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Disease Note.

Carrot Scab Caused by Streptomyces spp. in Michigan. L. E. Hanson, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. M. L. Lacy, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Plant Dis. 74:1037. Accepted for publication 3 July 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-1037A.

In 1988, carrots (Daucus carota L. subsp. sativus (Hoffm.) Arcang. 'Chancellor') sampled from a loamy sand soil in Montcalm County, Michigan, had a high percentage of roots with corky, scablike lesions. Root tissue was surface-disinfested by immersion in a 60 C water bath for 30 min. An actinomycete isolated from scab lesions was characteristic of Streptomyces scabies Lambert and Loria (1), except that spore surfaces were echinulate rather than smooth. Scab developed on 78% of Chancellor carrots grown in steam-sterilized potting mix infested with spore suspensions of this strain. Symptoms were identical to those observed on carrots from the field, and a Streptomyces sp. identical to the strain used in soil infestation was reisolated from lesions. Each of three soils was infested with a different strain of S. scabies isolated from and pathogenic on potato; 40% of carrots in one soil developed typical scab symptoms. The etiology of carrot scab has recently been reported to be a complex of abiotic factors in the United States, whereas S. scabies has been reported as the causal agent in Canada and Europe.

Reference: (1) D. H. Lambert and R. Loria. Int. J. Syst. Bacteriol. 39:387, 1989.