Connecticut Chestnut Research: Breeding and Biological Control
By Dr. Sandra L. Anagnostakis
Department of Plant Pathology and Ecology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P. O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8489 Fax: (203) 974-8502
Experiment Station History
The Connecticut Agricultural Experiment Station was founded in 1875. Offices and laboratories are in the city of
Chestnut breeding work was begun early in the
My early work at The Experiment Station included studies of the basic genetics of the blight fungus (and the system of vegetative compatibility that restricts hyphal fusion and the transfer of biocontrol viruses from one strain to another), and tests of extracellular enzymes produced by this fungus (1, 2, 13).
Chestnut Breeding in
Our breeding plan was first based simply on making hybrids of blight resistant Asian trees with susceptible American trees and testing the hybrids for resistance to Chestnut Blight Disease (4). When it became clear that at least two genes were responsible for this resistance, we began a back-cross breeding program based on the plan of Charles Burnham (7). Asian trees are crossed with American trees, and the hybrids (partially blight resistant) are crossed to American trees again. If there are two resistance genes, one out of four of the progeny from these back-crosses have one copy of both resistance genes, giving them partial resistance. If there are three genes for resistance, one out of eight of the progeny will have one copy of all three resistance genes. these trees with partial blight resistance are crossed again to American. This repeated back-crossing increases the percentage of American genes in the hybrids, and selecting for partial resistance insures passage of the resistance genes. A final cross of two trees with partial resistance should result in one of sixteen trees having two copies of two resistance genes (or one of sixty four trees having two copies of three resistance genes), which will make them fully resistant to the chestnut blight fungus (7). We bag female flowers in late June to protect them from pollen, and put selected pollen on the flowers in July and cover them back up again. Many hybrids are male sterile; the catkins form but the flowers never bloom to produce pollen. This is only seen in interspecific hybrid trees, but is a feature valued by nut growers who want to plant orchards of male sterile trees with a few pollen producing trees for yields of nuts which are uniform.
We have what is probably the finest collection of species and hybrids of chestnut in the world for use in this breeding program, with all seven species represented. The breeding work has been greatly helped by a genetic map prepared by Thomas Kubisiak (11). He has molecular (RAPD) markers for the chromosomal regions (“genes”) associated with resistance to chestnut blight, which could make selection much easier in the future. Trees of two kinds are being chosen: for timber (tall and straight, with little energy put into forming nuts) and for nut production (short and spreading with maximum energy put into forming large, good tasting nuts). Both kinds of trees must have resistance to Chestnut Blight Disease and be well adapted to our climate. We are starting to select our trees for resistance to Ink Disease, caused by the root pathogen Phytophthora.
Biological Control of Chestnut Blight Disease
In the late 1950’s, a chestnut recovery phenomenon was discovered and studied by Jean Grente in
We started testing a population replacement experiment with transgenic strains in 1997, spraying chestnut sprouts at least twice a year for four years with conidia from three transgenic strains, and spraying chestnuts in the control plot with water. We still cut back competing trees to allow the chestnuts to have the best chance to grow, and monitor the trees to see whether we have effected a stable biological control. For the last two years we have recovered none of these transgenic strains from the two test plots, but hypovirulence viruses have been found in isolates from many cankers. Soon we will begin looking for evidence that a biological control can spread through the forest around the plots.
Synthesis of Breeding and Biological Control
Chestnut seeds of four kinds of hybrids were planted at the Connecticut State Nursery in the spring of 1998. the resulting 500 chestnut trees were lifted in the spring of 2000 and 100 of them were planted in a clear-cut in Prospect, CT on land owned by the Town of Prospect and managed by the Connecticut Water Company, and 25 were planted in a clear-cut at Sessions Woods Wildlife Area in Burlington. These are being evaluated for survival under forest competition conditions. Of the remaining seedlings, the 200 best were planted in an orchard at our substation in
Ozark chinquapins (Castanea ozarkensis) are timber trees found on the Ozark Plateau in
The crosses that have produced blight-resistant trees for timber have, by necessity, used a rather narrow genetic base, even though different trees were used as parents in each generation. Since many of the native populations of American chestnuts in the
The Next Problem
Of course, no project is ever quite "finished." The oriental Chestnut Gall Wasp, Dryocosmus kuriphilus, was introduced into the
Since we now live in a world where travel and transport of pests and pathogens is all too easy, global communication and cooperation is our hope for the future.
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