Forage Seed and Cereal Research Unit, U.S. Department of Agriculture Agricultural Research Service, Corvallis, OR 97331.
Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331.
Plant Dis. 2020 May;104(5):1400-1406. doi: 10.1094/PDIS-10-19-2067-RE. Epub 2020 Mar 20.
Phosphonate (phosphite; HPO) is fungicidal against oomycetes and certain other organisms. The Fungicide Resistance Action Committee has deemed phosphonate to be at low risk of resistance development, and reduced sensitivity to phosphonate has been reported only occasionally in plant pathogens. Reduced sensitivity to the fungicide fosetyl-Al was documented in the hop downy mildew pathogen, , in the early 2000s, but disease caused by insensitive isolates could still be managed commercially if the fungicide rate was doubled from 2.24 to 4.48 kg/ha. In this research, we document the occurrence of isolates of . in Oregon that possess even higher levels of insensitivity to fosetyl-Al and other phosphonate fungicides. The median estimated effective concentration required to reduce infection by 50% (EC) for isolates collected from two farms reporting disease control failures was 2.7% (vol/vol) phosphonate (range = 1.6 to 164.2), which was 1.6 times (range = 0.9 to 96.0) the maximum labeled rate of the phosphonate fungicide utilized. In contrast, the median EC for isolates obtained from experimental plots that have received only a single application of a phosphonate fungicide was 0.6% (vol/vol) phosphonate (range = 0.11 to 2.3) or 0.3 times the maximum allowable rate. Sensitivity of isolates to a phosphorous acid fungicide, fosetyl-Al, and a plant nutrient product containing an unspecified level of phosphorous acid were linearly related. Insensitivity to the maximum allowable rate of a phosphorous acid fungicide was widespread within and among hop farms in Oregon. Among 54 isolates assayed for phosphonate insensitivity, 96% had EC values that exceeded the maximum allow rate of the fungicide used in the assays. Field studies conducted in 2 years further demonstrated that a phosphorous fungicide, a nutrient product containing phosphorous acid, and fosetyl-Al failed to provide commercially acceptable suppression of downy mildew when applied at the maximum allowable rates and even double these rates, whereas fungicides with different modes of action provided 91% or greater disease control. The whole of this research indicates that has been selected to tolerate fosetyl-Al and other phosphonate fungicides at rates four times greater than those used earlier to obtain satisfactory suppression of downy mildew. This finding has implications for management of the disease not only in Oregon but also, in other production regions should insensitive isolates be introduced on infected planting material.
膦酸盐（亚磷酸盐；HPO）对卵菌纲和某些其他生物具有杀菌作用。杀菌剂抗性行动委员会认为膦酸盐发展抗性的风险较低，并且仅偶尔在植物病原体中报道对膦酸盐的敏感性降低。在 21 世纪初，在啤酒花霜霉病病原体 中记录到对杀菌剂乙膦铝的敏感性降低，但如果将杀菌剂的用量从 2.24 千克/公顷增加到 4.48 千克/公顷，则仍可商业上控制由不敏感分离物引起的疾病。在这项研究中，我们记录了在俄勒冈州发现的 分离物的发生情况，这些分离物对乙膦铝和其他膦酸盐杀菌剂的敏感性甚至更高。从两家报告疾病控制失败的农场采集的分离物的估计有效浓度 50%（EC）需要减少感染的中位数为 2.7%（体积/体积）膦酸盐（范围为 1.6 至 164.2），这是标记的膦酸盐杀菌剂最高用量的 1.6 倍（范围为 0.9 至 96.0）。相比之下，从仅接受一次膦酸盐杀菌剂处理的实验田获得的分离物的中位 EC 为 0.6%（体积/体积）膦酸盐（范围为 0.11 至 2.3）或最高允许用量的 0.3 倍。分离物对亚磷酸的杀菌剂膦酸二乙酯和含有未指定亚磷酸水平的植物营养产品的敏感性呈线性相关。对最大允许用量的亚磷酸杀菌剂的不敏感性在俄勒冈州的啤酒花农场内部和之间广泛存在。在对 54 个分离物进行的膦酸盐不敏感性测定中，96%的分离物的 EC 值超过了测定中使用的杀菌剂的最大允许用量。在两年的田间研究中进一步表明，当以最大允许用量甚至两倍的用量施用亚磷酸杀菌剂、含有亚磷酸的营养产品和乙膦铝时，不能提供商业上可接受的霜霉病防治效果，而具有不同作用模式的杀菌剂则提供了 91%或更高的病害防治效果。整个研究表明， 已被选择耐受乙膦铝和其他膦酸盐杀菌剂的剂量是以前获得满意的霜霉病防治效果所需剂量的四倍。这一发现不仅对俄勒冈州，而且对其他生产地区也有影响，如果引入对感染种植材料具有抗性的不敏感分离物，该发现将对疾病的管理产生影响。
