All over eastern North America right now, chestnut breeders are pollinating tree flowers.
"So here is actually some flowers," Retired forester John Scrivani explains.
They’re beautiful.
"And they’re the male, well probably all you see now are male flowers."
We're in a field of neck high grass in the Lesesne State Forest in Virginia. We’re looking at one of the East’s largest plantings of blight-resistant chestnut trees. The chestnut blight is a fungus accidentally brought to North America on imported Asiatic trees in the late 1800s, and it’s devastated our wild American species, rendering it functionally extinct. Since 1969, more than 12,000 hybrid chestnuts with American and Chinese and Japanese genetics have been planted here as part of an effort to breed blight resistance into our native trees.
"The initial idea was that you could create a American-Chinese, American-Japanese, or some kind of combination of hybrids that would, have the resistance of the Asian species but hopefully grow like Americans," said John.
The American trees are taller than the Asian species and produce smaller nuts, which were beloved before the spread of the blight. But producing a hybrid tree with blight resistance that grows like an American tree has proved very challenging:
"There was actually a study that the U.S. Forest Service put a bunch of these hybrids in forest locations and most of them just got overtopped by other trees and they were good as orchard trees but not really growing well in the forest."
Here at Lesesne, though, foresters have cut down competing trees, so that the canopy is filled with chestnuts blooming.
"So what we do is get up in the tree somehow, bucket truck or ladder or whatever, and find those female flowers, they look like little pineapples. You emasculate—the term is, you cut off the part that has the male flowers of the catkin, and then you put a little paper bag, white paper bag which is actually what’s used for corn pollination on top of or over the twig so there’s nothing in there but the female flower, and I usually leave a few leaves in there as well."
This isolates the female flowers on breeding trees from the huge sea of male pollen that’s all over the area once the catkins release. This way, breeders like John can control the crosses they’re making, selecting trees that not only have strong American genetics, but that also have high levels of blight resistance. John points to a tree with a large swollen area on its bark and says this is actually what he likes to see.
"This is what’s called a swollen canker. So the tree has created callous tissue, which is compartmentalizing out the blight."
He further explained, "So the blight actually grows only in the bark, and it produces oxalic acid. And that’s — oxalic acid kills the xylem cambium, and that’s what kills the tree. So if it’s killing the tree what’ll happen is the wood is dying and it’ll be a sunken canker. And I’ll try to find one. So this is like actually a sign of some level of resistance."
How much resistance though, is hard to say. When breeders first started trying to create a blight-resistant tree in the 1920s, they thought blight resistance might be controlled by two or maybe three genes. But it’s turned out to be much more complicated.
"Here I think there's like 27, if I remember correctly, sources of resistance."
And that’s just at this site, on these trees. And so, as breeders continue to look for blight resistant mostly American chestnuts, pollination continues each summer all over eastern North America, flower by flower.
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