New Kinds of Tobacco

by E. E. CLAYTON

DURING THE 300-odd years that tobacco has been cultivated intensively in this country disease problems have multiplied. They are not little problems. Root rots, blue mold, and leaf blights plague every producing area. Some of them cause the leaf to ripen too early and force growers to harvest much' tobacco while it is still green. Diseases also lower the yield and compel growers to rest their best fields 3 to 5 years and to grow the crop on land that is not so well adapted to tobacco.

To meet this situation we need new varieties of good quality that can withstand the common diseases. But the work of breeding for disease resistance can go forward only as rapidly as adequate sources of resistance to the major diseases are established. The discovery of a new type of disease resistance rarely receives much attention except in the scientific world. Once discovered, the new kind of disease resistance is combined with desirable yield and quality through hybridization and selection. The final products are spectacular new varieties. Before 1920 we had plants resistant only to black root rot. During the next 10 years plants that could withstand black shank were developed. A little later mosaic resistance was obtained.

Beginning in 1934 the Department of Agriculture undertook an extensive search for new types of disease resistance. Collections were made throughout Mexico, Central America, and South America—more than 1,000 of them. They were tested systematically, and those that showed promise of disease resistance were tested further at field stations.

A major object was to find a tobacco that would not be attacked by the Granville ( bacterial) wilt disease. Four years of searching and testing revealed the first wilt-resistant tobacco ever found, and it has been designated as T. I. (for Tobacco Investigations) 448. T. I. 706, from which selections were obtained that were highly resistant to both root knot and nematode root rot, was obtained in the same manner. The two collections thus provided resistance to three major diseases. To get them took a year of laborious travel by two trained collectors and 6 years of intensive testing and selection by scientists and other skilled workers. The work of testing and selection was conducted at five locations simultaneously. Adequate resistance was now available to black root rot, black shank, mosaic, bacterial wilt, root knot, and nematode root rot, diseases that are estimated to cause an annual loss of at least 10 percent of the crop, or about $23,000,000 in 1940 values.

The search for healthier varieties of tobacco led ultimately to the many wild relatives of tobacco, some distant and some rather close, that offer types of immunity that do not exist in the cultivated tobacco. Blue mold and wildfire resistance were the two most important additional needs. Fortunately, some of the wild species proved immune to blue mold; still others were immune to wildfire. Crosses between separate species present many complications; the discovery of resistance to wildfire and blue mold in species related to tobacco represents only a small start toward the solution of these problems. However, the opportunities from crosses between cultivated tobacco and the wild species of Nicotiana are not limited to disease resistance. It is probable that desirable quality characters, such as better color, aroma, and leaf composition may also be had from the wild tobacco.

In breeding tobaccos resistant to disease, it is important to understand the inheritance of the resistance, the way the resistant plants react to the parasite, and the degree of resistance. In many diseases we find all degrees of resistance, from high susceptibility to practical immunity. Black root rot is a good example. Roots of various genotypes were grown in the same infected soil; all the roots of one, the highly susceptible T. I. 706, were destroyed, whereas none of the roots of T. I. 89 were diseased. In between were several commercial varieties that graded from the slightly resistant variety 400 to the distinctly resistant Havana 211.

Even a slight amount of disease resistance often has great practical value. The 400 variety in an area such as western North Carolina, where only weak strains of the root-rot parasite exist, is a valuable rootrot-resistant variety. Obviously, it is desirable to use in breeding work the. highest degree of disease resistance available. Resistance to one disease does not mean resistance to other diseases. T. I. 706, which is so susceptible to black root rot, is our best source of root knot resistance at the present time.

Most resistant tobaccos are invaded by the parasites, but for reasons not clearly understood the parasites are not able to grow freely after they are inside the resistant plant. A good example is T. 1. 448, which often shows slight wilt symptoms early in the season; later it recovers and shows no further signs of disease. Susceptible plants are invaded and killed. The root knot nematodes penetrate the roots of T. I. 706, but, once inside, the nematodes literally starve to death.

Different breeding methods are suited to different crops. With tobacco, the good quality of the present disease-susceptible varieties must be preserved—any changes that are made should clearly be improvements. In many instances we want to keep the leaf size and shape of present varieties because they are adapted to machinery or to special uses. The general breeding procedure followed, consequently, has been to pick the susceptible type into which it is desired to introduce disease resistance, and to pick the best source of that resistance. The two are crossed, and in subsequent generations, lines are selected that possess the full resistance of the original resistant parent. In rare instances it is possible also to obtain suitable growth characters and quality in combination with this resistance at this stage. Usually it is necessary to backcross to the quality parent and go through the process of selection again. This backcrossing may need to be repeated anywhere from two to ten times before the desired combination is obtained. The secret of rapid progress in breeding disease-resistant varieties is an abundance of facilities in greenhouse and field for the critical testing of much material, and the careful elimination of all but the most highly resistant plants and lines.