Effect of Free Moisture on Soybean Stem Canker Development

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Abstract
The influence of duration and type of free moisture on soybean stem canker development was studied by inoculating plants of breeding line J77-339 with ascospores and conidida of Diaporthe phaseolorum var. caulivora and maintaining free moisture on the plants [Glycine max] in mist chambers in the greenhouse. Wetting periods of 0-144 hr preceded exposure of plants to either dry or discontinuous wetting (8 hr/day) treatments for the remainder of the experiment. An additional treatment consisted of continuous wetting for the entire experiment. The pathogen was isolated from symptomless plants 7 days after inoculation in all treatments except one not receiving moisture. Isolation frequency declined for all treatments 30 days after inoculation. More than 24 hr of free moisture preceding dry treatment was required for stem canker development. Increases in incidences of stem canker and dead plants with wetting duration were sigmoidal for the dry treatment. Increase in canker number per plant for the dry treatment was quadratic. The length of the incubation period declined linearly with wetting duration from 48 to 144 hr preceding both dry and discontinuous moisture treatments. Maximum levels of stem canker incidence and severity and minimum incubation periods were achieved both with discontinuous wetting after 48- and 96-hr wetting periods and with continuous wetting for the entire experiment. Stem canker incidence, canker number, and incubation period, but not incidence of dead plants, for dry treatments approached levels achieved with discontinuous wetting treatments at 144 hr wetting. Moisture requirements for stem canker development were not rigid. Disease incidence and severity and length of the incubation period were related more to the total moisture duration than to the type (continuous vs. discontinous).