Plant disease

This article is from the January 2007 issue of

published by The American Phytopathological Society

For more information on this and other topics related to plant pathology, we invite you to visit APSnet at www.apsnet.org

Phytophthora Species Associated with Diseased Woody Ornamentals in Minnesota Nurseries
B. W. Schwingle, J. A. Smith, and R. A. Blanchette, Department of PlantPathology, University of Minnesota, 495 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108
species identification in several recent studies (5,6,25,31,49). However, ITS sequences do not clearly resolve all Phytophthora species (15,37), so other investigators have used additional genome regions, including the nuclear beta-tubulin (β-tub) gene and mitochondrial encoded cytochrome c oxidase (cox) Iand II genes, to differentiate between them (33,37). The objective of the study presented here was to survey, isolate, and identify indigenous and exotic Phytophthora species present in Minnesota ornamental nurseries. Detecting P. ramorum was of key concern, since its host range includes several native and important plants in Minnesota (2,44,51). Species identification was carried out using directsequencing of ITS rDNA, β-tub, and coxI genes and also by studying morphological characteristics. Although this work was done in Minnesota, it has nationwide importance since Minnesota nurseries ship ornamental plants to nearly every state (40). MATERIALS AND METHODS Study sites. Minnesota ornamental nurseries were surveyed in 2002 and 2003 solely to detect P. ramorum, a pathogen which hasdestroyed entire tracts of California oak forests (45) and threatens forests of the eastern United States (2). This work was conducted by the Minnesota Department of Agriculture, the University of Minnesota, and the USDA Forest Service. Phytophthora isolates were obtained only from symptomatic rhododendrons. Twenty-one nurseries in the fall of 2002 and 24 nurseries in the spring of 2003 were surveyed byMinnesota Department of Agriculture personnel. Samples collected during the surveys were identified to genus by staff in the Plant Disease Clinic, University of Minnesota. P. ramorum was not detected. Phytophthora isolates obtained were identified to species in this present study. Fifteen ornamental nurseries in or near Minneapolis and St. Paul, MN, and one nursery in Duluth, MN, were sampledbetween April and September in 2004 and 2005. The largest nurseries were visited several times each year, whereas the smaller nurseries were sampled twice in the course of the study. Plants that were previously confirmed as hosts or species associated with P. ramorum (2) were the focus of this study. Nonhosts of P. ramorum, however, were sampled occaPlant Disease / January 2007 97

ABSTRACTSchwingle, B. W., Smith, J. A., and Blanchette, R. A. 2007. Phytophthora species associated with diseased woody ornamentals in Minnesota nurseries. Plant Dis. 91:97-102. Phytophthora species are responsible for causing extensive losses of ornamental plants worldwide. Recent international and national surveys for the detection of P. ramorum have led to the finding of previously undescribed Phytophthoraspecies. Since no previous Phytophthora surveys have been carried out in Minnesota, surveys of ornamental nurseries were performed over 4 years to isolate and identify the Phytophthora species causing diseases of woody plants in Minnesota. Species were identified by direct sequencing of internal transcribed spacer (ITS) rDNA, β-tub, and mitochondrial coxI genes. Species associated with diseasedornamental plants include P. cactorum, P. cambivora, P. citricola, P. citrophthora, P. hedraiandra, P. megasperma, P. nicotianae, and the previously identified but undescribed taxon P. Pgchlamydo. The most common species encountered were P. cactorum, P. citricola, and P. citrophthora. Two additional isolates obtained did not match known species. One was similar to P. alni subsp. alni, and the other...