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Persistence of Fusarium oxysporum f. sp. vasinfectum in Fields in the Absence of Cotton. Shirley N. Smith, Department of Plant Pathology, University of California, Berkeley, California 94720; William C. Snyder, Department of Plant Pathology, University of California, Berkeley, California 94720. Phytopathology 65:190-196. Accepted for publication 28 August 1974. DOI: 10.1094/Phyto-65-190.

When formae speciales of Fusarium oxysporum, which cause vascular wilts, are introduced into a conducive soil, the pathogen usually persists there indefinitely. In a California cotton field soil planted for several years to cereals, the population of the pathogen, F. oxysporum f. sp. vasinfectum, increased more rapidly in the absence of cotton than when previously planted to cotton. Small barley roots remaining after the grain crop was harvested contained high populations of the pathogen. Likewise, roots and crowns of yellow nutsedge contained populations of the pathogen in epidermal and outer cortical cells which were as much as 20 times greater than those of nonpathogenic F. oxysporum. Nonsuscepts of the cotton pathogen, such as barley, may increase populations of F. oxysporum f. sp. vasinfectum under field conditions at a faster rate than does the continuous planting of cotton. Thus a crop rotation that does not take into consideration the manner in which the Fusarium wilt pathogen persists in field soil cannot be relied upon to reduce its numbers in the field.