POPULATIONS OF FUSARIUM OXYSPORUM F. MELONIS AND THEIR RELATION TO THE WILT POTENTIAL OF TWO SOILS

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Abstract
The relation of soil populations of the muskmelon wilt fungus, Fusarium oxysporum f. melonis, to the wilt potentials of a yellow Fox sandy loam soil (Fsl) and a dark Colwood loam (Cl) was investigated. In either soil a direct relationship existed between the size of the population of the fungus and wilt incidence. Notwithstanding this relationship, with the same population the greater incidence of wilt in Fsl than in Cl showed that a factor or factors other than population affect the wilt potential. Whereas mean populations of field soils obtained at the site of wilted plants ranged upward to 3300 per gram, they declined steadily during the 9-month interval between crops. During this interval random samples of field soils yielded mean populations of 228 and 268 per gram of Fsl and Cl, respectively. Of the F. oxysporum colonies isolated at the end of harvest, about 70% from plant sites and approximately 21% from intersites were pathogenic. Two to eight months later only 12 to 15% of F. oxysporum isolates obtained by random sampling of infested field soils were pathogenic.