By FLIPPO GRAVATT
Dead chestnuts on the Blue Ridge Plateau Virginia.
AMONG the trees of the world, chestnuts, Castanea spp., are outstanding because they produce large crops of tasty nuts and numerous valuable products such as durable posts, lumber, and tannin-extract wood. The forest stands of the American chestnut, however, have been destroyed by two serious fungus diseases, blight and phytophthora root rot (ink disease). Both these diseases are serious on European chestnut also. In part of Europe the ink disease has seriously damaged that species and now the blight, already found in Italy and Spain, is an immediate menace to chestnut throughout that continent. In contrast, Asiatic species of chestnut are resistant to both these diseases.
Chestnut blight is caused by a fungus named Endothia parasitica. This fungus grows in the bark, cambium, and outer wood of affected trees. Within the bark and cambium region the fungus growth appears as characteristic buff-colored fans (page 5). These fans indicate the presence of the blight. However, if the chestnut tissue has been injured or partially killed by freezing or some other factor, the blight fungus will grow through it without forming fans. On the outer bark surface the fungus produces reddish-brown fruiting bodies about the size of pinheads and of two types. Under moist conditions sticky tendrils of spores ooze from one kind of fruiting body. These spores are spread by birds, animals, and insects; the other type of fruiting body produces wind-borne spores.
Usually the first symptom of the disease to be noted outwardly is the dying of a limb to which are attached yellow-brown leaves that contrast with the green of the rest of the tree. On examination of such a limb, a girdling blight canker will be found. Sometimes the cankered area is swollen, and sometimes it is sunken (page 6); but slicing away the bark will always disclose the fungus fans if the blight is the primary cause of the canker. On some trees, even of susceptible species, the blight fungus may grow extensively in the outer bark for four or five years before it extends into the cambium region and then kills the tree.
There are four species of the genus Castanea in Asia. The Chinese chestnut, Castanea mollissima, is a medium-size tree much used for orchard and home planting. This species is reported by Chinese authorities to reach a height of 15 to 18 m. (50 to 60 ft.) in forests. It is not so upright growing as the American chestnut and shows quite a tendency to spread out when not crowded. In two plantings in the eastern United States, 22-year-old Chinese chestnuts growing on good sites are 15 m. (50 ft.) tall and are still rapidly increasing in height. Perhaps this species may do better as a forest tree in the United States than in China. The nuts range in size from 55 to 275 to the kilogram (25 to 125 or more to the pound) and are quite sweet, decidedly sweeter than Japanese chestnuts. Orchard production is mostly on a seedling rather than a grafted variety basis. The range of the tree in China extends from north of the Great Wall in the vicinity of Peiping south to Yunnan, and to the Kwangsi and Kwangtung provinces. The Chinese chestnut is also planted in northwestern Korea.
The Chinese chestnut shows general resistance to the blight in both China and the United States of America. Under good growing conditions, where the trees are not weakened by such factors as freezing, defoliation, or crowding, most of the trees are highly blight-resistant. The disease may be present in the outer bark of the trunk, especially in limb crotches and near the ground, without extending into the cambium region. However, on limbs that are weakened by shading, the fungus may grow into the cambium region and hasten death of the part. Abundant reddish-brown fungus fruiting bodies are then produced on the dead limb. Small trees are sometimes girdled and seemingly killed back to the ground by the blight, especially when they are weakened by late spring freezes, but most of them send up vigorous sprouts. Some such sprouts that have been under observation in the United States for 15 or 20 years have shown no further damage from the blight.
In China there are at least two other species of the genus Castanea, namely, the Henry chinkapin, Castanea henryi, and the Seguin chestnut, Castanea saguinii. The Henry chinkapin is a tall forest tree, up to 30 m. (100 ft.) in height. Because it produces a small single nut in the bur, it is classified as a chinkapin. This tree grows in central and eastern China but is not frequent. The blight has not been reported on it in China, but in the United States this species is more susceptible than the Chinese chestnut. The Seguin chestnut is a small, bushy plant. It is very prolific, produces three nuts to the bur, and under some conditions continues blooming even into the fall. The blight has not been reported on this species in China, and in the United States the species is very resistant to the disease.
The Japanese chestnut, Castanea crenata, is widely distributed in Japan and southern Korea as a forest tree and is extensively planted in Japan as an orchard tree. Many horticultural varieties are known and their nuts vary greatly in size. Some large ones run 11 to the kilogram (5 to the pound), while some of the wild forest Japanese chestnuts are about the size of the American chestnut and run 275 to the kilogram (125 or more to the pound). The blight is widely scattered in Japan but is not abundant. It sometimes injures orchard trees. In the United States the Japanese chestnut has been more susceptible to the blight than the Chinese chestnut and in general it is less hardly.
