Evaluation of Resistance to the Beech Scale Insect (Cryptococcus Fagisuga) and Propagation of American Beech (Fagus Grandifolia) by Grafting
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Ramirez et. al.·Silvae Genetica (2007) 56-3/4, 163-169 Evaluation of Resistance to the Beech Scale Insect (Cryptococcus fagisuga) and Propagation of American Beech (Fagus grandifolia) by Grafting By M. RAMIREZ1), J. LOO2) and M. J. KRASOWSKI3) (Received 25th April 2006) Abstract (EHRLICH, 1932), and have since been spreading through Scions collected from diseased trees and from those much of the natural range of American beech. Through- without symptoms of beech bark disease (BBD) were out New Brunswick, where the disease is well estab- cleft-grafted in 2003 and 2004 onto rootstock of lished, approximately 95% of beech trees are affected unknown resistance to BBD. Grafting success varied (D. McPhee, personal communication, 2004). among genotypes and year (30% in 2003 and 12% in Scale insects feed by inserting their stylets into the 2004), and improved with increasing rootstock diameter. bark parenchyma (WAINHOUSE et al., 1988), predisposing Successful grafts were used to test resistance to the beech scale insect, Cryptococcus fagisuga (the initiating trees to fungal infection (SHIGO, 1972). The initial effect agent of BBD) by introducing eggs onto the bark of of infection spreading to new areas is massive mortality, scions and allowing time for the emergence of all devel- and is known as the “killing front” (SHIGO, 1972). In the opmental stages of the insects. Significantly fewer “aftermath forest,” the insect and fungus become insects colonized scions collected from putatively resis- endemic and cause growth reduction, tree deformation, tant trees than those collected from diseased trees. In declines in wood quality and mast production, as well as some cases, where egg placement overlapped a portion premature death of affected trees (HOUSTON, 1975). The of the rootstock, insect colonies developed on the root- severe negative consequences of BBD have significantly stock but not on the scion collected from resistant trees. reduced the ecological value and economic potential of Occasionally, scions from putatively resistant trees were colonized, whereas some of those from diseased trees American beech in the affected areas, caused problems were not. When scions from putatively resistant trees for forest management, and lessened the aesthetic value were heavily colonized, only adult insects were present of forest sites (HOUSTON, 1999). and no eggs or other life stages of the insect were found. In stands affected by BBD for many years, some trees The findings indicate that the extent of resistance to the free of BBD symptoms occur as single individuals or in scale insect (hence to BBD) ranges from partial to total small groups surrounded by diseased trees (EHRLICH, resistance. 1932; SHIGO, 1964; HOUSTON, 1982). Isozyme analyses Key words: Beech bark disease, disease resistance, inoculation, reveal that the groups often consist of genetically identi- rootstock diameter, scion. cal trees for the loci examined, implying a clonal origin (root suckering). In other cases, BBD-free trees appear Introduction to be closely related, perhaps being full-sibs (HOUSTON and HOUSTON, 1987). These observations suggest genetic American beech (Fagus grandifolia Ehrh.) is an resistance to BBD, but the nature and extent of resis- important component of hardwood and mixed forest tance of individual trees is unknown, as is the mode of stands of eastern North America. This shade-tolerant, inheritance. slow-growing tree (Farrar, 1995) is a source of food and shelter for many wildlife species and is especially impor- Inoculation trials on BBD-free trees conducted by tant for sustaining populations of black bear (Ursus HOUSTON (1983) suggest that some of the symptom-free americanus Pallas) and marten (Martes americana Tur- trees are truly resistant to the beech scale attack. With- ton) (JAKUBAS et al., 2005). out the initial attack by the insect, the fungus does not cause much damage (SHIGO, 1964). When HOUSTON Beech bark disease (BBD) is caused by the interaction (1982, 1983) inoculated the bark of trees with and with- of the woolly beech scale insect (Cryptococcus fagisuga out disease symptoms with scale eggs, insect colonies Lind.) with one or two species of Nectria fungus, princi- established on susceptible trees but failed to establish or pally Nectria coccinea var. faginata Lohman, Watson & reproduce on most BBD-free trees. In a similar experi- Ayers (EHRLICH, 1932). The insect and probably the most ment, Houston (1982) was able to inoculate young (2-, common fungal agent (MAHONEY et al., 1999) were intro- 3-, and 4-year-old) seedlings of American beech with the duced to Nova Scotia from Europe sometime before 1890 scale insect and proposed that his inoculation method could be useful for screening young plant material for 1) Faculty of Forestry and Environmental Management, Univer- sity of New Brunswick. P.O. Box 44555 28 