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Effects of Chitin and Chitosan on the Incidence and Severity of Fusarium Yellows of Celery

The effects of chitin and chitosan on disease incidence and severity of Fusarium yellows of celery and on populations of Fusarium oxysporum were investigated between 1994 and 1996. Field experiments were conducted at two locations with a history of severe Fusarium yellows. Disease incidence and severity were significantly reduced by pre-plant chitin amendments to soil. Chitosan applied as a root dip alone did not reduce disease incidence but significantly (P < 0.05) reduced disease severity when used with a tolerant celery cultivar. Standard soil dilution methods were used to enumerate populations of soil microflora. Chitin increased bacterial and actinomycete populations in soil in 2 of the 3 years of study. The effects of potential biocontrol agents recovered from chitin-treated plots in 1995 were studied in 1996; enriching the transplant medium with isolates of bacteria and actinomycetes 4 weeks and 1 week prior to transplanting did not alter the established equilibrium in the field, and no biocontrol effect was observed. Chitin amendments to soil or chitosan treatment of transplants did not reduce soil populations of F. oxysporum. Whether these treatments affected the F. oxysporum f. sp. apii subpopulation within the F. oxysporum population could not be determined.