BREEDING AND SELECTING PEST-RESISTANT TREES

RUSSELL B. CLAPPER, JOHN M. MILLER.

Genetics has given us a good new tool to use against the diseases and insects of trees the selection and breeding of trees for resistance to pests. It is a long job. The time that a tree crop takes to produce seed and to mature exceeds the span of a human generation. Natural forces, aided now and then by man, have determined through the ages which forest species should survive, and these are the species with which the forester, the geneticist, and the forest pathologist now work.

Epidemics of introduced parasitic fungi stimulated interest in the development of healthier trees. Forty years ago the Department of Agriculture employed Walter Van Fleet to breed chestnut trees that would resist the introduced blight fungus. Since then several agencies have taken up the work of breeding and selection, for the most part to obtain vigorous, fast-growing specimens for lumber and other products. More recently, greater emphasis has been placed on developing trees resistant to particular fungus and virus diseases. The development of new forms resistant to insect enemies, however, has scarcely made a beginning.

The need for the work is clear enough. Besides the losses we have incurred, in some regions of the United States forest planting is coming into use as the surest and quickest method of reproducing the desired wood crops. Planting makes it possible to control the kind and variety of tree that occupies the site and gives special emphasis to the need for careful selection of the planting stock. It costs no more to plant the resistant trees, if they are available, than to plant ordinary stock.

In the development of trees resistant to a particular disease or insect enemy, the same principles of selection and genetics apply that are employed in the development of new, vigorous, and fast-growing tree forms. The tree breeder, however, usually desires both resistance and vigor in his final selection, but when the laws of heredity decree differently, the breeder faces a difficult problem. The solution of such problems requires knowledge of several sciences, especially genetics, plant pathology, entomology, and forestry.

The breeder first attempts to select trees that show resistance to the particular pest under study. Resistant selections are propagated by grafting or by cuttings. Seed from such selections is collected and thousands of seedlings are grown in nurseries where they may be tested against the pest, or the seedlings may be transplanted to testing plots where they can be tested at a more suitable age.

Sometimes selection results indicate that no individuals of the particular species or of related species are resistant. It is then necessary to import seed of foreign species for testing. The related foreign species, however, may possess no worthy characteristic other than that of resistance. The breeder must combine this character of resistance with the desirable characters of the susceptible species. The first step to bring about this combination is to produce a hybrid by crossing a resistant tree with a susceptible tree.

Hybrids obtained from the first crossing of two varieties or species are known as first filial (F₁) generation hybrids. If an F, tree sets seed by its own pollen (selfing), or if two or more F₁ trees are crossed with one another (sib-mating), the resulting hybrids belong to the second (F₂) generation. The F₂ and subsequent generations are called the segregating generations because all the characters, visible and invisible, that were present in the F₁ trees segregate out among the various trees of the later generations.

RESISTANCE to a pest may be inherited in one of three ways. If resistance is inherited as a dominant character, all the F₁ trees will be dominantly resistant and most of the F₂ trees will be similarly resistant. Resistance may be inherited as an incomplete dominant, in which instance the F₁ trees will be more or less intermediate in their resistance to the pest. The F₁ trees as a group will not show the resistance of the resistant parent nor the susceptibility of the other parent. In this type of inheritance the second and subsequent generations will produce a lower proportion of resistant trees than the first type of inheritance produces. If the breeder meets either one of these types of inheritance, he will have comparatively little difficulty in obtaining trees with a satisfactory degree of resistance. But susceptibility may be inherited as a dominant character. The first-generation trees will be susceptible and will have no value except for further breeding to obtain second-generation trees. The second generation in this instance must consist of large numbers of trees because the proportion of resistant specimens will be exceedingly small.

In agricultural crop breeding, the breeder usually fixes the type by inbreeding so that it reproduces more or less true from seed. The tree breeder cannot afford to fix his hybrid types.

Tree hybrids usually lose vigor when inbred, and the process of inbreeding trees requires too long a time. When the tree breeder obtains maximum resistance in his hybrids in combination with other desirable characters, he is ready to plant them on appropriate sites for final testing. Since his hybrids, in general, will not breed true, the question arises as to the nature of the progeny from these hybrids when they are planted in the wood lot and in the forest. Part of the progeny may be resistant but not vigorous, another part may be vigorous but susceptible, and another part may be both vigorous and resistant. The tree breeder can determine the theoretical proportions of these progeny types because he knows the way in which characters are inherited in the species with which he works.

