Transmission of Xanthomonas gardneri to tomato seedlings during transportation and transplanting.
Xanthomonas gardneri is the common causal agent of bacterial spot affecting field tomatoes in Ontario, Canada but spread during seedling transportation and transplanting is poorly understood. Spread during transportation and irrigation was determined by placing symptomatic seedlings at the top of a simulated plug trailer with healthy seedlings 30.5, 61.0, 91.5, and 122 cm below to assess pathogen movement with different irrigation treatments. The incidence of seedlings with symptoms 14 days after irrigation in the top to bottom (3.9%) and bottom to top (5.4%) treatments was greater than the dip treatment (0.4%). Symptoms were found at all distances below the inoculation point. A second set of experiments looked at spread in a transplanter by placing wet or dry symptomatic seedlings through the transplanter prior to healthy seedlings. Epiphytic X. gardneri was found on healthy seedlings from all diseased treatments 14 days after treatment, indicating transmission of X. gardneri during transplanting. A third set of experiments assessed the impact of transplanter transmission in the field by monitoring symptom appearance in initially healthy seedlings planted in rows with either machine-transplanted diseased plants or hand-planted diseased plants. There was no difference in the time to symptom appearance, except for one plant position which was not in the direction of transplanting. Thus, factors other than contaminated transplanters may be more important in X. gardneri transmission under field conditions. Best management practices for bacterial spot in field tomatoes should consider irrigation practices in plug trailers, while the contribution of contaminated transplanters to epidemics is likely limited.