U.S. Department of Agriculture, Agricultural Research Service, Horticultural Crops Research Laboratory, Corvallis, OR 97330.
Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331.
Plant Dis. 2018 Dec;102(12):2560-2570. doi: 10.1094/PDIS-04-18-0666-RE. Epub 2018 Oct 22.
Rhododendrons are an important crop in the ornamental nursery industry, but are prone to Phytophthora root rot. Phytophthora root rot is a continuing issue on rhododendrons despite decades of research. Several Phytophthora species are known to cause root rot, but most research has focused on P. cinnamomi, and comparative information on pathogenicity is limited for other commonly encountered oomycetes, including Phytophthora plurivora and Pythium cryptoirregulare. In this study, three isolates each of P. cinnamomi, P. plurivora, and Py. cryptoirregulare were used to inoculate rhododendron cultivars Cunningham's White and Yaku Princess at two different inoculum levels. All three species caused disease, especially at the higher inoculum level. P. cinnamomi and P. plurivora were the most aggressive pathogens, causing severe root rot, whereas Py. cryptoirregulare was a weak pathogen that only caused mild disease. Within each pathogen species, isolate had no influence on disease. Both P. cinnamomi and P. plurivora caused more severe disease on Cunningham's White than on Yaku Princess, suggesting that the relative resistance and susceptibility among rhododendron cultivars might be similar for both pathogens. Reisolation of P. cinnamomi and P. plurivora was also greater from plants exhibiting aboveground symptoms of wilting and plant death and belowground symptoms of root rot than from those without symptoms. Results show that both P. cinnamomi and P. plurivora, but not Py. cryptoirregulare, are important pathogens causing severe root rot in rhododendron. This study establishes the risks for disease resulting from low and high levels of inoculum for each pathogen. Further research is needed to evaluate longer term risks associated with low inoculum levels on rhododendron health and to explore whether differences among pathogen species affect disease control.
杜鹃是观赏苗圃业的重要作物，但容易受到疫霉根腐病的影响。尽管经过几十年的研究，疫霉根腐病仍然是杜鹃面临的一个持续存在的问题。已知有几种疫霉属物种会导致根腐病，但大多数研究都集中在肉桂疫霉上，而对其他常见卵菌如多黏疫霉和腐霉 cryptoirregulare 的致病性的比较信息有限。在这项研究中，使用了肉桂疫霉、多黏疫霉和腐霉 cryptoirregulare 的每个种的三个分离株，分别在两个不同的接种体水平下接种杜鹃品种坎宁安的白和亚库公主。所有三种物种都引起了疾病，特别是在较高的接种体水平下。肉桂疫霉和多黏疫霉是最具侵略性的病原体，引起严重的根腐病，而腐霉 cryptoirregulare 是一种较弱的病原体，只引起轻度疾病。在每个病原体物种内，分离株对疾病没有影响。肉桂疫霉和多黏疫霉在坎宁安的白上引起的疾病比在亚库公主上更严重，这表明在杜鹃品种中，相对抗性和易感性可能对这两种病原体都相似。从表现出地上萎蔫和植物死亡以及地下根腐病症状的植物中重新分离出的肉桂疫霉和多黏疫霉也比没有症状的植物多。结果表明，肉桂疫霉和多黏疫霉，而不是腐霉 cryptoirregulare，是导致杜鹃严重根腐病的重要病原体。本研究确定了每种病原体在低水平和高水平接种体下导致疾病的风险。需要进一步研究以评估低接种体水平对杜鹃健康的长期风险，并探讨病原体物种之间的差异是否会影响疾病控制。
