Transmission from seed to seedling and secondary spread of Xanthomonas campestris pv. campestris in Brassica transplants: Effects of dose and watering regime

The effects of inoculum load and watering regime on the transmission of Xan thomonas campestris pv. campestris from seed to seedlings of cauliflower we re investigated. Seed, inoculated with different concentrations of bacteria , was sown in commercial module trays and subjected to four different water ing regimes: high frequency overhead spray, low frequency overhead spray, h igh frequency capillary and low frequency capillary. Visible symptoms were recorded and leaf washings were carried out to detect the pathogen on sympt omless plants. The effects of treatments on symptoms and on the proportion of contaminated but symptomless plants was similar. Initially, they were in fluenced only by the dose of bacteria with little difference between the wa tering regimes, but later the proportion of plants with symptoms was greate r for plants subjected to overhead watering, due to spread and secondary in fection. Generalised linear models were fitted to the data relating the pro portion of symptomless contaminated plants or the proportion of plants with symptoms, p, to the mean dose of bacteria per seed, d, and the number of o verhead waterings, noh. The equations were: p = 1-exp(-0.014 . d(0.32). noh (0.045)) for symptomless contaminated/infected plants and p = 1-exp(-0.0056 . d(0.44). noh(0.014)) for plants with symptoms. These models indicated th at the one-hit probability for transmission of the pathogen (i.e. with/with out visible symptoms) was 0.014 and for infection (i.e. with visible sympto ms) was 0.0056.