SURVIVAL OF PHYTOPHTHORA-COLOCASIAE IN-FIELD SOIL AT VARIOUS TEMPERATURES AND WATER MATRIC POTENTIALS

A large number of the zoosporangia of Phytophthora colocasiae incorpor ated into moist soils germinated by zoospore discharge, and/or lysed i n the soil during the first 5 days of incubation, decreasing the initi al number of colony-forming units from 1 x 10(4) to 1 x 10(2) per g of soil in all treatments. Eighteen days after incorporation, the viable zoosporangia present in moist soils had thickened their cell walls an d germinated only directly, often producing smaller zoosporangia. A fe w thick-walled chlamydospores were observed, and they germinated only directly. Zoosporangia in soils at -1,500 J/kg matric potential surviv ed longer than 107 days, and the amount of viable zoosporangia present at that time was approximately 0.1 x 10(2) CFU/g of soil. Apparently the great majority of the thin-walled zoosporangia produced on V8 agar , when incorporated into moist soil, germinated indirectly in the firs t 5 days of incubation. Zoosporangia that did not germinate became res ting zoosporangia by increasing their wall thickness or by producing c hlamydospores. These enabled the pathogen to survive in soil at -1,500 J/kg matric potential for more than 3 months. However, in the absence of the host, the pathogen is predicted to survive less than 1 year du e to its lack of saprophytic ability to colonize nonhost tissues.