Risk of Spreading Soft Rot Through Cutting Dips Against Whiteflies in Greenhouse-Grown Poinsettia.

Dipping is a quick and cost-effective technique to reduce pest infestations on unrooted cuttings of greenhouse ornamental crops. Large numbers of cuttings are immersed in an insecticidal treatment, e.g., biopesticides and/or insecticidal soap, before they are stuck in the growing medium and rooted. This research investigated the risk of cross-contamination of poinsettia cuttings with subsp. , a potentially devastating pathogen causing soft rot, through the dipping process. Sampling at four commercial greenhouses showed that subsp. was present in all dip suspensions during and after processing poinsettia cuttings; concentrations up to 1 × 10 CFU/ml were detected. A laboratory experiment determined that subsp. -infected cuttings could contaminate clean dip suspensions to similar levels. These results indicated that there is potential for disease transfer during dipping. The potential for cross-contamination of healthy cuttings was evaluated by immersing poinsettia cuttings in dip suspensions artificially inoculated with subsp. (from 1 × 10 to 1 × 10 CFU/ml). Disease incidence increased as subsp. concentrations in the dip suspension increased and the variety 'Prestige Red' was more susceptible than 'Freedom White.' However, even at the highest subsp. concentration of 1 × 10 CFU/ml, the proportion of diseased cuttings was low at 6% for var. 'Freedom White,' but higher at 21% for var. 'Prestige Red.' We conclude that subsp. transfer among unrooted poinsettia cuttings through the dipping process is relatively low although some varieties are sensitive to high levels of inoculum. Even so, strict sanitation practices are still important to prevent build-up of inoculum in the dip treatment.