from strawberry, Fragaria X ananassa Duch., are pathogenic only to the plants of genus Fragaria. Although differences in susceptibility to powdery mildew is known among strawberry cultivars, no physiological races of the fungus have been reported [2].

In the present study, we collected monoconidial isolates of the fungus and investigated difference in their pathogenicity to the selected strawberry cultivars. We also conducted genetic studies on the resistance of several strawberry cultivars to the fungus.

Materials and methods
Conidia of mass population of the fungus were scattered onto agar plates and, under microscope, single conidia were picked up with a syringe. Each conidium was transferred to the lower surface of a leaf disk prepared from young leaves of the same cultivar on which the mass population had originally colonized. The inoculated leaf disks were incubated at 20°C, 12-h photoperiod in a moisturized Petri dish. An emerging colony of the mildew on a leaf disk was maintained as a monoconidial isolate on a whole young leaf at the same condition and subcultured every 10 days.

For the pathogenicity test, freshly formed conidia of each isolate were scattered onto the lower surface of the leaf disks from nine strawberry cultivars, Toyonoka, Nyohoh, Reikoh, Himiko, Hokowase, Doner, Kogyoku, Harunoka and Fukuba. The degree of mildew infection was evaluated 7 days after inoculation.

Results and conclusions
We obtained 42 monoconidial isolates from commercial strawberry fields in 11 prefectures of Japan. Thirty-one out of the 42 isolates were pathogenic to all the nine strawberry cultivars tested, while 11 isolates did not show pathogenicity to Toyonoka or Harunoka. We designated the former group as race 1 and the latter race 0.

Against race 0, progenies obtained from the cross between 'Toyonoka' and 'Nyohoh' segregated into 1:1 and S1 of 'Toyonoka' into 3:1 for resistant:susceptible, respectively, whereas S1 of "Nyohoh" were all susceptible. All the progenies were susceptible to race 1. These results clearly indicate the existence of physiological races in Japanese population of S. aphanis var. aphanis on strawberry, which correspond to a major gene for resistance in host cultivars.