The Genes That Mean Better Tobacco

E. E. Clayton.

From all over the world American plant breeders have collected hundreds of specimens of tobacco to test for factors that might be useful in developing disease-resistant tobacco plants.

Many came from Mexico, Central America, and South America, where tobacco is a native plant. Among the specimens were 59 other species of Nicotiana that, with N. tabacum, make up the genus. From Australia a group of Nicotiana species was obtained that represents a "lost" branch of the family, separated from the main American stock countless ages ago. They were wild, stunted, ill-smelling plants of no commercial value but of great scientific value. Some American species obtained from our own far West grew only a few inches tall. Other South American relatives were the so-called tree tobaccos. Types of cultivated tobacco were obtained in endless variety.

The studies have been going on since 1912, when James Johnson, at the Wisconsin Agricultural Experiment Station, began what might be termed scientific tobacco breeding, with definite procedures and objectives.

Some success has been attained. We have Havana and burley varieties that are resistant to black root rot, flue-cured varieties that resist wilt, and flue-cured and cigar-wrapper types that resist black shank. A beginning has been made in combining, in one variety, resistance to more than one disease.

The goal is to breed varieties that resist all major diseases.

To do that one has to know how much and which resistance is available, and that is why plant explorers went to the uninhabited interior of Australia, native Indian villages in remote sections of South America, and to many other places to get their thousand-odd collections of tobacco plants. We hope the study of all those specimens will yield two kinds of information.

The first is information about where the genes the carriers of heredity for resistance to each disease are located and whether they can be used. Much resistance is not usable because the species cannot be crossed, there are undesirable linkages, or inheritance is too complex to permit backcrossing.

The second type of basic knowledge concerns the parasitic potentialities of the disease-producing organisms: Is the organism in question a single race or is it a group of races that have varying parasitic ability.

Although breeding and selection work were carried on long before the underlying scientific principles had been worked out, James Johnson added greatly to our knowledge by his attempts to control black root rot by breeding better varieties. He found that in varieties and seed collections occasional plants appeared to be resistant. From that selection and testing, Havana 142 was developed, the first root rot resistant variety to be introduced. It is still grown extensively.

Resistance to black root rot depends on many genes. Furthermore, the root rot fungus is an assemblage of races that differ in their pathogenicity. Some are weakly parasitic. Others are strongly parasitic. If a series of varieties possessing resistance is tested with different races of the fungus, one finds that a variety may be highly resistant to one race and quite susceptible to another.

Resistance is a matter of degree. A variety may be slightly resistant it is only a little less severely damaged by the disease than the variety recognized as susceptible. In similar fashion, varieties may be moderately or highly resistant. The ultimate is immunity, which means complete freedom from disease. Immunity is not a matter of degree.

The varieties resistant to black root rot so far released are at least moderately susceptible to some races of the fungus. Some people report that certain varieties are "losing their resistance." Actually they have lost nothing; it is merely that the constant growing of one variety makes easier the multiplication of fungus races to which it is susceptible. With polygenic resistance which depends on the accumulation effect of a number of genes; it would be possible to find genes with which to meet this situation temporarily.

Upper: Root knot nematodes caused the swellings or galls on the roots. Nematodes attack and damage seriously many kinds of field, garden, and ornamental plants. Lower: Nicotiana longiflora, an unpromising relative of tobacco, provided the gene of immunity to wildfire. Genes conveying immunity may be found in other wild plants.

We have evidence, however, that linkage problems make the accumulation of high-level root rot resistance extremely difficult or impossible. For example, in 1935, certain tobacco collections having high-level resistance to root rot were crossed with susceptible burley, and a program of back-crossing and selection was started. The highly resistant selections from those crosses consistently proved to be low in yield or otherwise undesirable, and the variety finally released (Burley 1) was not nearly so resistant as the original highly resistant parent line.

Among the species of Nicotiana that are related to cultivated tobacco are some that are immune to black root rot and some that are moderately or highly susceptible. So, in addition to the work with polygenic resistance that made possible such varieties as Havana 142, Kentucky 16, Burley 1, and Connecticut 15, plant breeders began the work of transferring a black root rot immune reaction from wild species to the cultivated tobacco. The species selected was N. debneyi, a native of Australia. The immune reaction provides resistance far beyond the parasitic ability of any race of the fungus; so, in a practical sense, it eliminates the fungus race problem. The transfer of black root rot immunity from N. debneyi to N. tabacum was in its final; and successful stages in 1953. When immunity to root rot is available and is incorporated in our commercial tobacco varieties, one more problem will have been solved.

The study of the tobacco collections made in Central America disclosed that some had marked resistance to root knot nematode disease. Subsequent backcrossing to commercial varieties and careful selection made it possible to increase the original resistance to root knot, but all the highly resistant lines proved to be low in yield because their leaves were small. So now we have a good level of root knot resistance, the inheritance of which depends on many genes. Because of unfavorable linkages involving small leaf size, the resistance so far has not been usable. The difficulty is being overcome, however.

Plants resistant and susceptible to root knot are invaded freely by the nematodes. Once they are in the roots of resistant plants, however, the nematodes cease development after a time, and few eggs are produced. Gall formation also is reduced.

The resistant plant, in a sense, is a nematode trap. At McCullers, N. C., in 1950, winter peas were sown in the field following resistant and susceptible tobacco. The peas grew luxuriantly when they followed resistant tobacco, but after the susceptible tobacco they were severely attacked by root knot and made little growth. It seems that planting tobacco resistant to root knot reduces the number of nematodes in the soil, and hence can protect other crops that follow on the same land.