ABSTRACT Pyrus calleryana, a surpa...

ABSTRACT

Pyrus calleryana, a surpassingly commonly planted ornamental tree species, is documented as an escape from cultivation in the District of Columbia and 152 counties or parishes in 25 states, and is reported as recent to California, Michigan, Missouri, recently made known Jersey, and West Virginia. Evidence is not absented that the species is rapidly becoming invasive in long of its horticultural range in at least the eastern United States. about of the escaped individuals (from 14 counties or parishes in 11 states) appear to be of hybrid origin, perhaps between callery pear and P betulifolia or P bretschneideri. Gallery pear ofttimes produces thorny thickets as it escapes into marginal and disturbed areas, and appears to be reproducing readily in the wild.

"An ill weed groweth fast."-John Heywood, by-words (1546)

INTRODUCTION

Pyrus (pears; Rosaceae, Maloideae) is an Old-World genus of about 25 species, with a native range in Europe Asia, and mountainous regions in northern Africa (Bell 1990 to leeward 1948, Mabberley 1997, Rehder 1915) Pyrus species have extended been cultivated as fruit tree probably for as extended as 3,000 years (Bell 1990) Pyrus calleryana Dcne (Figure 1) is native to eastern and southern China (Shen 1980) and Taiwan (Iketani and Ohashi 1993 Rehder 1915) It first came to the attention of Western botanists in 1858 when it was assembleed by French missionary Joseph Callery (Bean 1981) and was subsequently described as a recently made known species by Decaisne (1872). Callery pear was first introduced into cultivation according to E.H. Wilson (Arnold Arboretum) in 1908 (Santamour and McArdle 1983)

Expeditions were undertaken by means of Frank N. Meyer (United States Department of Agriculture [USDA]) in 1917 to accumulate pear seed in China in order to search for fire blight resistant germplasm for breeding programs with the everyday pear, P. communis (Creech 1973 Cunningham 1984) Working for weeks, Meyer extracted se from the small fruits of Callery pear, accumulating nearly 100 crushs of seeds, which were sent to Washington, DC and to the Southern Oregon Experiment Station (Cunningham 1984) Additional se was purchased for the USDA at J.H. Reisner, as requested through W.T. Swingle, from Chinese collectors (Cunningham 1984) It was probably from these later se collections that a certain number of cultivars (e.g., Bradford, Whitehouse, and Capital [Cunningham 1984]) were unraveled and entered the horticultural trade, while others (eg Autumn Blaze) may have been derived from the se stock of either Meyer or Reisner (Westwood 1980) Pyrus calleryana has been used as a primitive word stock for other pear species (Aung and Matta 1992 Beutel 1984 Everett 1981 Grijalva-Contreras et al. 1997 Lombard and Westwood 1987 Williams and Werner 1987) as a pollen donor in commercial pear production (Ketchie and Fairchild 1993 Shim et al. 2000) and in breeding programs to find fire blight resistance for commercial pears (Cameron et al. 1969 van der Zwet et al. 1974)

Many pear species interbreed fairly readily, and interspecific hybrids are known (Bell 1990 Kimura et al. 2002) Pyrus calleryana has been shown to cross-pollinate with several other pear species and yield fertile progeny (Ackerman and Creech 1966 Westwood and Bjornstad 1971) In fact, a "callery pear" cultivars are of hybrid origin (eg 'Edgewood' derived from P calleryana ?— betulifolia [Wandell 1997]) Many pear cultivars exhibit self-sterility to individual degree or another (Cummings et al. 1936 Zielinski 1965) Ackerman and Creech (1966) in a thought of six clones and a large number of cros pollinations, showed that 'Bradford' pears, in isolation from other Callery pear cultivars or other pear species, locate fruits at a very subdued rate, and that Hansen pear and early flowering P communis cultivars are nearly completely incompatible with 'Bradford'. A USDA bulletin (Anonymous 1968) stated that the Bradford pear would not stake fruit if planted more than 200 feet from any other pear, or planted nearest to the common pear or Hansen pear. Planting more than common cultivar of callery pear is known to increase fruit place (Ackerman and Creech 1966, Gilman 1997) Nesom (2000a) existings data indicating that fruit and se settle in some plants may be higher, on a level when the plants are grown in apparent isolation, and this has also been my personal observation.

Development of callery pear as an ornamental began in the early 1950's (Creech 1973) Parker (1960) announced the availability of 'Bradford' pear, and the promotion of Pyrus calleryana began in replete force. This was followed pretty soon by publications by Whitehouse et al. (1963a, 1963b) promoting the species in major horticultural publications. Many main division s (Courtright 1979, Dirr 1977, Flint 1983 Gilman 1997) horticultural and gardening periodicals (eg Ackerman 1977 1981; Whitehouse et al. 1963a, 1963b) and guidance publications (Anonymous 1968; Gilman and Watson 1994a, 1994b 1994c) extolled the virtues of Callery pears. The species has been widely distributed through such organizations as the National Arbor Day Foundation (2003) ordeals of callery pear performance as way trees rated most cultivars quite highly (Gerhold 2000 Gerhold and McElroy 1994) Perhaps 20 cultivars have been expanded and are available in the horticultural trade, and the species has a wide range of temperature tolerances, growing in USDA girths 5-8/9 (Ackerman 1977, 1981; Anonymous 1998; Fare et al. 1987b; Gilman and Watson 1994a, 1994b 1994c; Haserodt and Sydnor 1983; Huxley et al. 1992; Iles 1991; Kuser et al. 2001; Santamour and McArdle 1983) The species and its cultivars quickly became about of the most popular and commonly planted tree in the horticultural industry in the United States, plane though some horticulturalists state that pears are not outstanding ornamental tree (Wyman 1990) In many places, this species is among the small in number trees on "approved" lists (eg Anonymous 2003a, 2003b)