Disease Incidence-Severity Relationships on Leaflets, Leaves, and Fruit in the Pecan-Venturia effusa Pathosystem

Standard Area Diagrams for Pecan Leaf Scab: Effect of Rater Experience, Location, and Leaf Size on Reliability and Accuracy of Visual Estimates

Assessments of the severity of scab (Venturia effusa), an economically significant disease of pecan, are critical for determining pecan cultivar susceptibility, disease epidemiology, and integrated disease management approaches. We developed a standard area diagram (SAD) set to aid in assessments of pecan leaflet scab. Leaflets with scab lesions were harvested and scanned using a flatbed scanner at 600 dpi, and Fiji (ImageJ) was used to determine the actual percent disease severity. The SADs had 10 leaflets ranging in severity from 0.2 to 48.9%. Forty "small" (1.34 to 7.43 cm2) and 40 "large" (7.67 to 25.9 cm2) leaflet images were randomized for rater assessments. The images were assessed twice by 36 raters, first without and then with the SADs as a guide. Data were subjected to analysis using Lin's concordance correlation coefficient (LCC, pc) to determine the accuracy of ratings and by intraclass correlation coefficient (ICC) analysis to determine interrater reliability. The effects of rater experience, rater location, and leaflet size were also determined. The SADs significantly improved the agreement between raters and the actual values (LCC, pc = 0.70 and 0.84 without and with the SADs, respectively). The reliability of estimates was improved (ICC = 0.54 and 0.82 without and with the SADs, respectively). The effect of rater location on overall concordance was significant without and with the SADs based on an analysis of variance using a generalized linear model and lsmeans separation (P < 0.05). A generalized linear mixed model analysis revealed that there was a significant interaction between rater location, experience, and the use of the SADs, with some raters having greater improvement in generalized bias and concordance. Raters had a significantly better accuracy when rating "small" leaves (LCC, pc = 0.86) compared with "large" leaves (LCC, pc = 0.82) when using the SADs, highlighting the impact of psychophysics on field evaluations of plant disease severity. The proposed SADs will serve as an improved tool for performing pecan leaflet scab assessments by the pecan research community.

Understanding the ramifications of quantitative ordinal scales on accuracy of estimates of disease severity and data analysis in plant pathology

The severity of plant diseases, traditionally defined as the proportion of the plant tissue exhibiting symptoms, is a key quantitative variable to know for many diseases but is prone to error. Plant pathologists face many situations in which the measurement by nearest percent estimates (NPEs) of disease severity is time-consuming or impractical. Moreover, rater NPEs of disease severity are notoriously variable. Therefore, NPEs of disease may be of questionable value if severity cannot be determined accurately and reliably. In such situations, researchers have often used a quantitative ordinal scale of measurement-often alleging the time saved, and the ease with which the scale can be learned. Because quantitative ordinal disease scales lack the resolution of the 0 to 100% scale, they are inherently less accurate. We contend that scale design and structure have ramifications for the resulting analysis of data from the ordinal scale data. To minimize inaccuracy and ensure that there is equivalent statistical power when using quantitative ordinal scale data, design of the scales can be optimized for use in the discipline of plant pathology. In this review, we focus on the nature of quantitative ordinal scales used in plant disease assessment. Subsequently, their application and effects will be discussed. Finally, we will review how to optimize quantitative ordinal scales design to allow sufficient accuracy of estimation while maximizing power for hypothesis testing.

A Comparison of Ground-Based Air-Blast Sprayer and Aircraft Application of Fungicides to Manage Scab in Tall Pecan Trees

Pecan scab (caused by Venturia effusa) is a destructive disease of pecan in the southeastern United States. Susceptible cultivars must be sprayed with fungicide every 10 to 21 days to ensure yield and kernel quality. Fungicide is most often applied using large orchard air-blast sprayers. Pecan trees grow tall, and air-blast sprays result in a gradient in spray deposition and consequently of scab. Aerial fungicide application is also practiced. Disease distribution and spray deposition of the two methods have not been compared but will provide information aiding decisions on spray application methods. We compared air-blast, aerial, and air-blast + aerial applications for efficacy controlling scab at five heights in the canopy of 25-m cultivar Schley pecan trees. There was a negative relationship between scab severity and height in control trees, a positive linear relationship with height in air-blast treated trees, and a generally negative linear relationship between scab severity and height in aerially treated trees. Air-blast + aerial treatments resulted in low severity of scab at all heights. Spray deposition on water-sensitive cards indicated a declining gradient with height using an air-blast sprayer, whereas aerial applications resulted in a low deposition at all sample heights. Air-blast sprays tended to result in less good control at heights >12.5 m, and aerially treated trees at ≤7.5 m. The results provide insight into the efficacy and advantages of these methods for applying fungicide to control scab in tall pecan trees; further research is needed to better understand the impact of frequency and timing of these two methods.