Spatial variation and temporal dynamics of fungicide sensitivity in Venturia effusa within a pecan orchard

Authors: STANDISH, JEFF - University Of Georgia; BRENNEMAN, TIM - University Of Georgia; Bock, Clive; STEVENSON, KATHERINE - University Of Georgia

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/28/2020
Publication Date: 2/1/2021
Citation: Standish, J.R., Brenneman, T.B., Bock, C.H., Stevenson, K.L. 2021. Spatial variation and temporal dynamics of fungicide sensitivity in Venturia effusa within a pecan orchard. Plant Disease. 105: 377-383. https://doi.org/10.1094/PDIS-04-20-0889-RE.
DOI: https://doi.org/10.1094/PDIS-04-20-0889-RE

Interpretive Summary: The spatial and temporal variation in fungicide sensitivity of Venturia effusa, cause of pecan scab was studied in a pecan orchard in southwestern GA. The orchard was divided into quadrats and samples were collected from each quadrat to be tested for sensitivity to fentin hydroxide, propiconazole, and thiophanate-methyl fungicides. Insensitivity to all three fungicides was significantly lower in 2016 compared with 2015, but significantly greater for fentin hydroxide and thiophanate-methyl in 2017. There was clustering in sensitivity to propiconazole in 2017 and to thiophanate-methyl in 2015 and 2017, but not for any other fungicide by year combinations. Omnidirectional spatial dependency was observed for sensitivity to propiconazole and thiophanate-methyl in 2017. A separate sampling was conducted in all 3 years to identify an appropriate sampling size and pattern for fungicide sensitivity screening. Results showed that in most cases, a leaflet sample size of 45 in three groups 15 was sufficient for quantifying sensitivity of two of the three tested fungicides in this study. These results reveal that considerable biological variation in fungicide sensitivity exists in orchard-scale populations of V. effusa and that the spatial characteristics of those populations may differ in space depending on the growing season.

Technical Abstract: An 18-ha commercial pecan orchard was sampled over 3 years to study the spatial and temporal variation in fungicide sensitivity of Venturia effusa, cause of pecan scab. The orchard was divided into an 8×8 grid of 64 quadrats, each containing nine trees (unless there were missing trees), and samples were collected from each quadrat to be tested for sensitivity to fentin hydroxide, propiconazole, and thiophanate-methyl. Insensitivity to all three fungicides was significantly lower in 2016 compared with 2015, but significantly greater for fentin hydroxide and thiophanate-methyl in 2017. Significant spatial autocorrelation was observed in sensitivity to propiconazole in 2017 and to thiophanate-methyl in 2015 and 2017, indicating clustering, but not for any other fungicide by year combinations. Omnidirectional spatial dependency was observed for sensitivity to propiconazole and thiophanate-methyl in 2017. In both instances, the semivariance increased linearly with lag distance; however, the range of spatial dependence was greater than 276.5 m and could not be estimated accurately. Additionally, a separate sampling was conducted in all 3 years to identify an appropriate sampling size and pattern for fungicide sensitivity screening. Results showed that in most cases, a leaflet sample size of 45 in three groups 15 was sufficient for quantifying sensitivity of two of the three tested fungicides in this study. These results reveal that considerable biological variation in fungicide sensitivity exists in orchard-scale populations of V. effusa and that the spatial characteristics of those populations may differ in space depending on the growing season.