Specific data have been obtained relating to several types of winter injury of Oriental chestnuts and hybrids. This information is limited to the performance of mostly young trees and to a comparatively small number of locations.
The fall freeze that occurred in mid-November, 1940, was studied in detail by Bowen S. Crandall,[9] formerly of this Division. Widespread damage occurred to Oriental chestnuts in the central parts of South Carolina, Georgia, and Alabama. Temperatures before the freeze had been mild, and heavy rains in early November had broken a drought. On the nights of November 15 and 16, temperatures of 12° and 14° F. were reported by various farmers, and a drop to 20° F. was general on the night of the 16th. The damage to chestnuts by this freeze was increased because of the mild temperatures and heavy rains that preceded the freeze. The chestnut trees were not able to attain complete dormancy. Those trees, however, that were growing on uplands or on sites that were well air-drained suffered much less damage. Apparently equal damage was inflicted to Chinese and Japanese chestnuts.
[Footnote 9: Crandall, Bowen S. Freezing injury to Asiatic chestnut trees in the South in November, 1940. Plant Disease Reporter 27:392-394. October 1, 1943.]
On one farm near Columbus, Ga., four plantings were located at different elevations. The planting at the lowest elevation, maintained as a well cultivated orchard, suffered almost 100 per cent loss from this fall freeze. The trees at the highest elevation, in a forest planting, were practically uninjured. The damage from this freeze varied from killing of buds and shoots to killing of complete trees. Many owners of chestnut plantings did not notice the damage until the following spring. Fortunately fall freezes of this magnitude occur only infrequently.
In the winter of 1944, this Division lost 23 per cent of its hybrids at Glenn Dale, Md., from freezing following abnormally high temperatures. The hybrids had been fertilized in October of the preceding year, but the effect on the extent of freezing damage is not known. The months of November, 1943, through March, 1944, were characterized by extremely variable temperatures. For example, in November a minimum of 15° F. occurred on the 17th, a maximum of 72° on the 19th; in December a maximum of 66° on the 3rd, a minimum of 2° on the 16th; in January a minimum of 8° on the 17th, a maximum of 74° on the 27th; in February a minimum of 11° on the 2nd, a maximum of 72° on the 25th; in March a minimum of 8° on the 10th, a maximum of 81° on the 16th.
The extremes of temperatures in any one of these months may have been sufficient to cause damage to chestnut, although the extent of damage is influenced by the physiological conditions within the tree. The usual type of injury to the hybrids was a killing of the cambial cells extending from the base of the trunk up to varying heights. The cambial region was grayish-black and the inner bark was sappy and greenish-brown. More trees were injured and killed on the lower portions of the plot than on the higher portions.
This catastrophe afforded opportunity to study resistance of the hybrids to freezing. In the lower part of the plot there were several 3-year-old American chestnut seedlings that were not damaged. Sixteen per cent of first generation hybrids of Chinese and American chestnut were killed. Chinese by American backcrossed with Chinese were killed to the extent of 36 per cent. Chinese by Japanese chestnut of the second generation were killed to the extent of 35 per cent.
Despite this extensive killing of hybrids by extreme variations of winter temperatures, older Chinese and Japanese chestnuts on slightly higher ground in the same plot suffered no visible injury. These old trees have rough bark, which may serve as an effective insulator against extremes of temperature. In 1944, there was no damaging late spring frost, and these old trees produced the largest nut crop in their history.
Winter temperatures of -25° F. or lower are usually injurious to Oriental chestnuts. A few reports of chestnuts surviving temperatures of -25° F. have been recorded, but usually Oriental chestnuts do not thrive in those northern States or regions where such temperatures occur.
Many of our cooperators report that late spring frosts frequently cause failure of chestnut crops. Damaging frosts in late spring occur more frequently and over greater areas than early fall frosts or extreme winter temperature variations. A late spring frost in 1945 reduced the chestnut crop at Glenn Dale, Md., from 50 bushels expected to 3 bushels actual. A freeze of 24°F. on the nights of April 4 and 5 was sufficient to inflict this damage after two weeks of abnormally warm weather. Many of the trees were visibly injured, with wilted or dried unfolding buds. Other trees on higher ground were not visibly affected, yet they produced no crops.