This study investigated the effect of CaCl2 and Ca(NO3)2 on fungal growth of Phytophthora sojae isolates, disease reduction on two cultivars of Glycine max (L.) Merr. cv. Chusei-Hikarikuro (black soybean) and cv. Sachiyutaka (white soybean), and zoospore release. A concentration of 20–30 mM CaCl2 or 30 mM Ca(NO3)2 led to a slight decrease of the growth rate of two isolates on PDA; however, 0.4 and 4 mM of CaCl2 and Ca(NO3)2 increased growth. The application of 4 mM CaCl2 or more than 4 mM Ca(NO3)2 before inoculation greatly inhibited infection in the two soybean cultivars. Disease suppression recorded in laboratory experiments using pathogen mycelium was due to the response of plant tissues rather than a direct inhibition of pathogen hyphal growth by the application of calcium. Furthermore, Ca(NO3)2 was more effective than CaCl2. The calcium contents in plants increased at the time of inoculation. The extent of disease reduction was related to an increased calcium uptake by plants of the two cultivars, except for some cases involving Chusei-Hikarikuro. Results showed that the effective element in reducing Phytophthora stem rot was calcium and that differences existed between the two cultivars in terms of the mechanisms of calcium uptake and the effect on disease suppression. The presence of 4–30 mM CaCl2 and Ca(NO3)2 decreased the release of zoospores from isolates on lima bean agar, although 0.4 mM CaCl2 and Ca(NO3)2 significantly induced zoospore release. These results suggest the possibility of applying a solution containing more than 4 mM of calcium to decrease the incidence of disease in agricultural fields by the inhibition of zoospore release.
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