Standard area diagrams for pecan leaf scab: Effect of rater experience and location, and leaf size on reliability and accuracy of visual estimates

Authors: Hilton, Angelyn; Bock, Clive; Wang, Xinwang; JO, YOUNG-KI - Texas A&M University; CONNER, PATRICK - University Of Georgia; RANDALL, JENNIFER - New Mexico State University; Chatwin, Warren

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2024
Publication Date: 1/26/2024
Citation: Hilton, A.E., Bock, C.H., Wang, X., Jo, Y., Conner, P., Randall, J., Chatwin, W.B. 2024. Standard area diagrams for pecan leaf scab: Effect of rater experience and location, and leaf size on reliability and accuracy of visual estimates. Plant Disease. 108:1820-1832. https://doi.org/10.1094/PDIS-09-23-1947-RE.
DOI: https://doi.org/10.1094/PDIS-09-23-1947-RE

Interpretive Summary: Plant disease assessments are performed to determine the susceptibility of plant hosts to disease, how the disease spreads, and disease management approaches. Pecan scab is considered the most economically significant disease of pecan. Previous efforts to categorize pecan scab severity in different pecan cultivars were often inaccurate and unreliable. There is a critical need to improve and develop new protocols to rate pecan scab severity more accurately. Leaf images with varying amounts of pecan scab, measured by image analysis software, were used to develop a new field guide to assist raters. A comparison between ratings performed with and without the aid of the field guide showed that the new field guide significantly improved the accuracy, precision, and reliability of disease ratings. The new field guides will be valuable tools for scientists studying pecan scab disease and identifying scab-resistant pecan cultivars.

Technical Abstract: Plant disease assessments of scab (Venturia effusa), an economically significant disease of pecan, are critical for determining pecan cultivar susceptibility, epidemiology of the pathogen, and integrated pest management approaches. We developed a Standard Area Diagram (SAD) to aid in plant disease assessments of pecan leaflet scab. Leaves with scab lesions were harvested and scanned into the computer using a flatbed scanner at 600 dpi, and Fiji (ImageJ) was used to determine the true percent disease coverage. The SAD was developed with 10 leaflets ranging in severity from 0.2% to 48.9%. Forty 'small' and forty 'large' leaflet images were randomized for evaluator assessments. The images were assessed twice by 37 evaluators, first without and then with the SAD as a guide. Data were subjected to analysis using Lin’s Concordance Correlation Coefficient (LCCC) to determine the agreement of ratings between evaluators and the true percent disease coverage, and an Intra-class Correlation Coefficient (ICC) analysis was used to determine interrater reliability. The effects of evaluator experience and leaflet size were also determined. The SAD improved the agreement between evaluators and the true values (LCCC, pc = 0.69 and 0.84 without and with the SAD, respectively). The reliability of estimates was improved, and there was no significant difference found between evaluators when the SAD was used. Evaluators had a significantly better agreement when rating 'small' leaves (LCCC, pc = 0.86) compared to 'large' leaves (LCCC, pc = 0.82), highlighting the impact of psychophysics on field evaluations of plant disease severity. The new SAD will serve as an improved tool for performing pecan leaf scab assessments by the pecan research community.