Nematode root rot, the common meadow nematode disease, has been studied in connection with resistance to root knot. Many of the selections resistant to root rot also show good resistance to meadow nematodes. The resistance is being preserved so that if varieties with resistance to root knot are perfected, they will also have a good level of resistance to root rot.

The story of developing plants resistant to black shank begins with the work of W. B. Tisdale. At the Florida Agricultural Experiment Station he intercrossed and selected many varieties in order to develop a resistant commercial shade tobacco out of the Florida-grown types, which were largely of Cuban origin. The variety Florida 301 was used as the source of resistance when breeding work was started with flue-cured tobacco in 1931 in western North Carolina. The resistance of poi to black shank is of the usual polygenic type.

More than 20 years of work have gone into the flue-cured breeding program with black shank resistance. Several problems have arisen. It has become necessary to breed burley, dark-fired, and Maryland and Pennsylvania broadleaf varieties that resist black shank. Flue-cured varieties have been produced that have the major part of the black shank resistance of the Florida 301, but they are not grown widely because they tend to be low in yield and quality. The high-yielding and better quality varieties have turned out to have only moderate resistance to black shank. The kind of resistance used in 1953 is least effective in seedlings. Resistance increases as the plants grow older. The roots of these moderately resistant varieties are freely invaded by the black shank fungus, and young plants are readily killed. Root invasion in older plants results usually in death of only a part of the root system, and there is slight or pronounced stunting. one answer to this situation is higher level resistance or immunity, but apparently neither can be obtained in flue-cured tobacco with the Florida 3o1 type of resistance, so we have had to look elsewhere.

We have long known that Nicotiana rustica is highly resistant or immune to black shank. More recently that was found to be true with regard to N. longiflora and N. plumbaginifolia. Work on the transfer of high resistance to black shank from one or more of these species has been started. Successful interspecific crosses have been made with the three species.

The moderate level of black shank resistance coming from Florida sot has made possible such commercial varieties as Oxford 1, Vesta 47, and Dixie Bright 101. Efforts to increase the level of that resistance have been fruitless because the more highly resistant types have proved to be lower in yield and quality. The related wild species of Nicotiana provide a new prospect for obtaining gene material with which to build new varieties with higher resistance. Resistant varieties constitute the only effective method for combating black shank. Consequently better resistance is highly important.

The need for resistance to Granville wilt became so critical that in 1934 and 1935 the Department of Agriculture sent men to Mexico, Central America, and South America to collect tobacco Seed. The aim was to get material that could be searched for wilt resistance. Out of a thousand seed collections so obtained, one showed good resistance. A selection from it, T. I. 448A, was the Source of wilt resistance that has now been incorporated into Oxford 26, Dixie Bright 101, and Golden Wilt. The resistance is polygenic. There is evident linkage between one or more of the resistance genes and some plant-growth characters; namely, short, broad leaves and height of plant. The linkages already have been broken to some degree, however. There is also a favorable linkage between resistance to wilt and resistance to black shank.

The behavior of the T. I. 448A type of wilt resistance is interesting. The resistance is lowest in young plants, which, under field conditions, may be freely invaded. Early in the summer it is not unusual to find a field of wilt-resistant tobacco with many plants showing symptoms of wilt that would kill plants of a susceptible variety. Rarely do more than 5 percent of the resistant plants die, however. Most recover and show no ill effects except one or two malformed lower leaves. Some roots of the resistant plants are killed, but usually not enough to affect normal growth. Wilt resistance is scarce. All the Nicotiana species tested are as susceptible as cultivated tobacco. Resistance constitutes the only effective method for combating the wilt.

Fusarium wilt is serious only in limited areas. Resistance is available within the cultivated tobacco species. It may be found in various types. The Robinson strain of Maryland Broadleaf is resistant to fusarium wilt in Maryland. In the flue-cured area the varieties resistant to Granville wilt, Golden Wilt, Oxford 26, and Dixie Bright 101, have fusarium resistance, which derives from T. 1. 448A. Fusarium resistance is incorporated in Kentucky into Kentucky 35. All fusarium resistance is inherited on a polygenic basis.

Collections from all parts of the world have been surveyed with the aim of finding resistance to blue mold. Some varieties from Argentina have a slight amount of resistance but not enough to be of practical use. In related species of Nicotiana native to western North America there is no resistance to blue mold. High resistance is found in some of the Australian group and in some South American species. As a start toward the transfer of the resistance to cultivated tobacco, N. debneyi, an Australian species, was crossed with tobacco. By a series of backcrosses, plus selection, it has been possible to transfer resistance from this source into the tobacco genome. Additional resistance is available in the South American species, N. plumbaginifolia and N. longiflora, in case it is needed. Ultimately the tobacco grower may be relieved of the trouble and expense of using fungicides against blue mold.

Wildfire and blackfire, from the viewpoint of resistance, may be regarded as a single bacterial leaf spot disease. The organisms that cause them are similar. