Vega, B., and Dewdney, M. M. 2014. Distribution of Qol resistance in populations of tangerine-infecting Alternada alternata in Florida. Plant Dis. 98:67-76.Chemical control, based on copper and quinone outside inhibitor (Qol) fungicides, has been essential for the management of brown spot of citrus, caused by Alternarla alternata. However, Qol control failures were detected recently in Florida. From 2008 to 2012, 817 monoconidial isolates of A. alternata from 46 citrus orchards were examined for sensitivity to azoxystrobin (AZ) and pyraclostrobin (PYR). Of the isolates, 57.6% were resistant to both fungicides, with effective concentration to inhibit 50% growth (EC,,,) values greater than 5 |ag/ml for AZ and 1 |ig/ml for PYR. The mean EC5,, values for sensitive isolates were 0.139 and 0.020 pg/ml for AZ and PYR, respectively. The EC50 values of both fungicides were highly correlated [P < 0.0001), indicating cross resistance. The proportion of resistant isolates differed sig-nificantly (P < 0.0001) among cultivars and with Qol applicadon frequency (P < 0.0001). However, resistance was not significantly related (P = 0.364) to disease severity in the field (low, moderate, and high) or isolate virulence {P = 0.397). The molecular basis for Qol resistance was determined for a subset of 235 isolates using polymerase chain reaction restricdon fragment length polymorphism of the cytochrome b gene. All resistant isolates showed the point mutadon G143A. Based on the presence of one or two introns, isolates were classified as profile I and profile II, respectively. The resistance frequency was significantly higher (P < 0.0001) in isolate profile II, suggesdng a higher selecdon pressure for resistant population profile II.Alternaria brown spot (ABS), caused by Alternarla alternata (Fr.;Fr.) Keissl., is a serious disease of tangerine cultivars {Citrus reticulata Blanco) and their hybrids in commercial citrus orchards in humid subtropical and semiarid growing regions (58). This fungus produces the host-selective ACT toxin (32,33), which plays an important role in host specificity and isolate virulence (23,34). Florida has 7,000 ha of tangerine and tangerine hybrids for freshfruit production (21). More than two-thirds of the tangerine cultivars planted in Florida are susceptible to ABS, including 'Minneola', 'Nova', and 'Orlando' tángelo; 'Dancy' and 'Sunburst' tangerine; 'Murcott' tangor, and some Dancy hybrids such as 'Lee' (56). The fungus can affect leaves, twigs, and young fruit, cause considerable yield loss, and produce blemishes on fruit that reduce marketability (56,58). Management of ABS includes the use of cultural methods such as disease-free nursery stock, wide tree spacing, reduction of excessive vegetadve growth, the elimination of overhead irrigation, and fungicide applications (16,57). Although cultural practices reduce disease severity, ABS control is largely dependent on fungicide applications (8,16,56). Copper-based fungicides (Fungicide Resistance Action Committee [FRAC] code Ml), ferbam (FRAC code M3), and ...