Bolton Melvin D, Rivera-Varas Viviana, Del Río Mendoza Luis E, Khan Mohamed F R, Secor Gary A
United States Department of Agriculture-Agricultural Research Service, Northern Crop Science Laboratory, Fargo, ND, and the Department of Plant Pathology, North Dakota State University, Fargo.
Department of Plant Pathology, North Dakota State University, Fargo.
Plant Dis. 2012 Dec;96(12):1749-1756. doi: 10.1094/PDIS-03-12-0255-RE.
Cercospora leaf spot (CLS) of sugar beet is caused by the fungus Cercospora beticola. CLS management practices include the application of the sterol demethylation inhibitor (DMI) fungicides tetraconazole, difenoconazole, and prothioconazole. Evaluating resistance to DMIs is a major focus for CLS fungicide resistance management. Isolates were collected in 1997 and 1998 (baseline sensitivity to tetraconazole, prothioconazole, or difenoconazole) and 2007 through 2010 from the major sugar-beet-growing regions of Minnesota and North Dakota and assessed for in vitro sensitivity to two or three DMI fungicides. Most (47%) isolates collected in 1997-98 exhibited 50% effective concentration (EC) values for tetraconazole of <0.01 μg ml, whereas no isolates could be found in this EC range in 2010. Since 2007, annual median and mean tetraconazole EC values have generally been increasing, and the frequency of isolates with EC values >0.11 μg ml increased from 2008 to 2010. In contrast, the frequency of isolates with EC values for prothioconazole of >1.0 μg ml has been decreasing since 2007. Annual median difenoconazole EC values appears to be stable, although annual mean EC values generally have been increasing for this fungicide. Although EC values are important for gauging fungicide sensitivity trends, a rigorous comparison of the relationship between in vitro EC values and loss of fungicide efficacy in planta has not been conducted for C. beticola. To explore this, 12 isolates exhibiting a wide range of tetraconazole EC values were inoculated to sugar beet but no tetraconazole was applied. No relationship was found between isolate EC value and disease severity. To assess whether EC values are related to fungicide efficacy in planta, sugar beet plants were sprayed with various dilutions of Eminent, the commercial formulation of tetraconazole, and subsequently inoculated with isolates that exhibited very low, medium, or high tetraconazole EC values. The high EC isolate caused significantly more disease than isolates with medium or very low EC values at the field application rate and most reduced rates. Because in vitro sensitivity testing is typically carried out with the active ingredient of the commercial fungicide, we investigated whether loss of disease control was the same for tetraconazole as for the commercial product Eminent. The high EC isolate caused more disease on plants treated with tetraconazole than Eminent but disease severity was not different between plants inoculated with the very low EC isolate.
甜菜尾孢叶斑病（CLS）由真菌甜菜尾孢菌引起。CLS的管理措施包括施用甾醇脱甲基化抑制剂（DMI）类杀菌剂戊唑醇、苯醚甲环唑和丙硫菌唑。评估对DMI类杀菌剂的抗性是CLS杀菌剂抗性管理的主要重点。于1997年和1998年（对戊唑醇、丙硫菌唑或苯醚甲环唑的基线敏感性）以及2007年至2010年从明尼苏达州和北达科他州的主要甜菜种植区采集分离株，并评估其对两种或三种DMI类杀菌剂的体外敏感性。1997 - 1998年收集的大多数（47%）分离株对戊唑醇的50%有效浓度（EC）值<0.01 μg/ml，而在2010年未发现该EC范围内的分离株。自2007年以来，戊唑醇的年度中位数和平均EC值总体上一直在增加，且EC值>0.11 μg/ml的分离株频率从2008年到2010年有所增加。相比之下，自2007年以来，丙硫菌唑EC值>1.0 μg/ml的分离株频率一直在下降。苯醚甲环唑的年度中位数EC值似乎稳定，尽管该杀菌剂的年度平均EC值总体上一直在增加。虽然EC值对于衡量杀菌剂敏感性趋势很重要，但尚未针对甜菜尾孢菌对体外EC值与植物体内杀菌剂药效丧失之间的关系进行严格比较。为探究这一点，将12个表现出广泛戊唑醇EC值范围的分离株接种到甜菜上，但未施用戊唑醇。未发现分离株EC值与病情严重程度之间的关系。为评估EC值是否与植物体内的杀菌剂药效相关，用戊唑醇的商业制剂Eminent的各种稀释液喷洒甜菜植株，随后接种表现出极低、中等或高戊唑醇EC值的分离株。在田间施用量和大多数降低的施用量下，高EC值分离株引起的病害明显多于中等或极低EC值的分离株。由于体外敏感性测试通常使用商业杀菌剂的活性成分进行，我们研究了戊唑醇与商业产品Eminent在病害防治效果丧失方面是否相同。高EC值分离株在接种了戊唑醇处理的植株上引起的病害比接种Eminent的植株更多，但在接种了极低EC值分离株的植株之间，病情严重程度没有差异。
