Effect of Cultivar, Inoculum Dose, and Strain ofClavibacter michiganensesubsp.sepedonicumon Symptom Development in Potatoes

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Abstract
Inoculation of potato (Solanum tuberosum) seed pieces with Clavibacter michiganense subsp. sepedonicum (=Corynebacterium sepedonicum) through wounds beneath sprouts invariably resulted in infection in cultivars Green Mountain, Norland, and Superior at inoculum doses from 10 to 106 cfu/seed piece, but incidence of infection varied from 70 to 100% in cultivars Katahdin, Ontario, and Russet Burbank. Infection did not affect field stand. Latent period (time from inoculation and planting to wilting of foliage) was significantly shortened by increasing inoculum dose in 1984 but not in 1981. Time of cultivar maturity correlated positively with length of latent period. The difference between time of symptom onset in infected plants and time of senescence in controls was significantly reduced by increasing inoculum dose. Stunting was a distinct symptom of bacterial ring rot in some cultivars. Strain SS43 of C. m. subsp. sepedonicum had a significantly shorter latent period than strain SS13 and caused more stunting than strain SS13 in Russet Burbank but not in Katahdin or Norland. Variation in latent period and severity of stunting may affect disease detection in the context of seed potato certification.