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Abstract
Pecan scab, caused by Venturia effusa, is the most economically damaging disease affecting pecan (Carya illinoinensis) in the southeastern United States. Managing scab requires fungicides and is often the greatest operating cost for commercial growers. The work detailed here aimed to characterize the biology of fungicide-resistant V. effusa with the goal of improving fungicide use patterns throughout Georgia pecan orchards. An intron downstream of position 143 in the V. effusa cytochrome b gene was identified in all sequenced isolates from these studies indicating that the risk of quinone outside inhibitor resistance is relatively low. A novel glycine to serine amino acid substitution was identified in a subset of 14 isolates and a detached leaf assay was developed to screen sensitivity of isolates to azoxystrobin. Isolates with G137S were less sensitive than wild-type isolates. The relationship between fungicide sensitivity determined using in vitro bioassays and fungicide efficacy in the field was explored but a consistent quantitative relationship was not observed. Sensitivity and efficacy results obtained from fentin hydroxide reveal that relative insensitivity values between 0.6 and 40.9% could occur without causing a likely control failure, although efficacy was reduced compared with other treatments. Interestingly, when tebuconazole insensitivity values were between 34.6 and 69.3%, a control failure was likely on trees treated with tebuconazole. Spatial and temporal dynamics of fungicide sensitivity were examined in a commercial orchard over 3 years. Sensitivity values differed in each year and were spatially dependent for propiconazole and thiophanate-methyl in 2017, but in no other instance. A separate study revealed that in most cases, a sample size of three groups of leaflets to be sufficient for sensitivity testing, although a fourth group could improve precision. Fitness attributes were compared, and phenotypic stability was assessed between isolates sensitive and insensitive to fentin hydroxide and tebuconazole. Conidial germination was negatively correlated with fentin hydroxide insensitivity, and tebuconazole insensitivity was negatively correlated with hypersensitivity to 1.0 mM H2O2. Tebuconazole sensitivity did not differ over consecutive generations but fentin hydroxide sensitivity did, indicating that insensitivity is likely a temporary adaptation of the pathogen to the fungicide.