A survey of the distribution of <i>Pseudomonas amygdali</i> pv. <i>loropetali</i> present on and in stems of <i>Loropetalum chinense</i> shrubs in ornamental nurseries

Bacterial gall of Loropetalum chinense, caused by Pseudomonas amygdali pv. loropetali (PAL), was reported to have become an invasive problem in nurseries due to a lack of proven control methods. We surveyed six nurseries to assess the frequency of loropetalum plants with and without galls and to assess PAL density on and in woody twigs in three vertical strata (base, mid-height, and new growth sections) of plants. Quantitative counts were determined using a quantitative real-time polymerase chain reaction (qPCR) method involving primers and probe specific for PAL. The whole plant frequency of galled plants within a single cultivar block of loropetalum varied from 8 to 92%, with three nurseries having moderate levels of 27 to 39%. No consistent relationship was observed between gall frequency and PAL abundance across canopy strata. The pattern of mean DNA copy number of PAL in twigs of the three vertical plant strata varied between nurseries, with the highest copy number occurring more frequently in the base-stratum and least frequently in the new-stratum. The presence of PAL in the phyllosphere of loropetalum plants indicates a risk for spreading the pathogen on vegetatively propagated stem cuttings.