Effects of systemic fungicidal seed dressings on the incidence of Phytophthora megasperma, nodulation, and nitrogen fixation in two soybean cultivars

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Abstract
Two cultivars of soybeans, Davis and Ross, resistant and susceptible to Phytophthora root rot, respectively, were grown in a field trial where there was a moderately high level of Phytophthora megasperma f. sp. glycinea. The efficacy of metalaxyl and Dowco 444 as fungicidal seed dressings, their interaction with host resistance and the effect of plant health on nodulation and nitrogen fixing activity (acetylene reduction) were assessed. The cultivar Davis showed a relatively high level of field resistance in the absence of fungicidal protection; this was manifest in increased emergence and a lower incidence of disease in the surviving plants. In contrast cv. Ross had virtually no field resistance. In both cultivars, disease incidence was lower where fungicides were used. Metalaxyl was more effective than Dowco 444 on cv. Ross but not on cv. Davis. This suggests that the better performance of Dowco 444 on Davis was through an enhancement of existing resistance mechanisms of the host. Nitrogen fixing activity was generally higher in Davis than in Ross with a positive correlation between nitrogen fixing activity and nodule number. Disease incidence was negatively correlated with nitrogen fixing activity indicating the dependence of nitrogen fixation on root health. Neither fungicides suppressed nitrogen fixing activity at the rates used.