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Abstract. In 1992, three elm (Ulmus spp.) cultivais reported to have tolerance to Dutch elm disease (DED) were planted in a field plot in Atherton, California, U.S. Cultivars included 'Prospector', 'Frontier', and 'American Liberty'. A fourth cultivar, 'Valley Forge', was added to the plot in 1999. American elms grown from seed were planted as controls. In the 10-year period from 1992 10 2002, tree height and trunk diameter were measured annually, and inspections for DED infections, elm leaf beetle activity, and other pests were conducted. In August 2003, one 'American Liberty' was found to be infected with DED. This is the only tree that has shown infection symptoms since the beginning of the study. Elm leaf beetle activity was highest in 1993 and 1994, causing substantial injury to 'American Liberty', 'Frontier', and controls. 'Prospector' sustained little injury. Other pest activity has been slight in most years. In 2002, 'Frontier', 'American Liberty', and controls were found to have equivalent trunk diameter growth, and all were significantly larger than 'Prospector'. After 10 years, 'Frontier' was tallest [11.3 m (37 ft)], 'Prospector' was shortest [6.9 m (23 ft)], and 'American Liberty' and controls were in between [9.6 and 10.3 m (32 and 34 ft), respectively]. 'Valley Forge' was monitored for 3 years and then removed from the study because of poor structure.
Key Words. Dutch elm disease; elm cultivars; disease resistance; elm leaf beetle; pest tolerance.
Over the past 50 years, Dutch elm disease (DED) has caused a serious decline in the elm population in California, U.S. Although total mortality is not known, records indicate that 2,849 trees infected with DED were removed from 1975 to 1990, with virtually all the cases coming from the San Francisco Bay Area and Central Valley (California Department of Forestry and Fire Protection 1990). Although DED was first detected in California in 1975, isolations from annual rings indicate that it may have been present since 1953 (Tidwell 1982). Consequently, cases occurring before 1975 are not recorded, but the number may approach that from 1975 to 1990. Many more elms have died since 1990, but complete records have not been maintained. Collectively, from 1953 to the present, thousands of elms have been removed because of DED. With new infections continuing to...