Visual assessment of phony peach disease: evaluating rater accuracy and reliability

Authors: JOHNSON, KENDAL - University Of Georgia; BRANNEN, PHILLIP - University Of Georgia; Chen, Chunxian; Bock, Clive

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2023
Publication Date: 4/19/2024
Citation: Johnson, K.A., Brannen, P.M., Chen, C., Bock, C.H. 2024. Visual assessment of phony peach disease: evaluating rater accuracy and reliability. Plant Disease. 108(4)/930-940. https://doi.org/10.1094/PDIS-11-22-2669-RE.
DOI: https://doi.org/10.1094/PDIS-11-22-2669-RE

Interpretive Summary: Phony peach disease (PPD) is a damaging disease of peach found predominantly in central and southern Georgia. It is becoming more prevalent. PPD is caused by a bacterial disease called Xylella fastidiosa subsp. multiplex. Management of the disease in the field relies on visual identification of PPD with subsequent removal of those trees, as the disease may be passed from tree to tree. Thus, accurate detection and rapid removal of symptomatic trees are crucial. We determined the accuracy of visual identification of symptoms compared to known infection based on molecular identification (ID) of the causal bacteria. Visual accuracy of PPD compared to the molecular ID was variable, but experienced raters were more accurate at identifying PPD trees. In three orchards mean rater accuracy for experienced and inexperienced raters was 0.882 and 0.805, 0.914 and 0.816, and 0.898 and 0.807, respectively (where 1.000 is complete accuracy). All raters misidentified some healthy (as PPD) and PPD (as healthy) trees. Almost all raters overestimated the incidence of PPD in the orchards (inexperienced raters more so). Reliability among raters was moderate to good, regardless of experience. Further research is needed to develop accurate and reliable methods of detection to aid management of PPD.

Technical Abstract: 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 2020 in ‘Scarletprince’ (SP2020). With no prior knowledge of qPCR results, all trees in each orchard were assessed by a cohort of 5 experienced and 5 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, mean rater accuracy for experienced and inexperienced raters was 0.882 and 0.805, respectively. For JP2020 mean rater accuracy for experienced and inexperienced raters was 0.914 and 0.816, respectively, and for SP2020 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, although, experienced raters had significantly lower FN rates compared to 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.