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The Sporangium of Pythium ultimum as a Survival Structure in Soil. M. E. Stanghellini, Department of Plant Pathology, University of California, Berkeley 94720, Present address of senior author: Department of Plant Pathology, University of Arizona, Agricultural Experiment Station, P.O. Box 1308, Mesa, Arizona 85201; J. G. Hancock, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 61:157-164. Accepted for publication 8 September 1970. DOI: 10.1094/Phyto-61-157.

Sporangia of Pythium ultimum persisted for 11 months in air-dried and moist field soil with little or no decrease in either the rate or percentage germination. The P. ultimum population, as estimated by plate count, remained stable throughout this period. Sporangial germination occurred in soil in 1.5 hr, and reached a maximum of about 80% 3 hours after the addition of nutrients. Germ tube growth was independent of soil moisture contents between field capacity and near saturation. Germlings grew through moist soil at about 300 µ/hour, and were capable of maintaining this rate provided adequate nutrients were available. Upon cessation of germling growth, germ tube and/or hyphal cell walls lysed. Two survival mechanisms capable of countering lysis were observed: (i) at low nutrient levels the protoplasm retracted from the tip of the germ tube, forming septa as it moved into the parent sporangium; and (ii) at high nutrient levels terminal and/or intercalary sporangia were formed. Both the parent and secondary sporangia were capable of regermination and germination, respectively. Quantitative population estimates, subsequent to sporangial germination and cell wall lysis in soil, indicated that these two survival mechanisms were capable of maintaining the P. ultimum population. Results indicate that sporangia of Pythium ultimum function as major survival structures and inocula in cultivated soils.