We believe that, in cutting out the diseased bark, it is advisable to cut out also a few of the outer annual rings of wood (of course tangentially), especially if the canker is one of long-standing, since we know that the fungus eventually penetrates the outer rings of wood. Since that is true, the canker might enlarge later on from this same source of infection. Further it may also be possible for spores or bits of mycelium to be transported upward in the sap stream and cause new infections higher up in the tree. A thorough painting of the cut surfaces should go far toward remedying this situation.
One can usually judge the extent of damage caused by the blight by the number and vitality of the basal shoots, a large number of basal shoots indicating a heavy attack. However, if the roots have been severely injured, perhaps by short-tailed mice, as sometimes happens, no basal shoots appear, in which case the tree is doomed.
If no blight is present, but one or more basal shoots appear (sometimes due to shrubby ancestors), it is advisable to inarch these as an insurance against possible trouble in the future.
This inarching process has not received the attention it deserves. There is absolutely no reason why, if this method is followed, there should be any death from blight in resistant hybrids or in Japanese or Chinese chestnuts, barring, of course, cases where roots are attacked by mice (or Phytophthora in warmer regions). Those of our trees in Connecticut which have been blighted have continued in health and nut-bearing ever since we began the inarching method in 1937 (Fig. 4). If the inarches become blighted, they can themselves be inarched, as shown.
[Illustration: Fig. 3. Veneer crown grafting on chestnut. Photo by B. W.
McFarland, Conn. Agric. Expt. Sta. May, 1952.]
Research on Blight Resistance
[Illustration: Fig. 4. Japanese-American Chestnut, 21 yrs. old, showing inarching begun 15 yrs. ago. Original trunk, long since dead and now rotting, shows in center. Kept alive and vigorous because valuable for hybrid vigor and future breeding. Sleeping Giant Chestnut Plantation, Hamden, Conn. Photo by Louis Buhle, Brooklyn Botanic Garden. Sept. 26, 1952.]
A study has been made of the factors that cause the Chinese and Japanese chestnut to be resistant to the Endothia canker, and a close correlation was found between the tannin content of the bark and the relative resistance of the three species, i.e., Chinese, Japanese and American chestnut. The total tannin concentration in the bark of the Asiatic species is only slightly higher than in the American, and native trees can be found with as high a concentration as is found in the Asiatic. A similar overlap in resistance does not occur and it is therefore clear that the total tannin concentration as such cannot account for resistance. There is, however, good evidence that the tannins in the Asiatic species, as a result of the way in which they are bound to other colloids in the cells, are more soluble than in the American species. This, of course, would have a marked bearing on the effectiveness with which the tannins could check the spread of the parasite. Furthermore, it has been found that the types of tannins in the three species differ. In the American and Japanese species they are a mixture of catechol and pyrogallol tannins, while they appear to be pure pyrogallol tannins in the Chinese species. Considering the specificity of the enzyme systems of fungi it is quite possible that different tannins show different degrees of toxicity to a certain fungus. The following hypothesis has been suggested to explain the relative resistance of the three species: In the American chestnut bark the concentration of the available toxic tannin never reaches a level where it can stop the advancing parasite. The tannins in the Japanese species, although of the same type as in the native tree, are more soluble and reach a level toxic to the fungus. In the Chinese trees all the tannins of the bark belong to the toxic pyrogallol groups, and this, combined with their high solubility, results in the high degree of resistance in this species (4).
The information available at present regarding the formation of tannins in plants is not conclusive. In some plants, apparently, they are formed in the leaves, and the presence of carbon dioxide and light is required; in other plants the tannin concentration can increase when the plants are grown in darkness (5). A more general formation of tannin in tissues with a high metabolic rate throughout the plant has also been suggested (3).
It would be important to know the centers of origin of the tannins in the chestnut, their translocation, and whether they are translocated through or over graft-unions. In other words, will a susceptible scion when grafted on a resistant rootstock become more resistant because antibiotic substances formed in the roots of the resistant rootstock are translocated into the scion?