Plant Disease (2018) 102, 2560-2570

Jerry E. Weiland, Carolyn F. Scagel, Niklaus J. Grünwald, E. Anne Davis, Bryan R. Beck and Val J. Fieland (2018)
Variation in disease severity caused by Phytophthora cinnamomi, P. plurivora, and Pythium cryptoirregulare on two rhododendron cultivars
Plant Disease 102 (12), 2560-2570
Abstract: 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.
(The abstract is excluded from the Creative Commons licence and has been copied with permission by the publisher.)
Link to article at publishers website
Database assignments for author(s): Niklaus J. Grünwald

Research topic(s) for pests/diseases/weeds:
general biology - morphology - evolution

---

Pest and/or beneficial records:

Beneficial	Pest/Disease/Weed	Crop/Product	Country	Quarant.
	Phytophthora plurivora	Rhododendron (crop)
	Phytophthora cinnamomi	Rhododendron (crop)
	Globisporangium cryptoirregulare	Rhododendron (crop)