Dineen Drive, Fred- resistance to the attack by the scale insect. ericton, NB E3B 6C2. E-mail: [email protected]. WARGO (1988) proposed that a low concentration of 2) Corresponding author: JUDY LOO, Natural Resources Canada, amino-form nitrogen in some BBD-free trees limits the Canadian Forest Service – Atlantic Forestry Centre. P.O. Box establishment and growth of the scale insect. Houston 4000, Fredericton, NB, E3B 5P7, Tel. (506) 452-3398, Fax: (2005) suggested that anatomical barriers might also (506) 452-3525. E-mail: [email protected]. contribute to the resistance (as shown for Fagus sylvati- 3) Faculty of Forestry and Environmental Management, Univer- sity of New Brunswick. P.O. Box 44555 28 Dineen Drive, Fred- ca by LONSDALE (1983)), but this has not been studied ericton, NB E3B 6C2. E-mail: [email protected]. for American beech and the cause of resistance to BBD Silvae Genetica 56, 3–4 (2007) 163 DOI:10.1515/sg-2007-0025 edited by Thünen Institute of Forest Genetics Ramirez et. al.·Silvae Genetica (2007) 56-3/4, 163-169 remains unidentified. Studies are underway to deter- trees were selected based on accessibility (roadside), mine modes of inheritance using controlled breeding availability (only 5% of all beech trees are disease-free) (KOCH and CAREY, 2004; LOO et al., 2005). and no visible signs of BBD (scale insects, cankers, or Vegetative propagation of resistant genotypes may Nectria fruiting bodies). Diseased trees were selected allow conservation of a BBD-resistant gene pool and randomly, and represent a control. retention or restoration of healthy beech to the forests of eastern North America. The feasibility of this approach Grafting depends on whether BBD-free trees truly demonstrate a Scions were collected in late February and early genetically based resistance, and whether American March of 2003 and 2004. Twigs (20 to 25 cm long) with beech can be propagated vegetatively on a large scale. two to three buds were cut from branches, packed into American beech is very difficult to propagate vegeta- plastic bags containing snow, brought to the Atlantic tively (DIRR and HEUSER, 1987). Limited success has Forestry Centre laboratory, and stored at 0°C for no been achieved with rooting of cuttings and micropropa- more than 2 days before grafting. Twenty scions per tree gation, but the plantlets did not survive the first over- (genotype) were grafted onto 1-year-old rootstock in wintering (BARKER et al., 1997; SIMPSON, 2001). Euro- 2003 and a mixture of 1- and 2-year-old rootstock in pean beech has been grafted successfully but with a low 2004 over a 2-week period each year. The rootstock was percentage of successful grafts (DIRR and HEUSER, 1987). grown from seeds collected from open-pollinated trees Grafting would not be a viable method for mass produc- free of disease symptoms but of unknown resistance to tion of resistant genotypes, but could be very useful for BBD. Seeds were germinated and potted in a mixture of resistance screening to BBD and development of seed peat moss, perlite, aggregate (small rock), and loam orchards to produce resistant seedlings. (2:2:1:1). The rootstock was carefully matched to the diameter of the scion. The grafting technique was top The main objective of this study was to test if selected cleft, matching closely the cambium of the scion and disease-free trees of southern New Brunswick origin are rootstock. Graft unions were wrapped with rubber genetically resistant to the beech scale. A secondary bands and coated with warm wax to hold the graft in objective was to evaluate factors affecting grafting suc- place and prevent drying. Grafts were placed in a green- cess. The trial compared results of inoculation of scions house under controlled conditions (initially, 70% humid- collected from disease-free and diseased trees, and eval- ity, 16-h photoperiod, 10–12°C, then the temperature uated the success in grafting American beech. was increased gradually to match outdoor conditions). The grafts were assessed for flushing of buds every week Materials and methods for 15 weeks. The diameter of the rootstock was mea- th Plant material sured in the 15 week and graft success was determined at that time. Successful grafts were kept in the green- Material for this study was collected from trees with house for 5 months and then placed in an outdoor shad- diameter at breast height (DBH) of 15 cm or more. ed area until the inoculation trials began. Twenty-two putatively resistant and five diseased trees were selected at nine locations in southern New Testing resistance to BBD Brunswick for scion collections (Fig. 1). Disease-free Eggs of C. fagisuga were collected in mid-July 2003 and 2004 from infested trees located in southern New Brunswick (Fig. 1). Bark disks 5 cm in diameter were cut from the trees and placed in a moist cooler for trans- port to the laboratory. Masses of wax containing adult scales and eggs were removed from the disks under a dissecting microscope, placed in a fine mesh screen (1 mm2) and gently teased with a soft brush to separate the eggs.