The Influence of Temperature and Water Potential on Asexual Reproduction byPythiumspp. Associated With Snow Rot of Wheat

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Abstract
Zoospores were released by sporangia of Pythium iwayamai on colonized wheat Triticum aestivum L. ''Nugaines'', leaf disks in deionized water at 1, 5, 10 and 15.degree. C, but not at 20.degree. C. P. okanoganense released zoospores from sporangia on colonized leaf disks at 1, 5 and 10.degree. C, but not at 15 or 20.degree. C. Fewer zoospores were released by P. okanoganense at 5 or 10.degree. C than at 1.degree. C, whereas P. iwayamai released relatively high numbers of zoospores at 1, 5 and 10.degree. C. Both fungi released more zoospores over 15 day at 1.degree. C than at higher temperatures. Empty sporangia with terminal evacuation tubes (indicating indirect germination) were observed on leaf disks maintained for 5 days in soil at 0.5.degree. C, but only at 0 bars matric water potential. P. iwayamai and P. okanoganense released zoospores from 21 and 9% of their sporangia, respectively, in flooded soil (water to a depth of 1 mm above the soil surface), and from 4 and 1% of the sporangia, respectively, in saturated soil. At 1.degree. C, sporangium production on wheat leaf disks was limited to osmotic water potentials (.PSI.O) above -8 bars, and zoospore release was limited to osmotic water potentials above -0.5 bars. Both Pythium spp. produced more sporangia and released more zoospores in snow melt water (.PSI.O = -0.035 bars) than in any other solution tested. Neither Pythium spp. released zoospores in 2 soil extracts (.PSI.O = -0.251 or -0.933 bars) nor in MgSO4 solutions (.PSI.O = -0.046 bars). More zoospores were released in dilute solutions of NaCl (.PSI.O = -0.150 bars) and NaNO3 (.PSI.O = 0.143 bars) than in deionized distilled water (.PSI.O = -0.002 bars). In nature, P. iwayamai and P. okanoganense probably produce zoospores only when sporangia at the soil surface are immersed in water from melted snow at near freezing temperatures.