Probabilities of detecting Clavibacter michiganensis subsp. sepedonicus in seed potato lots with serological tests
In potato certification schemes a zero tolerance is required for Clavibacter michiganensis subsp. sepedonicus (EPPO A2 quarantine pest), but the pathogen may not always be detected during visual inspection of seed crops. Serological methods (IF, ELISA) are available to detect latent infections in samples of stems or tubers. The probability of detecting ring rot in seed potato lots, although dependent on test efficacy, is also a function of sample size and incidence of infection. Sample size can be set empirically within the limits of practicability and cost effectiveness. The incidence of the disease is unknown, but generally tends to be low. Tests can be done on stems of mother plants or on the progeny tubers (seed potato tubers). But it has been shown that bacterial populations were higher in stems than in tubers. Studies were carried out in Canada to compare proportions of stems and progeny tubers that become infected on plants grown from seed pieces inoculated at different rates of inoculum with ring rot, in four different cultivars (cvs. Red Pontiac, Russet Burbank, Superior, Katahdin).
The incidence of asymptomatic stem and tuber infections were determined in the four potato cultivars at three levels of inoculum. At the high inoculum level, 51-93 % of the stems were affected at 80 days after planting, and 10-59 % of the tubers were infested at harvest. Probabilities of detecting Clavibacter michiganensis subsp. sepedonicus in stem and tuber samples under simple random sampling were calculated. The calculated probabilities were compared with those obtained in preliminary field experiments on two potato cultivars (cvs Red Pontiac, Russet Burbank). Results showed that probabilities of detection, calculated for a sample of 400 stems from an hypothetical field of 10 ha, planted with 330,000 seed tubers of which 1% were infected, where higher for all inoculum levels (low, medium and high) at 80 days after planting, than at 60 days. The actual detection levels for the two cultivars planted in the field with predetermined incidence levels of ring rot infected plants were reasonably close to predicted probabilities.
Sources
De Boer, S.H.; Hall, J.W. (1996) The probability of detecting Clavibacter michiganensis subsp. sepedonicus by indexing seed potato lots with serological tests.
Journal of Phytopathology, 144(9-10), 459-463.