Plant Growth Stimulants and Defense Activators Fail to Control Phony Peach Disease in Mature Peach Orchards

The bacterium Xylella fastidiosa subspecies multiplex causes phony peach disease (PPD), a serious disease of peach in the southeastern United States. The current management strategy for PPD is removal of symptomatic trees, but other management options should be explored. Field efficacy trials with plant growth stimulants and plant defense activators were conducted in orchards of ‘Julyprince’ in 2021 and 2022, and ‘Springprince’ in 2021. Treatments were applied either as a foliar or root drench application once a month for three consecutive months during the growing season. Treatments included Rio, ProGibb 40SG, Maximizer, Actigard 50WG, and K-phite 7LP. In 2021, ProGibb 40SG provided increased shoot growth in both varieties, but cycle threshold (Ct) values did not suggest that any treatment affected bacterial titer. In 2022, applications were initiated one month later and no significant effects on shoot growth parameters or bacterial titer were observed among treatments. The reduction in symptoms after ProGibb 40SG application and the fact that healthy trees responded similarly indicates the growth response was due to the plant responding to the hormonal treatment, and not an effect on the pathogen, the titer of which remained similar. Thus, ProGibb 40SG may have very limited value for improving PPD tree health.