Coming home and getting near the park, we again saw these strange-looking trees we had wondered about with Nick and Terry. I thought that maybe they were grafted, and at the park guard house I found out I was right.
It turns out the trees are English Walnuts, but the lower trunks are Black Walnuts. The English Walnut has a milder taste and is more popular, but the Black Walnut has a stronger root system. So they graft English Walnuts on Black Walnuts for the best of both. So now we know.
https://ourrvadventures.wordpress.com/2011/06/03/
Visitors to our orchards often ask about the tree trunks, particularly the large dark-colored part just above the ground. It is the junction between the rootstock and the upper, producing portion of the tree. English walnuts lack strong, disease-resistant roots so almost all English walnut varieties (and there are over 100) are grafted on a strong, vigorous, long-lived rootstock - usually black walnut or Paradox (a hybrid of English and black walnut).
In a nursery the rootstock grows for one year from the planted nut. Then it is whip grafted to the English walnut and typically grows one more year prior to being dug up during the winter and sold as bareroot in the spring. In our orchards the rootstock was planted and allowed to grow up to five years before the English walnut was grafted to the root.
Typically in nursery-grafted trees and vines the junction is close to ground level. However in walnuts the graft may be 3 to 5 feet above ground. The reasons being the black walnut rootstock stump may be very valuable when the orchard is finally removed and grafting at waist height as opposed to grafting at ground level is much easier on the back.
In many cases walnut orchards may last 80 to 100 years. When the orchard removal finally occurs the wood is much sought after for everything from car dashboards to gun stocks, furniture, cabinets and flooring.
http://www.lanogalerablog.com/2013/02/walnut-grafting.html
하지만 조심해야 한다.
These walnut trees have blackline, a serious disease caused by infection with cherry leaf roll virus (CLRV). In California, English walnut is usually grafted on Northern California black walnut or Paradox (English and black walnut hybrid) rootstock. Grafted walnut trees usually perform better than non-grafted trees, because the rootstock imparts tolerance to certain soil conditions and pests. One potential drawback to grafting trees is the possibility for incompatible interactions.
Blackline develops because the English walnut is tolerant to CLRV, whereas Northern California black walnut is intolerant of the virus. CLRV is transmitted in walnut orchards by infected pollen. The virus will proliferate and move in the tolerant English walnut. However, once the virus spreads down from the point of infection to the graft union, the rootstock’s cells will hyper-react to the presence of the virus by forming a layer of dead cells to “wall off the virus.” As the virus continues to spread to the graft union from above, eventually, a strip of dead cambium cells will form at the graft union, making a characteristic “blackline.” The death of cambium cells between the scion and rootstock will eventually girdle the tree.
Blackline of English walnut symptoms include trees with declining vigor, an abundance of suckers, and the necrotic strip or blackline (visible after bark is removed at graft union). For English walnut grafted on Paradox rootstock (top picture), blackline can appear thicker than a line – more like a canker – extending from the graft union down. These trees often collapse suddenly in the summer. For English walnut grafted on Northern California black walnut rootstock (bottom), the blackline is very distinct. These trees are marked by a slow decline in tree health and by a proliferation of suckers. Control measures include removing symptomatic trees from the orchard and re-planting or re-grafting with walnut trees that flower at a different time than the existing walnut trees (which are potentially infected). Additionally, in areas with very high levels of CLRV-infected pollen, own-rooted English walnut trees may be planted.
http://www.ohmyfungi.com/viruses/
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