Experiments were undertaken to elucidate the cause of reduction of spinach root rot by nitrate nitrogen application. It was confirmed in vitro that nitrate as Ca(NO₃)₂⋅4H₂O has an effect on the inoculum potential of the pathogen rather than on susceptibility of the host.
Accumulation of secondary zoospores of Aphanomyces cochlioides and infection of hypocotyls of host plants were observed in nitrate solutions at nitrogen levels of 0-200ppm, but not at levels of more than 300ppm. This phenomenon at high concentration of nitrate was the result of encystment and death of secondary zoospores. Inhibition of zoospore production from mycelia was also observed at high concentration of nitrate. Encystment and death at 0.75bar (N: ca. 300ppm) and 1.00bar (N: ca. 400ppm), respectively, were observed in nitrate solutions, but not in controls of sugar solutions, suggesting that nitrate toxicity is more closely responsible for the motility of secondary zoospores than increased osmotic pressure.