Variation in disease severity caused by Phytophthora cinnamomi, P. plurivora, and Pythium cryptoirregulare on two rhododendron cultivars

Authors: Weiland, Gerald - Jerry; Scagel, Carolyn; Grunwald, Niklaus - Nik; Davis, Elizabeth; Beck, Bryan

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/22/2018
Publication Date: 6/1/2018
Citation: Weiland, G.E., Scagel, C.F., Grunwald, N.J., Davis, E.A., Beck, B.R. 2018. Variation in disease severity caused by Phytophthora cinnamomi, P. plurivora, and Pythium cryptoirregulare on two rhododendron cultivars. Plant Disease. https://doi.org/10.1094/PDIS-04-18-0666-RE.
DOI: https://doi.org/10.1094/PDIS-04-18-0666-RE

Interpretive Summary: Rhododendrons are an important part of the ornamental nursery industry, but are prone to root rot. Phytophthora cinnamomi, Phytophthora plurivora, and Pythium cryptoirregulare are known to cause root rot, but there are no studies comparing how much disease each pathogen causes in rhododnedron. In this study, three isolates of each pathogen were used to inoculate two rhododendron cultivars, 'Cunningham's White' and 'Yaku Princess' at a low and high inoculum level. Pythium cryptoirregulare caused mild disease while both Phytophthora species caused severe disease. Disease was more severe at the high inoculum level, but there were no differences among isolates within a pathogen species. The cultivar 'Cunningham's White' was more susceptible to P. cinnamomi and P. plurivora than 'Yaku Princess'. Both Phytophthora species were reisolated more often from plants exhibiting symptoms of severe root rot whereas P. cryptoirregulare was reisolated more frequently from visually asymptomatic plants. Results show that both P. cinnamomi and P. plurivora are important pathogens causing severe root rot in rhododendron.

Technical Abstract: Rhododendrons are an important component of 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 P. 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. Phytophthora cinnamomi and P. plurivora were the most aggressive pathogens causing severe root rot, whereas P. 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 P. cryptoirregulare, are important pathogens causing severe root rot in rhododendron. This study also 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.