Abstract
Black root rot caused by Aphanomyces cochlioides Drechsler is a soilborne disease of sugar beet. It severely reduces sugar beet production and at present, this damage is a serious problem in the Hokkaido region. Because of a lack of an effective method for the control of this disease, breeding of resistant cultivars is strongly requested. For effective breeding of resistant cultivars, we developed an artificial inoculation method for the assay of the resistance using pot culture and zoospore inoculation. One-month-old young sugar beet plant was inoculated with zoospore suspension (30,000 zoospores / plant) and transplanted to 1/5000a wagner pot (one plant / pot). Then, the plant was grown in a greenhouse and wet treatment (300ml water / pot daily, for 10 days) was conducted twice; first from the day after transplanting and second from 30 days after transplanting. Roots were rated for disease (0-5 scale, 0 = no symptom, 5 = root rotted completely) at 90 days after transplanting. In the inoculated roots clear symptoms were observed and Aphanomyces were detected from the lesion area, but in the non-inoculated roots few symptoms were observed. The disease indexes were 2.6 in ‘Kabutomaru (susceptible cultivar)’, 1.9 in ‘Monohomare (medium susceptible cultivar)’ and 0.9 in ‘Yukihinode (resistant cultivar)’. This difference of index was statistically significant and agreed with the result of the infested field test. This inoculation method might be useful for evaluating root rot resistance and is expected to contribute to screening and breeding of resistant lines.
