Publication Type:Journal Article
Source:Plant Disease (2016)
Since 2010, extensive crown transparency of European beech (Fagus sylvatica) has been recorded in Southern Sweden, with P. cambivora, P. plurivora, and P. cactorum as well as associated climatic triggers believed to be the major factors involved. In 2015, a different Phytophthora species was isolated from three F. sylvatica trees showing characteristic bleeding canker symptoms in Pildammsparken in Malmö, Sweden. Small sections of inner phloem were dissected at the interface between necrotic and healthy tissue, directly plated onto PAR(PH)-V8 Selective medium containing 0.250 g/liter sodium ampicillin, 0.01 g/liter benomyl, 0.05 g/liter hymexazol, 0.1 g/liter PCNB, 0.05 g/liter pimaricin, and 0.01 g/liter rifamycin) and incubated at 20°C in darkness. Growing hyphae were later transferred to PDA media and incubated at 20°C in the dark. The colony pattern was stellate. The average radial growth rate at 20 °C was 0.6 cm/day. These morphological features are similar to that previously described for P. gonapodyides (Corcobado et al., 2010; Erwin & Ribeiro, 1996; Jung et al., 1996). The isolates grown on PDA formed irregularly branched hyphae. Nonpapillate, elongated-ovoid to obpyriform sporangia (approximately 50 x 40 µm) were produced after 7 days by incubating on V8 agar culture (non-sterile soil water). DNA was extracted from mycelia and the identity of P. gonapodyides was confirmed by sequencing the internal transcribed (ITS) spacer region of the rDNA with the primers ITS4/ITS6 [GenBank Accession Nos. KX055998, KX055999, KX056000]. Blast search showed 99-100% identity with reference sequences of P. gonapodyides deposited in GenBank (NCBI). Pathogenicity on F. sylvatica was confirmed in a greenhouse experiment using two different isolates of P. gonapodyides. Mycelial plugs of 3-week-old cultures of each isolate grown on PDA were inoculated on the stems of 1-year-old seedlings at two locations, separated by 20 cm, by aseptically excising a small 5-mm flap of bark, inserting the mycelial plug along the cambium and adding a droplet of Milliq H2O before sealing the wound with Parafilm®. Ten seedlings were inoculated for each isolate. Controls consisted of stem-wound inoculation with a sterile plug of PDA, and no wounding at all. After 7 weeks, lesion length was measured. All seedlings inoculated with P. gonapodyides showed distinct necrotic lesions, whereas control treatments did not show any symptoms of disease. The average cumulative lesion length on seedlings inoculated with P. gonapodyides was significantly higher than on the wounded control treatment (25.4 mm, ±1.66 vs. 0.4 mm, ±0.01; P<0.0001 for Isolate 1 and 28.9 mm, ±1.42 vs. 0.4 mm, ±0.01; P<0.001 for Isolate 2). Koch’s postulates were fulfilled by successfully reisolating the pathogen from infected seedlings. To our knowledge, this is the first report of P. gonapodyides causing disease on trees in Sweden. P. gonapodyides has been noted previously in Denmark (Erwin and Ribeiro, 1996), and has been traditionally regarded as a weak parasite with saprophytic abilities, usually associated with aquatic environments such as rivers, riparian areas and wetlands (Brasier et al., 2003). We cannot exclude that recent climatic triggers such as high summer precipitation coupled with mild winter temperatures have favored conditions to multicyclic spread of P. gonapodyides via zoospores, or that increased average age of beech stands has contributed to their higher susceptibility to weaker pathogens. Damage caused by P. gonapodyides however might be underestimated as it also can inhibit seed germination, and is known to cause root rot and stem lesions in Q. robur and Q. ilex (Jung et al. 1996; Corcobado et al. 2010). The newly reported damage caused by P. gonapodyides on F. sylvatica trees in Southern Sweden is alarming particularly since beech is a dominant and ecologically important hardwood species that is widely used in forestry and as a planted ornamental in urban and landscape settings. Furthermore, other hosts such as Alnus glutinosa, Quercus petraea, Q. robur (Jung et al., 1996), Picea abies, Betula spp., Acer spp., Tilia spp., (Jung et al., 2009) and Salix spp. (Brasier et al., 2003), also common in southern Sweden, may face an increased risk of damages caused by P. gonapodyides in the future.