An Ornamental Swiss Stone Pine (Pinus cembra) in Wisconsin is a Host of the Shoot Blight Pathogen Diplodia pinea
Authors: Oblinger B. W. , Smith D. R. , Stanosz G. R. 
Published: July 2009

Swiss stone pine (Pinus cembra) is a slow-growing, hardy tree native to high-mountain regions of Europe from the Alps to the Carpathians. It also is planted as an ornamental in North America. Shoot blight and branch dieback were observed in the fall of 2008 on a single, 25- to 30-year-old Swiss stone pine growing on the University of Wisconsin–Madison campus. This tree is located between two mature Austrian pines (P. nigra) that exhibit symptoms of Diplodia blight and show signs of the conifer pathogen Diplodia pinea. Approximately 20% of the Swiss stone pine shoots were affected with needles and stems killed before full elongation. Symptom development appeared to have progressed from tips into older portions of branches with several years' growth often heavily resinous and necrotic. Five samples each of needles, stems, and cones bearing erumpent, black pycnidia were collected for microscopic examination. Each sample yielded conidia consistent with those of D. pinea (2). Using tannic acid agar (TAA) (1) on which autoclaved pine needles were placed to induce sporulation, this fungus was cultured from all 15 samples. The identity of the pathogen was confirmed as D. pinea with species-specific PCR primers (4) that allow differentiation from the similar fungus D. scrobiculata. Single-conidial isolate 09-03 from the affected Swiss stone pine was used to inoculate potted seedlings of this species in a greenhouse. Growing shoots of 12 seedlings were wounded by removing a needle fascicle and then were inoculated by placing on the wound a 5-mm-diameter plug cut from an actively growing colony on water agar (WA). Noncolonized WA plugs were placed on five wounded control seedlings, and five nonwounded control seedlings were used. Seedlings were covered with plastic bags to maintain high humidity for 2 weeks and then the bags were removed. The initial symptom, present 1 week after inoculation, was chlorosis of the bases of current-year needles near the point of inoculation. Affected needles became necrotic and pycnidia were visible on some by 10 days after inoculation. Needle chlorosis, necrosis, and dark discoloration of vascular tissue had developed on 11 of 12 inoculated seedlings by 6 weeks after inoculation, but not on wounded or nonwounded control seedlings. At that time, one or more symptomatic needles and a stem segment from each inoculated seedling and comparable material from control seedlings were surface disinfested and placed on TAA. The pathogen was cultured from needles of 10 of 12 inoculated seedlings and from stems of all inoculated seedlings. The fungus was not cultured from needles of control seedlings, but was cultured from stems of 2 of 10 control seedlings, one wounded and one nonwounded. D. pinea often severely damages species in the Pinus subgenus Diploxylon (two- and three-needle pines), but it is much less frequently reported as a cause of damage to hosts in the subgenus Haploxylon (five-needle pines), which includes Swiss stone pine. Although an unidentified Diplodia species was listed among fungi cultured from a healthy shoot of P. cembra (3), to our knowledge this is the first report of D. pinea as a pathogen of Swiss stone pine.

References: (1) J. T. Blodgett et al. For. Pathol. 33:395, 2003. (2) E. Punithalingam and J. M. Waterston. No. 273 in: Descriptions of Pathogenic Fungi and Bacteria. Commonwealth Mycological Institute, Kew, Surrey, England, 1970. (3) G. R. Schnell. Eur. J. For. Pathol. 17:19, 1987. (4) D. R. Smith and G. R. Stanosz. Plant Dis. 90:307, 2006.