Georgia is the largest pecan producer in the United States (U.S. Department of Agriculture, 2009). A substantial amount of Georgia’s production is from orchards planted 60 to 100 years ago. Typically, these orchards were planted at 14.2 m × 14.2-m spacing, requiring removal of a portion of the trees to avoid excessive crowding. Many cultivars in these orchards are obsolete and perform poorly. In addition, disease or canopy loss has rendered many trees unproductive or their nut value is less than production costs.
Old orchards may be renewed by removing all trees and establishing new orchards with currently recommended cultivars. However, this approach eliminates cash flow for 6 to 8 years and profitability is unlikely for at least 9 years (Foshee et al., 1997). Orchards are renewed by selective removal of inferior trees (Goff and Browne, 2004) followed by interplanting recommended cultivars (seedling rootstock grafted to scion cultivar) or seedling rootstocks for later grafting (Wells, 2013). This approach maintains cash flow and reduces competition for remaining established trees, frequently resulting in increased total production, improved nut quality, and less disease pressure from pecan scab [Cladosporium caryigenum (Ellis and Langl.) Gottwald].
Cultivar pecan orchards are started by planting seed at the desired orchard spacing to produce a rootstock followed by grafting to acceptable cultivars, planting nursery-produced seedlings at orchard spacings followed by grafting or planting nursery-grafted trees (rootstock grafted to scion cultivar). Some advantages and disadvantages are obvious for each tree type. For instance, seed or seedlings are inexpensive, decisions regarding the scion cultivar can be delayed, and limited availability of the preferred cultivar is avoided (normally graft or bud wood is not limiting). Disadvantages include the expense of grafting and, depending on the skill of those involved, may require 3 or more years to successfully graft all seedlings. The time required to begin production may be longer using seed or seedlings. One significant advantage, particularly in northern production areas, of establishing seedlings and then grafting is greater cold-hardiness when grafted higher on the rootstock trunk (Sparks and Payne, 1977). Observations also suggest most seedling trunks (juvenile growth) are less prone to damage than cultivar trunks (adult phase growth) when mechanically thinning fruit. In hurricane-prone areas of the Southeast, whip-grafted trees that are grafted near or below ground level are frequently planted too deep resulting in poor anchorage and are subject to wind-throw.
Data are unavailable comparing pecan orchard renovation with grafted vs. seedling trees; however, one study compared seed, seedling, and grafted pecan trees when establishing a new orchard (Thompson et al., 1982). Pecan yields during initial production were similar 3 of 5 years for trees started from seed, 1- or 3-year-old seedlings that were budded their third growing season, and nursery-budded trees. In walnut (Juglans regia L.), cumulative yield (8 years) was greater using grafted trees or planting seed and then grafting to the desired cultivar than those planted as seedlings and then grafted (Reil, 2001).
One problem encountered with interplanted pecan trees was a symptom called “mouse-ear or little-leaf,” a disorder formerly thought to be a manganese shortage (Gammon and Sharpe, 1956). In its most severe form, young trees were severely stunted and unproductive. Wood et al. (2004a, 2004b) conclusively demonstrated that mouse-ear was caused by nickel deficiency. Routine foliar application of a commercially available nickel product eliminated this problem.
Crown gall has the broadest host range of any plant pathogen with over 600 characterized plant hosts (DeCleene and Deley, 1976). This disease is widespread in most old Georgia pecan orchards with 25% to 60% of the trees infected (Bouzar et al., 1983). Galled trees are stunted and less productive. Diseased trees may have resulted from contaminated nursery stock (Moore, 1976) or the soil inhabiting bacterium was present when trees were planted. A common weed control practice before herbicides came into widespread use was soil disking. This practice repeatedly wounded roots increasing the likelihood of infection (Formica, 1989) and probably contributed to the pervasive infection common in these old orchards (Bouzar et al., 1983).
Removal of old trees is accomplished using several options. These range from removal with a chainsaw, tree cutter, bulldozer, or track hoe. Removal with a chainsaw or tree cutter leaves the stump and roots intact, whereas a bulldozer or track hoe removes the stump and a portion of the root system.
No data are available regarding performance of transplanted trees into an existing orchard. Concerns include transfer of crown gall from stumps of removed trees or remaining old trees and negative impacts of stumps or remaining trees on growth of transplanted trees. Because renovation practices include the use of nursery grafted trees and seedling trees grafted after establishment, both tree types were included in the study. We report performance of transplanted trees before production.
BouzarH.MooreL.W.SchaadN.W.1983Crown gall of pecan: A survey of Agrobacterium strains and potential for biological control in GeorgiaPlant Dis.67310312
DraperN.R.SmithH.1966Applied regression analysis. Wiley New York NY
EpsteinL.KaurS.McKennaJ.R.GrantJ.A.OlsonW.H.ReilW.O.2008Crown gall can spread between walnut trees in nurseries and reduce future yieldsCalif. Agr.62111115
FormicaJ.V.1989Crown gall neoplasms p. 497–512. In: Sirica A.E. (ed.). The pathobiology of neoplasia. Plenum Publishing New York NY
FosheeW.G.IIIGoodmanR.W.PattersonM.G.GoffW.D.DozierW.A.Jr1997Weed control increases yield and economic returns from young ‘Desirable’ pecan treesJ. Amer. Soc. Hort. Sci.122588593
HarmonM.E.FranklinJ.F.SwansonF.J.SollinsP.GregoryS.V.LattinJ.D.AndersonN.H.ClineS.P.AumenN.G.SedellJ.R.LienkaemperG.W.CromackK.JrCumminsK.W.1986Ecology of coarse woody debris in temperate ecosystemsAdv. Ecol. Res15133302
KluepfelD.A.McCleanA.E.YakabeL.E.MaccreeM.M.ParkerS.R.2010Detection and implication of early Agrobacterium tumefaciens infection of Paradox seeds and seedlingsActa Hort.861497500
MooreL.W.1976Latent infections and seasonal variability of crown gall development in seedlings of three Prunus speciesPhytopathology66101
MooreL.W.2002Disease caused by a bacterium: Crown gall p. 10–11. In: Teviotdale B.L. T.J. Michailides and J.W. Pscheidt (eds.). Compendium of nut crop diseases in temperate crops. APS Press St. Paul MN
OlsenS.R.SommersL.E.1982Phosphorus p. 404–430. In: Page A.L. R.H. Miller and D.R. Keeney (eds.). Methods of soil analysis. Part 2. Chemical and microbiological properties. Amer. Soc. Agron. and Soil Sci. Soc. Amer. Madison WI
ReilW.O.2001Comparison of Chandler walnut grown on both Paradox and northern California black walnut rootstock that were planted as seed, seedling or grafted treesActa Hort.544481488
U.S. Department of Agriculture20092007 Census of Agriculture. Natl Agri. Stat. Ser. Washington DC