American chestnut attacked by the blight.
Chestnut blight was first reported at New York City in 1904. It spread rapidly and eventually killed all the American chestnuts in its natural range in the eastern United States (page 6). Sprouts continue to come up from the base of the killed trees, and sometimes these sprouts bear crops of nuts. However, they in turn are usually killed before they reach a height of 6 m. (20 ft.).
In the southeastern United States there are in the genus Castanea several species of chinkapin, ranging in size from small, low, flat-growing shrubs to trees sometimes 30 cm. (1 ft.) or more in trunk diameter. All of these species are susceptible to blight, and large numbers have been killed by it. Blight has not yet been reported from the Ozark Mountains, west of the Mississippi River, where the largest of the species, Castanea ozarkensis, is native, but in time it undoubtedly will be found there.
From 1912 to 1914 an effort was made by the State of Pennsylvania, with the co-operation of the United States Department of Agriculture, to stop the spread of blight. The program was based on insufficient knowledge, because later studies showed that the disease was more widely distributed in different States than was suspected at that time and was exceedingly hard to control. The State control work undoubtedly delayed the spread of the blight for some years and gave owners of millions of acres of chestnut more time to market their timber stands. It was found that advance small spot infections could be kept from spreading, even though located in the midst of continuous growth of highly susceptible trees. However, very careful work was necessary, including burning of all parts and treatment of the peeled stump with creosote. Frequent reinspection for several years was necessary, as nearly always some infected trees were missed. The fungus fruits on untreated stumps, on chips or pieces of bark that are missed, on small twigs, and on exposed roots. Anyone attempting eradication, even on a less susceptible species like the European chestnut, must take extreme precautions.
The difficulties, but also the success, in holding the blight in check are shown by the work on the Pacific Coast of North America. The first infection was found on European and Japanese chestnuts at Agassis in British Columbia. The infected trees were cut out, but the disease reappeared on several other European chestnuts, which in turn were destroyed. A second infection was found in the State of Oregon, originating from a shipment of chestnut nursery stock from the infected eastern States. This was cut out in 1929 and again in 1934, and no further infection has been found there. A third infection, at Seattle, in the State of Washington, presumably originated from a planting of infected nuts of the American chestnut that came from the eastern States. The infected trees were destroyed and no further disease has been noted at Seattle.
In 1934 the blight was found in several irrigated orchards of European chestnuts in California. All infected trees were immediately burned by State authorities and all known chestnut trees in the State were inspected by State and Federal pathologists. Each year since then a few infected trees have been found in the previously infected orchards, and these have been burned. Inspection and control are supplemented by a rigidly enforced embargo that prevents shipments of chestnut nursery stock into California from the infected eastern States. Another important factor which should contribute to the successful stopping of the blight is the isolated character of the plantings, with no known susceptible host plants in the vicinity. In contrast there frequently were 4,0008,000 highly susceptible trees per km² (10,000-20,000 to the square mile) in the eastern chestnut forests. Such a concentration of susceptible trees provides ideal conditions for spread of the disease and makes it difficult for inspection to locate all infections. The inability to eradicate the disease in California has therefore been puzzling.
A point of interest about the blight in California is its apparent virulence on the seedling European chestnuts. Although the trees were growing under the best of conditions, the cankers continued to enlarge with only a little evidence of any resistance to the enlargement as indicated by callus formation. European chestnuts in the eastern United States have in general shown more resistance than the American chestnut to the blight before being killed, but none under observation has shown as much resistance as the ones reported by Professor A. Pavari in Italy.
The chestnut blight fungus grows and fruits also on a number of species of oak. Occasionally typical cankers form on twigs or branches of the chestnut oak, Quercus montana. The fungus gains entrance through wounds into the wood of a number of native oaks, grows inwards for several inches, but seems to cause little damage. The only native American oak that so far has been damaged is the post oak, Quercus stellata. In many areas this widely distributed tree, with a standing volume of over 23 million m³ ® (5,000 million bd. ft.), is seriously damaged by the blight. Some trees have their tops killed, some have open cankers along their trunks; on many others the fungus grows in the outer bark without reaching the cambium and doing any damage. However, a large proportion of the trees exposed to the blight for 25 years are not damaged. The blight has not been found on post oak outside the area where chestnut has been killed.
