Disease incidence–severity relationships on leaflets, leaves, and fruit in the pecan–Venturia effusa pathosystem

Authors: Bock, Clive; CHIANG, KUO-SZU - Chung Hsing University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/19/2019
Publication Date: 11/1/2019
Citation: Bock, C.H., Chiang, K. 2019. Disease incidence–severity relationships on leaflets, leaves, and fruit in the pecan–Venturia effusa pathosystem. Plant Disease. 103: 2865-2876. https://doi.org/10.1094/PDIS-11-18-1950-RE.
DOI: https://doi.org/10.1094/PDIS-11-18-1950-RE

Interpretive Summary: Scab is the most destructive disease of pecan in the southeastern USA. Incidence (I)-severity (S) relationships have not previously been characterized in this pathosystem, but incidence measures can save time and should have higher accuracy compared to estimates of severity. Ten scab-susceptible cultivars were assessed for I and S of scab on fruit (1972 trees) and foliage (compound leaves and leaflets, 1129 trees) between 2010 and 2014. Cultivar and year had a significant interaction with incidence of scab, and thus analysis of covariance (ANCOVA) is not appropriate. So, individual analyses were performed for each cultivar and year. Back-transformed values showed that the differences were generally numerically small. The observation that low severity persists until a high incidence of scab is achieved may limit the ability of incidence data to clearly differentiate treatment effects, even when based on the CLL transformation. But if found to be effective, and if used, it would reduce the labor requirements, and result in more accurate data being obtained as incidence estimates do not tend to suffer from the same subjective biases as do visual estimates of severity.

Technical Abstract: The most destructive disease of pecan in the southeastern USA is scab, caused by Venturia effusa. Incidence (I)-severity (S) relationships have not previously been characterized in this pathosystem, but incidence measures can save time and should have higher accuracy compared to estimates of severity. Ten scab-susceptible cultivars were assessed for I and S of scab on fruit (1972 trees) and foliage (compound leaves and leaflets, 1129 trees) between 2010 and 2014. Samples were assessed on a tree basis, and sample size ranged from 10 to 100 specimens per tree. The range in mean I and S was different depending on the organ (fruit I = 0 to 100%, S = 0 to 100%; compound leaves and leaflets, I = 0 to 100%, S = 0 to 10.1%, respectively). However, mean I could be 100% at a mean S <2.0% for fruit, compound leaves and leaflets. Data were transformed by complementary log-log prior to linear regression analysis. A linear regression model described the relationship between I and S per tree for fruit (P=0.0001, R2=0.61), compound leaves (P=0.0001, R2=0.82) and leaflets (P=0.0001, R2=0.91) for all cultivars. The analysis of regression model showed that both cultivar and year had a significant interaction with incidence of scab, and thus analysis of covariance (ANCOVA) is not appropriate. So, individual analyses were performed for each cultivar and year. Back-transformed values showed that the differences were generally numerically small. The observation that low severity persists until a high incidence of scab is achieved may limit the ability of incidence data to clearly differentiate treatment effects, even when based on the CLL transformation. But if found to be effective, and if used, it would reduce the labor requirements, and result in more accurate data being obtained as incidence estimates do not tend to suffer from the same subjective biases as do visual estimates of severity.