Assessment of Pathogenic Variation against Pitch Canker Pathogen, Fusarium circinatum in Pinus thunbergii and Responses of Natural Selection Pinus × rigitaeda to Branch Inoculation in a Seed Orchard

Southern Forest Research Center, Korea Forest Research Institute, Jinju 660-300, Korea(Received on April 12, 2010; Accepted on September 18, 2010)A half-sib family of two 4-year-old seedlings of Pinus×rigitaeda was inoculated with each of 20 Pinus thunbergiiisolates of Fusarium circinatum (syn. Fusarium sub-glutinans f. sp. pini) from two pitch canker damagedsites in Jeju Island, South Korea. Initial symptoms ofneedle damages were visible on most of the seedlings at18 days after inoculation. The 20 tested isolates were notsignificantly different in virulence, based on lesionlengths at the site of inoculation (P = 0.217). The mostvirulent isolate FT-7 showed the longest lesion length.Some seedlings began to die 46 days after inoculation.All seedlings were dead by 68 days after inoculationexcept two seedlings inoculated with each of isolates FS-2 and FS-13, respectively. Using the FT-7, 38-year-old 11P. × rigitaeda trees, which were survived from a seedorchard severely damaged by pitch canker, were ino-culated on branches in the seed orchard in Jeju Islandto assess differences in susceptibility to pitch canker.The 11 trees differed significantly (P < 0.001) in suscep-tibility to F. circinatum based on average lesion lengthsmeasured 56 days after inoculation. It is possible thatinduced resistance contributed to their capacity to limitlesion development. The susceptibility of natural selec-tion P. × rigitaeda trees are more likely affected by inter-action with F. circinatum rather than environmentalconditions. Keywords : Fusarium circinatum, Natural selection, Pinusthunbergii, Pinus × rigitaeda, SusceptibilityPitch canker is a serious disease of pines, which is causedby the fungus Fusarium circinatum Nirenberg & O’Donnell(syn. Fusarium subglutinans f. sp. pini; telemorph: Giberellacircinatum Nirenberg & O’Donnell) distributed in differentparts of the world (Correll et al., 1991; Dwinell et al., 1985;Gordon et al., 2001; Nirenberg and O’Donnell, 1998).Advanced symptoms of the disease result in crown diebackand resinous cankers on branches and main stems (Storer etal. 2002). Pitch canker was first reported in 1946 on Pinusvirginiana in North Carolina, USA (Hepting and Roth,1946), and it later appeared on Pinus elliottii in Florida,Pinus taeda in Mississippi, and on Pinus radiata inCalifornia where it has caused a significant epidemic in itsnative range (Dwinell and Phelps, 1977; Gordon et al.,2001; Storer et al., 1997). The pathogen is also found inJapan (Kobayashi and Muramoto, 1989), South Africa(Viljoen et al., 1994), Mexico (Guerra-Santos, 1999), SouthKorea (Lee et al., 2000), Chile (Wingfield et al., 2002),Spain (Landeras et al., 2005), and was recently reportedfrom Italy (Carlucci et al., 2007). In South Korea, pitch canker was first noticed as a causeof branch dieback on the exotic species Pinus rigida Mill.in Incheon city in the mid-1990s. Pitch canker may bealleged to have introduced into South Korea through tworoutes: one from the USA to Incheon city and the otherfrom Japan to Jeju Island. The disease has caused a seriousdamage mostly on three-needle pines, P. rigida and P. ×rigitaeda in the southwestern part of the Korean peninsulaand Jeju Island. Most infections appear to result fromweather-related wounds from wind that serves as entrypoints of micro- or macro-conidia. The seed orchards of P. rigida and P. × rigitaeda in JejuIsland have been damaged by pitch canker for about tenyears perhaps because of warm temperature and geographiclocation near to Japan where the disease has been establish-ed since the late 1980s (Kobayashi and Muramoto, 1989;Woo et al., 2005). Recently, the disease occurred moreoften on sapling stands of a native two-needle pine P.thunbergii on the roadside and a golf course in Jeju Island.The isolates of F. circinatum obtained from different pinehosts, for example, P. nigra, P. pinaster, P. sylvestris, and P.radiata in Spain (Perez-Sierra et al., 2007) and P. taeda, andP. elliottii in USA (Hodge and Dvorak, 2007), were testedfor virulence. However, virulence to P. thunbergii isolatesof F. circinatum has not been reported previously.Phenotypes of relatively resistant pines are characterizedby a reduced rate of lesion development and thus less