The Effects of Inorganic Elements on the Reduction of Phytophthora Stem Rot Disease of Soybean, the Growth Rate and Zoospore Release of Phytophthora sojae

The effects of several inorganic elements contained in B5 medium on Phytophthora stem rot disease reduction of Glycine max (L.) Merr. cv. Chusei-Hikarikuro, fungal growth of Phytophthora sojae isolate and zoospore release were investigated. Application of B5 solution and macro inorganic nutrients in the B5 medium prior to inoculation significantly inhibited infection, compared with controls. Various concentrations of KNO3, (NH4)2SO4, MgSO4, CaCl2 and NaH2PO4 in the presence of macro inorganic nutrients were investigated in an effort to determine the elements most effective in suppressing the incidence of disease. A concentration of 2.47–24.7 mm KNO3 and 0.1–10.2 mm CaCl2 greatly inhibited infection. Although mycelium growth of the isolate was affected by the potassium and calcium concentration, no significant relationship was observed between inhibition of the growth rate and disease reduction at 2.47 mm KNO3 and 0.1–5.1 mm CaCl2 application. Disease suppression recorded in laboratory experiments using pathogen mycelium was due to the response of plant tissues rather than a direct inhibition of pathogen fungal growth by the application of potassium or calcium. The extent of disease reduction was related to an increased potassium and calcium uptake by plants, suggesting that the effective elements in reducing Phytophthora stem rot were potassium and calcium. The presence of 2.47–247 mm KNO3 and 5.1– 10.2 mm CaCl2 decreased the release of zoospores, although 0.1–2.5 mm CaCl2 significantly induced zoospore release. These results suggest that applying a solution containing more than 2.47 mm of potassium and 5.1 mm of calcium can decrease the incidence of disease in agricultural fields by the inhibition of zoospore release