The population of spinach wilt fungus, Fusarium oxysporum f. sp. spinaciae, and the wilt incidence in soil.

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A high population of spinach wilt fungus, Fusarium oxysporum f. sp. spinaciae, was found in the fields where F. oxysporum population was high. In rhizosphere soil around diseased spinach roots, about 69% of F. oxysporum isolates were pathogenic on spinach seedlings, and the highest population of the pathogen (6, 600 propagules/g dry soil) was found. The population was relatively low in loam and loam clay soil, and also in drained rice paddy fields, but high in upland fields with light clay soil, especially in the field where spinach plant has been cultivated annually for over 6 years. The pathogen was also numerous in fields lower than 700m above sea level. The optimum temperature for hyphal growth of the pathogen was between 25 and 28C, and the wilt disease was severe above 25C. The occurrence of much large population of the pathogen and more severe wilt disease at the low-elevation areas might be due to the high soil temperature. The minimum spores to cause the wilt disease were between 10 and 100/g dry soil. The number of pathogen enough to cause the wilt disease was found in every field, even where the wilt incidence has never been found.