United States Department of Agriculture-Agricultural Research Service, Horticultural Crops Research Laboratory, 3420 NW Orchard Avenue, Corvallis, OR 97330.
Plant Dis. 2021 May;105(5):1505-1514. doi: 10.1094/PDIS-09-20-1960-RE. Epub 2021 Apr 7.
Phytophthora root rot, caused by many soilborne spp., is a significant disease affecting the $42 million rhododendron nursery industry. Rhododendron growers have increasingly reported failure by two systemic fungicides, phosphorous acid and mefenoxam, to adequately control root rot. Both fungicides may be applied as a foliar spray or soil drench but it is unknown how application method, fungicide chemistry, or pathogen diversity affects disease control. Therefore, two experiments were conducted to (i) determine whether differences in application method or fungicide chemistry affect control of root rot caused by and and (ii) evaluate the sensitivity of spp. and isolates from the rhododendron industry to each fungicide. Results demonstrated that soil drenches of either fungicide were more effective than foliar sprays for control of but were ineffective for . Furthermore, spp. and isolates varied in sensitivity to phosphorous acid and mefenoxam, and there were multiple fungicide-insensitive isolates, especially within . Differences in sensitivity were also observed among isolates from different nurseries and production systems, with some nurseries having less sensitive isolates than others and with container systems generally having less sensitive isolates than field systems. Our results provide three potential reasons for why fungicide control of Phytophthora root rot might fail: (i) the fungicide can be applied to the wrong portion of the plant for optimal control, (ii) there are differences in fungicide sensitivity among soilborne spp. and isolates infecting rhododendron, and (iii) fungicide-insensitive isolates are present in the rhododendron nursery industry.
疫霉根腐病，由许多土传病原菌引起，是影响价值 4200 万美元的杜鹃苗圃产业的重要病害。杜鹃种植者越来越多地报告，两种系统性杀菌剂——磷酸和甲霜灵，无法充分控制根腐病。这两种杀菌剂都可以作为叶面喷雾或土壤淋溶施用，但不清楚施用方法、杀菌剂化学性质或病原菌多样性如何影响疾病控制。因此，进行了两项实验，以确定（i）施用方法或杀菌剂化学性质的差异是否会影响疫霉根腐病的控制，（ii）评估疫霉菌种和来自杜鹃产业的分离物对每种杀菌剂的敏感性。结果表明，两种杀菌剂的土壤淋溶处理均比叶面喷雾处理更能有效控制，但对无效。此外，疫霉菌种和分离物对磷酸和甲霜灵的敏感性存在差异，并且存在多种对杀菌剂不敏感的分离物，尤其是在疫霉菌种中。不同苗圃和生产系统的分离物之间也观察到敏感性差异，一些苗圃的分离物比其他苗圃更不敏感，容器系统的分离物通常比田间系统的分离物更不敏感。我们的结果提供了三种可能导致杀菌剂防治疫霉根腐病失败的原因：（i）杀菌剂可以施用到植物的错误部位以达到最佳控制效果，（ii）感染杜鹃的土传疫霉菌种和分离物对杀菌剂的敏感性存在差异，以及（iii）杜鹃苗圃产业中存在对杀菌剂不敏感的分离物。
