First Report of Phytophthora occultans Causing Root and Collar Rot on Ceanothus, Boxwood, Rhododendron, and Other Hosts in Horticultural Nurseries in Oregon, USA Public Deposited

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  • Dead and dying Ceanothus sanguineus, C. velutinus, and C. integerrimus plants grown in a native plant nursery in Oregon for landscape restoration were reported in 2011. Plants were wilted with stem lesions above necrotic roots. Using selective media (Hansen et al. 2012), twelve similar Phytophthora isolates were obtained. DNA sequences of the cox 1, β tubulin, and the rDNA ITS regions were generated (Hansen et al. 2012). All isolates had identical ITS sequences (GenBank KP742989), and were identical to Phytophthora occultans (Man in’t Veld et al. 2014) (JX978155) and 99% similar to P. himalsilva (HM752784) in a BLAST analysis. They were also identical to P. occultans in cox 1 and β tubulin (KR028484 and KR028483). Isolates were homothallic, with smooth 30-µm-diameter oogonia, and slightly aplerotic oospores. Antheridia were mostly paragynous. Colonies were stellate on carrot agar, growing 6 to 7 mm/d at optimum temperature (25°C). Sporangia were ovoid to irregular and papillate. Morphology and growth were consistent with P. occultans. Collections of unidentified Phytophthora spp. from the OSU Plant Clinic and from other Oregon nurseries (J. Parke et al. 2014) revealed additional isolates with similar morphology and identical DNA sequences. P. occultans was identified from boxwood (Buxus spp.), rhododendron, Gaultheria shallon, Arctostaphylos uva-ursi, and Mahonia nervosa in addition to Ceanothus spp. Two inoculation trials were conducted: (i) Healthy 1-year-old plants of C. sanguineus and C. velutinus were stem wound inoculated with two isolates of P. occultans from Ceanothus, or with sterile agar. There were 3 to 5 replications for each host and the control. The test was repeated with addition of two isolates from boxwood. (ii) Boxwood (B. sempervirens) and rhododendron (R. catawbiense Alba) were stem wound inoculated with two isolates each of P. occultans from Ceanothus and boxwood. There were four replications of each host for each isolate. All plants were incubated at 20 to 22°C. In test 1, all isolates induced stem lesions and wilting on all inoculated plants of both Ceanothus species. Wilting began in 14 days and lesions, measured at 19 days, averaged about 150 mm. There were no symptoms on control plants. In test 2, lesions developed on rhododendron stems, often girdling the stem within 12 days. Most boxwood showed no foliar symptoms or only mild yellowing, although stem lesions averaging 3.5 cm in 7 weeks were present on all plants. P. occultans was reisolated from all hosts in both tests. P. occultans was recently described from Buxus nursery stock in The Netherlands (Man in’t Veld et al. 2014), and isolates with identical DNA sequences were reported from Germany and Romania (Nechwatal et al. 2014). This is the first report from North America. It appears that a single clone of P. occultans recently has been spread widely in the nursery trade. P. occultans is similar to P. himalsilva (Vettraino et al. 2011) and to other members of the poorly defined P. citrophthora clade. Phylogenetic analysis may revise species definitions. Nursery plants grown for wildland restoration are at high risk to carry exotic Phytophthora species into vulnerable landscapes. Forest restoration specialists must demand healthy stock from nurseries.
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  • Reeser, M. P. W., Sutton, W., Hansen, E., Goheen, E. M., Fieland, V., & Grünwald, N. J. (2015). First report of Phytophthora occultans causing root and collar rot on Ceanothus, boxwood, rhododendron, and other hosts in horticultural nurseries in Oregon, USA. Plant Disease, 99(9), 1282-1283. doi:10.1094/PDIS-02-15-0156-PDN
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