R. Linderman1, J. Parke2, and E. Hansen2
The discovery of Phytophthora ramorum infecting plants such as Rhododendron and Viburnum that are grown extensively in nurseries, raised the question of what its potential impact would be if introduced into production areas of the nursery industry in the Pacific Northwest. We predict very high risk because of the wide range of tree and shrub plant species and cultivars grown; because the climatic conditions in Oregon’s Willamette Valley would be optimum for the pathogen; because irrigation and fertilization practices might favor infections in the nursery; because plant material at all stages of growth is moved within and between nurseries; and because symptoms caused by P. ramorum and other P. species on any one host, such as rhododendrons, might be similar and therefore preclude its detection. Furthermore, symptoms caused by P. ramorum might not be the same on different hosts. As a result, infected symptomatic or symptomless plants might be shipped to other nurseries.
Our studies sought to determine (1) if there would be any distinguishing symptoms caused by P. ramorum on a range of ornamental plants compared to those caused by other P. species known to occur in Oregon (P. cactorum, P. syringae, P. citricola, P. hevea, P. parasitica, P. citrophthora, and P. cinnamomi); (2) how many plants would be potential hosts for P. ramorum compared to other P. species, (3) if P. ramorum is more virulent than other P. species on any given host plant, and (4) if the biological traits for P. ramorum, especially sporulation capacity, would increase its potential to have a major disease impact on nursery production.
Inoculation of detached leaves of all hosts with mycelial plugs of each P. species resulted in varied susceptibility and severity of symptoms based on visual ratings of lesion size. On any given host, lesions were essentially identical in appearance, but on hosts like laurels, “shot-hole” lesions developed compared to general necrosis seen on most hosts. Species of Rhododendron, Pieris, Vaccinium, Syringa, Prunus (Laurel), Cotoneaster, and Arctostaphylos were the most susceptible to the most P. species. Some plants were susceptible to P. ramorum but not other P. species (Viburnum plicatum ‘Tomentosum’ and V. davidii), while others were susceptible to other P. species but not P. ramorum. P. ramorum, P. citricola, and P. citrophthora were the most aggressive/virulent pathogens, often spreading throughout the entire leaf. Of the many plant species and cultivars tested by inoculation with mycelial plugs or by dipping leaves in a zoospore suspension, some were essentially resistant, others varied from low to moderate to highly susceptible. Sporulation (sporangia/zoospores and chlamydospores) by P. ramorum also varied on different hosts when leaf discs were inoculated by floating them on a zoospore suspension, and it was more profuse on infected tissues and on agar media than any other P. species. These results indicate (1) that a wider range of nursery and landscape plants should be surveyed for early detection of P. ramorum and other aggressive species in the nurseries; (2) that P. ramorum is as virulent as P. citricola and P. citrophthora on some hosts, uniquely virulent on some hosts, and less virulent on others; and (3) that the sporulation potential of P. ramorum exceeds that of most P. species, leading to the conclusion that P. ramorum would have very significant disease-causing potential in nurseries in the Pacific Northwest.
