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The American Chestnut Foundation’s Breeding Program. F. V. Hebard Research Farms Meadowview, VA Fred@acf.org.
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The American Chestnut Foundation’sBreeding Program F. V. Hebard Research Farms Meadowview, VA Fred@acf.org
The American chestnut tree was an important forest tree throughout its range. For instance, it comprised 25% of the standing timber in Pennsylvania at the turn of the century. Some of this was due to its rapid, abundant regrowth after cutting, and due to the fact that the valleys where it had been less prevalent had been cleared for field crops. Nevertheless it was quite important. This tree grew in North Carolina.
Canker Chestnut blight is a canker disease. A canker is defined as an area of dead bark. In chestnut blight cankers the bark is killed by the blight fungus, and is characterized by an orange discoloration on younger trees, typically with longitudinal cracks and slight sinking of the afflicted area. Cankers effectively girdle the trunk once they encircle it.
Cutting into a canker shows the dead brown bark, in comparison to the living white and green tissues in the surrounding area that has not been infected yet.
Much of the orange color is due to the stromata, which bear the fruiting bodies of the blight fungus.
Two types of spores are produced in the stromata. Conidia can be squeezed out in long tendrils like toothpaste, such as seen here. These can be spread to other trees in rain splash or stick to the feet of birds or other animals to be transported extremely long distances. Ascospores are the second spore type. The windborne ascospores are responsible for most spread of the disease over intermediate distances, from about 5 feet to 500 yards. Each stroma can produce thousands of both types of spores. Overall, billions and billions are produced on each tree.
Chestnut blight cankers are characterized also by the presence of buff-colored mycelial fans in the middle of the bark.
A photomicrograph of a mycelial fan growing through chestnut bark. myclelial fan
In American chestnut, mycelial fans form more rapidly, in greater numbers, and grow more quickly than in Chinese chestnut, which is resistant to blight. myclelial fan myclelial fan myclelial fan
Current status of American chestnut in our forests Chestnut is abundant in the understory of mature forest.
Trees in forest are old and slow growing (diameter increase of <1 mm/yr).
Fast-growing chestnut in 4-yr-old clearcut (note camera-shy man to left behind the pitch pine).
Trees are young and fast growing in clearcuts (diameter increase of >1 cm/yr).
Chestnut blight is rare on sprouts in mature forest, but becomes epidemic after clearcutting. The incidence of blight approaches 100%. The photo shows a single, blight-killed chestnut in a 9-yr-old clearcut, surrounded by younger sprouts which came up after the older sprout died.
All large chestnut trees are killed by blight in 9-yr-old clearcut.
Current status of American chestnut in our forests • Chestnut blight is rare on sprouts in mature forest, but becomes epidemic after clearcutting, incidence approaching 100%. Increase of blight incidence with years after clearcutting.
The epidemics occur due to a 10-fold increase in sporulation per canker, caused by the larger size of trees in young clearcuts than in mature forest. Increase of canker size with years after clearcutting.
Naturally Occurring Hypovirulence?: • Toward the end of the epidemic, a few trees survive blight longer than usual. A tree that survived several years longer than most trees at this site. Trees at this site were not released from competition; all the thousands of chestnut sprout clusters in the interior of this cut are now dead.
Normal canker that killed its sprout. “Big, ugly” canker on surviving tree.
When the chestnut blight fungus is isolated in pure culture from big, ugly cankers, some of the culture colonies are white.
This is in contrast to the normal yellow color of colonies of the chestnut blight fungus.
When inoculated back into American chestnut, white cultures give smaller cankers than yellow cultures. They are hypovirulent. In addition, the virus causing whiteness and hypovirulence can move from a white to a yellow culture. Here, the center canker, caused by a yellow culture, started out larger than those caused by the white cultures, but then stopped expanding. White hypovirulent cultures were isolated from it, and they were the original yellow culture infected with the virus from the original white culture. Yellow culture White culture White culture
Here, in a canker of the same age as the previous photograph, the inoculation with the yellow culture was not subtended by white cultures. The canker got much larger. Yellow culture No white culture
Naturally Occurring Hypovirulence?: • Longer periods of survival in clearcuts are associated with release of young sprouts from competition, as proposed by Griffin. We release trees to promote flowering, which increases with increased exposure to sunlight. This cut was 12 years old in 2001. Its trees were first released in 1996, resulting in a pure stand of chestnut on the hill in the foreground.
On surviving trees at sites released from competition, there is a tendency for less invasion of the callus ridges at canker margins than occurs on surviving trees at untreated sites.
Large, surviving American chestnut trees. Some trees have been surviving blight since the 1920s.
The Amherst tree, in Amherst County, Virginia, is probably about 150 years old, and was a large tree when chestnut blight first struck it in the 1920s. All large, surviving American chestnut trees are infected with hypovirulent strains of the blight fungus. In addition, some have low levels of blight resistance. Our sister organization, the American Chestnut Cooperators’ Foundation, is breeding these for blight resistance. We also work with large, surviving American chestnut trees in Meadowview.
Requirements for breeding • Breeding Method: the sequence of crosses to perform. • Materials: sources of resistance and flowering American chestnut trees. • Crossing Method: for making the next generation of progeny. • Cultivation Method: for growing the trees. • Screening Methods: for determining the blight resistance of progeny, and other traits.
Backcrossing Expected Proportion Chinese C x A ¯ F1 x A ¯ B1 x A ¯ B2 x A ¯ B3 x B3 ¯ B3-F2 x B3-F2 ¯ B3-F3 1/2 1/4 1/8 1/16 1/16 1/16
Materials • American chestnut trees flower in clearcuts
MaterialsSources of blight resistance • The old USDA breeding program determined that Chinese chestnut had the most blight resistance of all the chestnut species. In American forests, the Chinese chestnut itself cannot reach the overstory when competing with other tree species. We were able to jumpstart our program by using two first backcrosses from the USDA & Connecticut programs. • The low levels of blight resistance in some of the large, surviving American chestnut trees might also be useful in a breeding program, which we are exploring. • We also use Japanese chestnut as a source of resistance.
Crossing Method Flower being pollinated
We use standard horticultural techniques adapted to screening chestnut trees for blight resistance. These are high input horticultural methods, not silvicultural methods. • Our orchard spacings are designed to grow trees as fast as possible until they can survive being screened for blight resistance long enough to produce a crop. • Trees with intermediate levels of blight resistance need to be 3-4 years old, about 2 to 3 inches in diameter at breast height, to survive inoculation. • Trees with high levels of blight resistance can survive inoculation when they are only 1 or 2 years old, less than an inch caliper. • This requires a lot of land and effort compared to breeding field crops, and more time, of course. Cultivation Method
We work the ground up and lay plastic mulch. We then dig a hole down through the mulch with a bulb planter, fill the hole with potting mix and plant nuts about one-half-inch deep.
We protect the nut from critters with a cylinder rolled from aluminum flashing and cover the cylinder with a styrofoam cup to keep the elements out.
After one growing season, the trees are about 18 inches tall.
Screening Method • We determine the blight resistance of chestnut by giving them the disease, inoculating them with the blight fungus, and observing the resultant cankers. Cutting disks of agar and mycelium of the chestnut blight fungus from cultures in petri dishes.
