Abstract
The effects of the fruit growth stage on the susceptibility to and the latent period of scab fungus on Japanese pear cv. “Kosui” were investigated. The degree of fruit susceptibility to the scab disease was high from the fruit receptacle stage (before bloom) to the young fruit stage (about 2 weeks after bloom). Susceptibility to the disease quickly decreased from mid May (about 25 days after bloom) and was low until early June (about 50 days after bloom) then gradually increased again. Susceptibility reached its highest during early to mid July (about 75 to 90 days after bloom). The latent period of the disease was short, 14 to 21 days, during the stage of high susceptibility and long, about one month, during the stage of low susceptibility. The percentage of cracked fruits was highest in fruits inoculated during early to late May (about 15 to 30 days after bloom), and was little lower during mid June. From the standpoint of orchard management, fruit cracking caused by fungus infection during early May and mid to late June is practically important because of high fruit susceptibility to the disease. This indicates that the best time for controlling the disease on fruit of cv. “Kosui” with fungicides in the Kanto district is from prebloom to early May and again from mid June to mid July (about 55 to 90 days after bloom). The results obtained from this work may be revised for an earlier calendar timing to be adapted in the Kyushu district, warmer area in Japan, and for a later calendar timing to be adapted in the Tohoku district, cooler area.
