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.