Significance: Samples taken during the U.S. Phytophthora ramorum survey have revealed the presence of Phytophthora hedraiandra in Minnesota (Schwingle et al., 2006) nurseries. Phytophthora hedraiandra was first identified from a Viburnum sp. leaf spot sample collected in 2001 in the Netherlands (de Cock and Levesque, 2004). Since then it has also been identified in Spain (Moralejo et al., 2005) and Italy (Belisario et al., 2005) on Viburnum tinus. In the Minnesota nurseries, samples were taken from symptomatic Rhododendron and Quercus leaf and stem tissues. Isolates from Rhododendron were identified as P. hedraiandra with molecular techniques. Schwingle et al. (2006) successfully conducted Koch’s Postulates with the P. hedraiandra isolates and Rhododendron cultivar ‘Mikkeli’.
Issues of Concern: The full host range and distribution are not yet known. Diagnosis is difficult as P. hedraiandra closely resembles other species of the same genus. As seen with other Phytophthora species, symptoms could vary with host and easily be overlooked at early infection stages. In general, Phytophthora species are difficult to control due to their typically large host ranges and presence of survival structures in soil or plant debris (Erwin and Ribeiro, 1996).
Pathways: Movement of infected host plants, infested soil, rain splash, and perhaps infested irrigation water (Davidson and Shaw, 2003; Erwin and Ribiero, 1996) (not proven for Phytophthora hedraiandra but are possibilities as demonstrated with other Phytophthora species).
Hosts: At this time, only certain species of Rhododendron and Viburnum are known to be susceptible. Host range studies needed.
Detection Strategies: Symptoms mentioned in the disease reports include branch dieback, basal stem cankers, root rot, and leaf spots (Belisario et al., 2005; Moralejo et al., 2005; Schwingle et al.). As seen with other Phytophthora species, symptoms could be host-dependant and variable.
Belisario, A., Gilli, G., and M. Maccaroni. 2005. First report of Phytophthora hedraiandra on Viburnum tinus in Italy. British Society for Plant Pathology. http://www.bspp.org.uk/ndr/jan2006/2005-85.asp Davidson, J.M. and C.G. Shaw. 2003. Pathways of movement for Phytophthora ramorum, the causal agent of Sudden Oak Death. Sudden Oak Death Online Symposium. www.apsnet.org/online/SOD (website of The American Phytopathological Society). doi:10.1094/SOD-2003-TS. de Cock A. W. A. M. and C. A. Lévesque. 2004. New species of Pythium and Phytophthora. Studies in Mycology 50: 481-487. Erwin, D.C. and O.K. Ribeiro. 1996. Phytophthora Diseases Worldwide. Amer. Phytopathol. Society. St. Paul, MN. 561 pp. Schwingle, B.W., Smith, J.A., Blanchetter, R.A., Gould, S., Blanchette, B.L, and S.D. Cohen. 2006. First Report of Dieback and Leaf Lesions on Rhododendron sp. Caused by Phytophthora hedraiandra in the United States. Plant Dis. 90:109; published on-line as DOI: 10.1094/PD-90-0109A. http://www.apsnet.org/pd/searchnotes/2006/PD-90-0109A.asp Moralejo, E., Belbahri, L., Calmin, G., Lefot, F., Garcia, J. A., and E. Descals. 2005. First report of Phytophthora hedraiandra on Viburnum tinus in Spain. British Society for Plant Pathology. http://www.bspp.org.uk/ndr/jan2006/2005-90.asp