Asian Buxus species and the four hybrid varieties:
The Sheridan varieties are grown extensively here in Canada, as they are some of the few varieties in the market that combine winter hardiness with the valuable aesthetic ornamental traits. Currently there are three Buxus breeding programs of note: Herplant in Belgium (Better Buxus), Saunders Genetics in Virginia (NewGen) and USDA-ARS program at the national arboretum in Beltsville, Maryland. Dr. Didier Hermans of Herplant began breeding Buxus in 2007. They have bred a large collection of hybrids that are interspecific crosses between B. sempirvirens and different B. microphylla subspecies of Asian origin. Although not breeding specifically for BTM resistance, Herplant tested several Buxus genotypes for BTM resistance which included the four boxwood-blight- resistant varieties: Renaissance, Skylight, Heritage and Babylon Beauty. Once BTM larvae developed into caterpillars on the plants, they were able to determine the percentage of dead versus live caterpillars. The new varieties showed a remarkably high percentage of caterpillar mortality whereas B. sempirvirens remained a good food source for caterpillars. These tests concluded that
America, it seems natural that breeding programs will shift to focus on developing boxwood varieties that exhibit resistance to box tree moth. CNLA’s New Plant Development Committee (NPDC) has had multiple discussions on this topic over the past couple of years and contracted Dr. Daryl Somers this past spring to conduct a literature review on the topic of breeding BTM resistant boxwood. The committee was interested in the feasibility of using genetic modification to insert the Bacillus thuringiensis (Bt) toxin gene into the boxwood genome as one method for resistance breeding and asked that the process be investigated and outlined in the report. Historically, Buxus breeding has relied on chance seedlings and sport mutations to derive new varieties, or relied on open pollination of Buxus collections to generate seedlings. Sheridan Nurseries introduced three varieties in the early to mid 1970s (Green Gem, Green Mountain and Green Velvet) which were interspecific hybrids of B. sempervirens x B. microphylla var. Koreana. B. sempervirens is native to Europe, northwest Africa and southwest Asia while B. microphylla is native to Japan and Korea.
1) showed less pupae forming, 2) showed a high number of dead caterpillars and 3) remained relatively green in field tests under BTM pressure. Since they were not directly breeding for BTM resistance, these results were simply a side effect of using Asian species in their hybrid development and selecting hard for blight resistance. Similarly, the use of Asian boxwood parentage in the Saunders program is present with the intent to develop resistance to both BTM and blight. The USDA program was not working on BTM resistance and is focused heavily on blight resistance and other performance features. However, they have recruited a Post Doctoral researcher to begin work on genetic modification for BTM resistance. The obvious modification would be to add the Bt toxin expression into Buxus that would provide resistance to BTM just as the Bt spray does. Genetic modification involves the insertion of a specific DNA sequence
DECEMBER 2024 - JANUARY 2025 | 67
Made with FlippingBook - Online magazine maker