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Abstract
Two distinct subgroups (L2 and A−2) were recovered from irrigation reservoirs and a stream in Virginia, USA. After molecular, morphological and physiological examinations, the L2 subgroup was named Phytophthora aquimorbida and the A−2 designated as Phytophthora taxon ‘aquatilis’. Both taxa are homothallic. P. aquimorbida is characterized by its noncaducous and nonpapillate sporangia, catenulate and radiating hyphal swellings and thick-walled plerotic oospores formed in globose oogonia mostly in the absence of an antheridium. P. taxon ‘aquatilis’ produces plerotic oospores in globose oogonia mostly with a paragynous antheridium. It has semi-papillate, caducous sporangia with variable pedicels, but it does not have hyphal swelling. Analyses of ITS, CO1, β-tubulin and NADH1 sequences revealed that P. aquimorbida is closely related to P. hydropathica, P. irrigata and P. parsiana, and P. taxon ‘aquatilis’ is related to P. multivesiculata. The optimum temperature for culture growth is 30 and 20 C for P. aquimorbida and P. taxon ‘aquatilis’ respectively. Both taxa were pathogenic to rhododendron plants and caused root discoloration, pale leaves, wilting, tip necrosis and dieback. Their plant biosecurity risk also is discussed.
Acknowledgments
This research was supported in part by grants from USDA/NIFA ( 2005-51101-02337, 2010-51181-21140).
The only isolate of P. taxon ‘aquatilis’ was recovered from a bait sample received from Virginia Department of Forestry (T. Edgerton and C. Asaro). We thank G. Abad, C.M. Brasier, E.M. Hansen, M. Garbelotto, W.H. Ko, W. Man in’t Veld and M. Wingfield for providing reference isolates used in this study. We also thank P.M. Eckel for her assistance in coining the Latin name P. aquimorbida and M.E. Gallegly for critical reading of this manuscript.