SITES OF INFECTION BY PYTHIUM SPECIES IN RICE SEEDLINGS AND EFFECTS OF PLANT-AGE AND WATER DEPTH ON DISEASE DEVELOPMENT

Seedling disease, caused primarily by several species of Pythium, is o ne of the major constraints to water-seeded rice production in Louisia na. The disease, also known as water-mold disease, seed rot, and seedl ing damping-off, causes stand reductions and growth abnormalities. In severe cases, fields must be replanted, which may result in delayed ha rvests and reduced yields. To develop more effective disease managemen t tactics including biological control, this study was conducted prima rily to determine sites of infection in seeds and seedlings; effect of plant age on susceptibility to P. arrhenomanes, P. myriotylum, and P. dissotocum; and minimum exposure times required for infection and see dling death. In addition, the effect of water depth on seedling diseas e was investigated. Infection rates of seed embryos were significantly higher than those of endosperms for all three Pythium spp. The develo pment of roots from dry-seeded seedlings was significantly reduced by P. arrhenomanes and P. myriotylum at 5 days after planting compared wi th that of roots from noninoculated controls. Susceptibility of rice t o all three species was sharply reduced within 2 to 6 days after plant ing, and seedlings were completely resistant at 8 days after planting. There was a steep reduction in emergence through the flood water, rel ative to the noninoculated control, following 2 to 3 days of exposure to inoculum of P. arrhenomanes and I! myriotylum. In contrast, P. diss otocum was much less virulent and required longer exposure times to ca use irreversible seedling damage. Disease incidence was higher when se eds were planted into deeper water, implying that seedlings become res istant after they emerge through the flood water. These results sugges t that disease control tactics including flood water management need t o be employed for a very short period of time after planting. Also, gi ven that the embryo is the primary site of infection and it is suscept ible for only a few days, the disease should be amenable to biological control.