Influence of temperature and inoculate timing on the disease severity of Aphanomyces root rot by artificial inoculation in sugar beet [Beta vulgaris]

Root rot, which can cause severe yield loss in sugar beet (Beta vulgaris L.), is caused by the soil-borne fungus Aphanomyces cochlioides Drechsler (Ui and Nakamura, 1963). This study was initiated to clear the influences of growing temperature and inoculate timing on the disease severity by using zoospore inoculation and pot-culture. As materials, 2 or 3 sugar beet varieties (F1 hybrids), differ in resistance against Aphanomyces root rot, were sown in paper pots. One month later, the seedlings were inoculated with zoospore suspension (30,000 zoospores / plant) and transplanted to 1/5000-are wagner pots. After transplanting, the seedlings were grown in a glasshouse without temperature control or with heating. (controlled the lowest temperature at 15 deg C for the 30 days after inoculation, thereafter 20 deg C). Roots were harvested about three months after transplanting, ranked for disease severity (0-5 scale) and disease index (DI) was calculated by following formula; DI=sigma(rank x number of plants) / number of total plants. When the inoculated plants were grown in the greenhouse without temperature control, the difference in DI between years was highly significant. Since the growth temperatures in 2003 were low, conditions were unfavorable for the development of the disease. Consequently, the disease did not developed well in 2003 and DI was quit low. Moreover the difference in DI between cultivars was not clear. In contrast, the roots, grown in greenhouse with heating, showed many root rot symptoms, the average of DI was 2.4 and much higher than that in a test without temperature control (1.7). Moreover inter-cultivar differences in DI were also observed clearly. The influence of inoculate timing on disease severity was investigated. In the case of inoculation at one week before transplanting, the root showed many rot and the average of DI was 2.4. Moreover, cultivars were classified from susceptible to resistant more clearly. However, when the seedlings were inoculated at the next day of transplanting, the root showed little rot and the average of DI was quite low (1.7). Inter-cultivar difference in DI was also not clear.