In addition to the oaks, related species of Castanopsis from Asia and one from the Pacific Coast have been killed by the blight fungus in greenhouse inoculation tests. Sometimes the fungus grows as a saprophyte on red maple, Acer rubrum, shagbark hickory, Carya ovata, and staghorn sumac, Rhus typhina.
The search for American chestnut resistant to the blight is still unsuccessful. Thousands of trees have been reported as resistant; hundreds of the best of these have been propagated for further testing, which is still under way, but to date there is no selection considered sufficiently resistant to warrant propagation. Considering that the host chestnut covered millions of acres over a wide climatic range, it is quite remarkable that all trees seem to be so susceptible to the blight. In the areas where the blight has been present 25 years or more, a very few large trees are still alive and struggling against the disease, but each year a few more of them die. Sprouts are prevalent everywhere.
Mycelian fans of the chestnut blight fungus. On most specimens only scattered small fans will show, not an extensive fan growth as shown here.
The European chestnuts and their hybrids growing in orchards or as ornamentals in the eastern States have nearly all been killed by the blight. The most extensively planted variety, the Paragon, had been grafted on American sprouts and these were quickly killed.
Forest plantings of Chinese chestnuts made by Dr. J. D. Diller of the Division of Forest Pathology, U. S. Plant Industry Station, in the eastern United States have grown well on deep, fertile soils with good air drainage. On old, abandoned fields, however, the plantings nearly always failed. Even on good sites, the best results were obtained by girdling the present forest growth and allowing the chestnuts to grow up with protection of these girdled trees with less deterioration of site than is usual with clear-cutting. Trees grown from seed of certain selected trees of the Chinese chestnuts give better growth and are hardier than are those from other trees. The tannin content of the Chinese chestnuts grown in the United States equals or is slightly higher than that of the American chestnut, which still supplies most of the vegetable tannin produced in this country (Fig. 4)
A sunken and swollen canker on American chestnut.
Some State conservation, forest, and wildlife departments are now growing Chinese chestnuts and distributing them to farmers for woodland plantings. The Chinese chestnut seedlings are being widely planted as orchard and ornamental trees in the eastern United States. Two private nurseries, for instance, are each advertising 75,000 seedlings for sale this year. A limited number of trees of grafted varieties are planted now, but as the supply of grafted trees increases they will be more extensively used.
Map showing the rate at which chestnut blight spread over the Eastern United States. The dated lines show the extent of the heavy infection at the time indicated.
Breeding to combine some of the desirable characteristics of the different chestnuts is under way by Mr. R. B. Clapper of the Division of Forest Pathology, U. S. Plant Industry Station, and Dr. A. H. Graves of Hampden, Connecticut. Most of the F1 hybrids between the Asiatic chestnuts and the American chestnut have been susceptible to the blight, but the progeny of one cross made by Mr. Clapper is very promising. The Chinese chestnut used in this cross came originally from Tientsin in China. A few of the sixteen surviving plants of this cross are shown on page 7; though each of them has the blight fungus growing in the outer bark, so far these have not been damaged. It will be many years before the final value of this hybrid as a forest tree is determined, but it is by far the best one secured among many Asiatic and American hybrids. Unfortunately, crossing F1 Chinese-American or Japanese-American hybrids back on to the Asiatics to secure more resistance usually results in a loss of the vigorous upright growth habit of the American chestnut. Hybrids between the other Asiatic chestnuts and American chestnuts and chinkapins are under test but so far nothing of outstanding value has been secured.
The dead chestnut trees are chipped up at this plant, the tannin extracted, and the chips then used for pulp products.
Pollen from some of the Chinese hybrids and selected pure Chinese trees has been sent by air mail to Professor A. Pavari at Florence, Italy, for the past two seasons. Use of this pollen on bagged flowers of European chestnuts has given hybrid nuts both years. Nuts and scions from the United States have also been sent, but the viable pollen gives quicker results in combining the desirable qualities of the European chestnuts with the selected trees from this country. World-wide airmail service will especially facilitate tree breeding since most trees must grow many years before they produce pollen.
The tragedy of the destruction or threatened destruction of the chestnut in many lands should serve as a warning against the dangers of tree diseases spreading from one country to another, especially since the phenomenal increase of airplane transportation enhances this danger.
Hybrids between the American and Chinese chestnuts. Note the vigorous upright growth, characteristic of the American parent. Flowers are being bagged to secure the F2 generation.
Photos by courtesy U. S. Forest Service