Pathogen distribution, incubation period and seedling transmission resulting from secondary contamination of pepper seeds with Xanthomonas euvesicatoria

The role of externally contaminated pepper seeds on secondary contamination of healthy seed lots and its significance on pathogen distribution, incubation period and seedling transmission of Xanthomonas euvesicatoria was investigated. In two independent experiments, a contaminated seed was either retained or removed following thorough mixing with a healthy seed lot (n = 100 seeds / lot). Subsequent distributions of X. euvesicatoria populations in individual seeds within a seed lot (n = 50 seeds / lot) were determined by dilution plating on a semi-selective medium. The replicates of similarly treated lots (n = 100 seeds / lot) were planted under greenhouse conditions to assess the incubation period and seed-to-seedling transmission of pathogen. All the seed lots were contaminated with X. euvesicatoria, irrespective of retention or removal of the contaminated seed. Whether the contaminated seed was retained or removed, bacterial populations in all infested seed lots were positively and highly skewed and followed an inverse Gaussian distribution. Under favourable greenhouse conditions, the seed lots with the infested seed retained, had incubation periods of 12-21 days after planting (DAP) and seed-to-seedling transmission ranged between 10 and 62%. In contrast, seed lots with the infested seed removed, had incubation periods from 36-42 DAP and the percent seed-to-seedling transmission ranged from 8 to 64%. These results suggest that secondary contamination of healthy seeds from externally contaminated infested seeds can occur. The results also indicated that removal of primary-infested seeds from a seed lot does not change the infestation status of a seed lot; however, incubation periods of seedlings from such lots were longer than from seed lots where the primary-infested seed was retained. This could affect interpretation of results from seedling grow-out assays (SGO) used in seed health tests. The highly skewed distributions of bacteria in infested seed lots have implications on seed-to-seedling transmission and incubation periods under greenhouse conditions, which could also influence interpretation of SGO assays.