has gained only about 40 species from North America. Vermeij (1991) observed that asymmetry characterizes most, if not all, biotic interchanges between biogeographic regions. Possible explanations for this phenomenon are discussed later in this report.
Most plant pathogens in the United States have entered inadvertently and unnoticed as contaminants of agricultural commodities, nursery stock, cut flowers, timber, seeds, and other plant parts and in the soil of potted plants, freight containers, packing material, and vehicles (U.S. Congress 1993, Pimentel et al. 2000). Imported collections of wild germplasm might have also been infected with viruses (Kahn 1991, Lenné and Wood 1991). Imported military cargo might have also been the source of hitchhiking pests. Soil on military and other used vehicles might have been the source of the corn cyst nematode (Heterodera zeae) (Sardenelli et al. 1981).
Despite an increase in trade and travel over the last half-century, fewer than five new nonindigenous plant pathogens were introduced in each decade from 1940 to 1970 (U.S. Congress 1993). The enactment of state and federal plant quarantine laws in 1912 are likely to have prevented an increase in introductions of pathogens commensurate with the volume of trade. During the 1970s, however, the number of newly introduced plant pathogens jumped to 18. Possible explanations include the globalization of agriculture, increasing commercial air traffic, and increased movements of seeds and plant material by the private sector spurred on by new intellectual property rights to plant varieties. The number of newly introduced pathogens dropped to seven during the 1980s. As of 1991, there were an estimated 239 plant pathogens in the United States whose origins are outside the United States (U.S. Congress, 1993). This number probably reflects the nonindigenous origin of most economic crops in the United States, the natural hosts for their respective nonindigenous plant pathogens. However, figures on detected pathogens undoubtedly represent organisms that have been recognized because they are causing notable diseases of economically important plant species. For most new disease outbreaks, researchers do not know, without population-genetics studies, whether an outbreak is due to an invasion or a previously unrecognized endemic pathogen that suddenly increases because of a change in cropping practices or environment. It is unknown whether plant pathogens already established in the United States might have spread beyond their native hosts to introduced crops. It might also be assumed that some nonindigenous pathogens have been introduced but remain undetected. There is a disincentive to survey for these, however, because of concerns of how such findings might affect some U.S. exports of plant-related commodities. Nevertheless, new technologies with increased sensitivity, such as polymerase chain reaction-based techniques, will become increasingly useful in separating indigenous