Vega Byron, Dewdney Megan M
Citrus Research and Education Center, University of Florida, Lake Alfred.
Plant Dis. 2014 Jan;98(1):67-76. doi: 10.1094/PDIS-04-13-0449-RE.
Chemical control, based on copper and quinone outside inhibitor (QoI) fungicides, has been essential for the management of brown spot of citrus, caused by Alternaria alternata. However, QoI 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 μg/ml for AZ and 1 μg/ml for PYR. The mean EC values for sensitive isolates were 0.139 and 0.020 μg/ml for AZ and PYR, respectively. The EC values of both fungicides were highly correlated (P < 0.0001), indicating cross resistance. The proportion of resistant isolates differed significantly (P < 0.0001) among cultivars and with QoI application 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 QoI resistance was determined for a subset of 235 isolates using polymerase chain reaction restriction fragment length polymorphism of the cytochrome b gene. All resistant isolates showed the point mutation 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, suggesting a higher selection pressure for resistant population profile II.
基于铜和醌外抑制剂（QoI）类杀菌剂的化学防治，对于由链格孢引起的柑橘褐斑病的管理至关重要。然而，最近在佛罗里达州发现了QoI防治失败的情况。2008年至2012年，对来自46个柑橘园的817个链格孢单孢分离株进行了对嘧菌酯（AZ）和吡唑醚菌酯（PYR）的敏感性检测。在这些分离株中，57.6%对两种杀菌剂均具有抗性，其抑制50%生长的有效浓度（EC）值对于AZ大于5μg/ml，对于PYR大于1μg/ml。敏感分离株对AZ和PYR的平均EC值分别为0.139和0.020μg/ml。两种杀菌剂的EC值高度相关（P<0.0001），表明存在交叉抗性。抗性分离株的比例在不同品种间（P<0.0001）以及与QoI施用频率之间（P<0.0001）存在显著差异。然而，抗性与田间病害严重程度（低、中、高）或分离株毒力（P=0.397）无显著相关性（P=0.364）。使用细胞色素b基因的聚合酶链反应-限制性片段长度多态性方法，对235个分离株的一个子集确定了QoI抗性的分子基础。所有抗性分离株均显示出G143A点突变。根据内含子的有无，分离株分别被分类为I型和II型。II型分离株的抗性频率显著更高（P<0.0001），表明II型抗性群体面临更高的选择压力。
