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Relationship of Inoculum Density and Soil Types to Severity of Fusarium Wilt of Sweet Potato. Shirley N. Smith, Assistant Research Plant Pathologist, Department of Plant Pathology, University of California, Berkeley 94720; W. C. Snyder, Professor, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 61:1049-1051. Accepted for publication 1 April 1971. DOI: 10.1094/Phyto-61-1049.

Fusarium oxysporum f. sp. batatas was incorporated as chlamydospores into wilt-conducive and wilt-suppressive cultivated and noncultivated soils from California. Fusarium wilt on sweet potato planted in these soils was more severe in the wilt-conducive than in the wilt-suppressive soil, with the same number of chlamydospores in each soil. Non-cultivated suppressive soil was intermediate between the two cultivated soils in disease development on sweet potato. A few plants in the conducive soil became diseased within 4 weeks, when as few as 50 chlamydospores/g were present, but one-third of the plants in the suppressive soil remained disease-free when as many as 5,000 chlamydospores/g were present. Furthermore, the pathogen propagule counts doubled in both the conducive and the noncultivated suppressive soils at the 500/g level, but remained the same in the cultivated suppressive soil after two sets of cuttings were grown.