Effects of Temperature on Pythium Root Rot of Spinach Grown Under Hydroponic Conditions

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Abstract
The effect of nutrient solution temperature on the cyclic occurrence of Pythium aphanidermatum and P. dissotocum as causal agents of root rot of hydroponically grown spinach was investigated. Both species grew well and produced zoospores in comparable numbers at temperatures between 17 and 27.degree. C. Infecting hyphae of both species penetrated spinach roots, without appressorium formation, within 15 min after inoculation with zoospores at 23.degree. C, the transitional temperature that governed the predominance of either P. aphanidermatum or P. dissotocum. Both species also caused significant yield reductions at all temperatures tested (17, 21, and 27.degree. C) except P. aphanidermatum at 17.degree. C. However, the rapidity of symptom development varied between the 2 spp. Severe root rot and/or plant death occurred within 3-4 days after inoculation with P. aphanidermatum at 21 and 27.degree. C, whereas severe root rot, but no plant death, occurred only after 7 days of incubation following inoculation with P. dissotocum at 21 and 27.degree. C. Differences in pathogenicity at specific temperatures give a temporary competitive advantage to the favored species with respect to rapidity of host colonization and subsequent fungus reproduction. Metalaxyl, at 5 .mu.g a.i. [active ingredient]/ml in the nutrient solution, effectively controlled root rot.

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