RESUMEN
Phony peach disease (PPD), found predominantly in central and southern Georgia, is a re-emerging disease caused by Xylella fastidiosa (Xf) subsp. multiplex. Accurate detection and rapid removal of symptomatic trees are crucial to effective disease management. Currently, peach producers rely solely on visual identification of symptoms to confirm PPD, which can be ambiguous if early in development. We compared visual assessment to quantitative PCR (qPCR) for detecting Xf in 'Julyprince' in 2019 and 2020 (JP2019 and JP2020) and in 'Scarletprince' in 2020 (SP2020). With no prior knowledge of qPCR results, all trees in each orchard were assessed by a cohort of five experienced and five inexperienced raters in the morning and afternoon. Visual identification accuracy of PPD was variable, but experienced raters were more accurate when identifying PPD trees. In JP2019, the mean rater accuracy for experienced and inexperienced raters was 0.882 and 0.805, respectively. For JP2020, the mean rater accuracy for experienced and inexperienced raters was 0.914 and 0.816, respectively. For SP2020, the mean rater accuracy for experienced and inexperienced raters was 0.898 and 0.807, respectively. All raters had false positive (FP) and false negative (FN) observations, but experienced raters had significantly lower FN rates compared with the inexperienced group. Almost all raters overestimated the incidence of PPD in the orchards. Reliability of visual assessments was demonstrated as moderate to good, regardless of experience. Further research is needed to develop accurate and reliable methods of detection to aid management of PPD as both FPs and FNs are costly to peach production.
RESUMEN
Peach is an important specialty fruit crop in the United States, and phony peach disease (PPD), caused by Xylella fastidiosa subsp. multiplex, has been a major cause of yield loss since it was first observed in 1885. Under a federal eradication program, surveys of PPD were conducted from 1929 to 1972, when the program was terminated. No surveys have been conducted in approximately 50 years; therefore, the current prevalence of PPD in the United States is unknown, especially in the Southeast, where damage was previously most severe. To ascertain the status of PPD, we surveyed orchards in Alabama, Florida, Georgia, and South Carolina from June to August 2020 and, except for South Carolina and northern Georgia, PPD was prevalent. Trees in 17 orchards were subjected to confirmation of X. fastidiosa using the AmplifyRP XRT+ for X. fastidiosa to corroborate our visual assessments; based on these tests, PPD incidence in the orchards ranged from 0 to 30.5%. Ancillary written surveys of relative PPD presence and prevalence were sent to fruit pathologists from universities in 20 states where PPD was historically reported. Only 35.0% of respondents reported that PPD either currently or recently occurred in their state and, of these, three reported PPD to be of significant concern. The results of the physical and written surveys indicate that PPD remains prevalent mainly in the southeastern region of the United States but, in other states where previously reported, it is either not present or has very low prevalence when compared with historical accounts of the disease.
