Investigation of Wound Incidence on Stem Surface in Soybean Seedling

The occurrence of Phytophthora root and stem rot (PRSR) varies significantly depending on the presence or absence of wounds on the soybean hypocotyl as well as waterlogging and excessive soil moisture. However, the actual situation of wound incidence in the field and factors that cause wounds are not clear. To gain insight into the mechanism of PRSR in the field, we investigated the frequency and depth of naturally occurring wounds on soybean stems, and examined the factors that increase wound development. Wounds were observed in 1.7 to 7.1% of the individuals in the Kyoto University research field (Kizu farm in Kizugawa City, Kyoto farm in Kyoto City). The mean depth of wounds was 224 ± 193 µm (standard deviation), and 62% of the wounds were greater than 145 µm, which could accelerate PRSR occurrence significantly. Wounds were observed more in Kizu farm than in Kyoto farm, and the percentage of wounds in Kizu farm decreased with fine harrowing before sowing. Plants grown from seeds with wrinkle, seedcoat cracking, or seedcoat puffing tended to have more wounds. These results indicate that wounds of a depth that could promote PRSR occur with a considerable frequency under natural conditions, and that seed and sowing conditions are related to wound incidence. Further investigation is needed to determine the extent to which natural wounds contribute to PRSR occurrence.