The phenotypic stability, fitness components, and ability to cause disease of isolates of the Alternaria alternata tangerine pathotype resistant to quinone-outside inhibitors (QoIs) were studied. Stability of resistance to azoxystrobin (AZ) and pyraclostrobin (PYR) was determined after consecutive transfers on potato dextrose agar (PDA). The sensitivity to QoIs did not change significantly after 10 transfers on PDA compared with the initial sensitivity of all isolates tested. Fitness components evaluated in vitro were mycelial growth, conidial germination, and conidial production. Incubation period, number of lesions per leaf area, and virulence were determined with detached leaf assays using four cultivars: Dancy, Minneola, Murcott, and Sunburst. Variability in fitness components was observed among isolates within the same sensitivity group. As a group, no significant differences in the mean values of these fitness components were observed between resistant and sensitive phenotypes, except for virulence. Resistant isolates were significantly (P < 0.05) more virulent than the sensitive isolates on Dancy, Minneola, and Sunburst but not on Murcott (P = 0.3506). There was no significant correlation between individual fitness components and the level of sensitivity to AZ and PYR. Preventive applications of Abound (commercial formulation of AZ) at full field rates failed to control disease caused by QoI-resistant isolates under greenhouse conditions. Our results suggest that QoI resistance in A. alternata tangerine pathotype is stable in the absence of QoI selection pressure and that resistance development did not affect the fitness of resistant isolates.
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