Each tree-breeding project presents problems of its own. Examples of experimental work will be described to illustrate various methods of testing trees for resistance to particular pests, and to indicate the progress that has been attained. However, most of the selecting and breeding of trees for resistance to pests is still exploratory in only a few instances hybrids have been developed to the stage that permits planting them as replacements for their inferior parents.

WHITE PINES RESISTANT TO BLISTER RUST: A. J. Riker and associates at the University of Wisconsin, in cooperation with the Department of Agriculture, have tested selections of eastern white pine against the blister rust. One thousand grafts were made from 163 trees selected for their resistance to heavy natural infection for more than 15 years or for other special properties. Most of the grafts resisted artificial infections of the blister rust fungus. However, when 10,000 seedlings from the selected trees and commercial seedlings were subjected to natural and artificial infections, a high percentage of the seedlings were I infected with stem cankers within a year. Ray R. Hirt of the New York State College of Forestry, in cooperation with the U. S. Department of Agriculture, observed eastern white pines of various ages in the period of 1927-47 for resistance to the rust. He found varying degrees of rust resistance in a small percentage of the total pines observed. Those trees showing greatest resistance to rust are being, propagated by grafting and cuttings so that more extensive tests for resistance can be made. The low percentage of rust-resistant seedlings reported by Riker and Hirt indicates that rust resistance is not inherited as a dominant character.

The white pine blister rust fungus also attacks other five-needle species of pine. Forest pathologists are keenly interested in determining the relative susceptibility of native and exotic species of pine to the fungus. Seven species of pine were tested against rust by Ray R. Hirt, of the New York State College of Forestry; in the Northwest, nine species were tested by Thomas W. Childs and Jess L. Bedwell, of the Division of Forest Pathology.

The species of pine that showed resistance to blister rust were: Pinus cembra var. helvetica, P. armandi, P. griffithii, and P. koraiensis all are foreign species but are not known to have any timber value. Those showing susceptibility in increasing degrees were: P. aristata, P. peuce, P. ayacahuite, P. flexilis, P. monticola, P. albicaulis, and P. lambertiana. Several trees of the latter species, commonly called sugar pine, have withstood infections from blister rust for 14 years and will be used as breeding and propagating material.

RESIN MIDGE: In the blister rust experiments, inherent resistance of selected pines was determined by inoculation tests. In the case of an insect parasite, the resin midge, we find that an external characteristic of the trees new shoots with dry, smooth bark is an indicator of resistance. The problem was approached by selecting for this particular character.

Resin midge resistance studies were carried on from 1930 to 1940 at the Institute of Forest Genetics, near Placerville, California. This undescribed species of resin midge (Retinidiplosis sp.) caused considerable damage to young planted ponderosa pines at the Institute and to natural reproduction throughout much of the western pine region during this period. The feeding habits of the larvae cause resin-filled pits in the thin bark of the stems and twigs, and these pits result in growth deformities and dwarfing of the trees. In time the heavily injured trees die.

A study of the stem characteristics of many trees revealed that the heavily infested trees were those that produced new shoots covered with a sticky, resinous film, a growth character of certain trees. Noninfested trees were those that produced new shoots with dry, smooth bark. The data collected showed that only 11.2 percent of the sticky-stemmed trees escaped injury, while 93.4 percent of the smooth-stemmed trees escaped injury entirely or were only lightly attacked. The next phase of these investigations will be to determine whether dry, smooth bark is inheritable and, if so, to produce trees with this characteristic for reforestation purposes.

RESISTANCE TO WEEVIL: Resistance in pine to another parasitic insect, a weevil, was obtained by crossing a resistant with a susceptible species. The insect (Cylindrocopturus eatoni) is the most important enemy of young planted pines in the brush fields of northern California where, in some areas, it killed 90 percent of the trees within 10 years after planting. It also killed natural reproduction that was restocking burned-over pine areas. The trees are killed by the larval mines that extend through the phloem and cambium areas and later into the wood. In nature, the weevil's preferred hosts are ponderosa pine and Jeffrey pine. A number of other species of pines, such as Coulter pine and sugar pine, appear to be immune to its attacks.

Studies were begun at the Institute of Forest Genetics in 1946 to determine whether a resistant variety of pine could be developed that would have the same desirable wood qualities as ponderosa and Jeffrey and at the same time survive weevil infestations during the early years of growth. A number of species, hybrids, and varieties of pines were tested by forcing the attacks of the weevil on them under cage control. Among the trees tested was a new hybrid pine first produced by geneticists at the Institute in 1939 by crossing Jeffrey pine with a natural hybrid of Coulter pine.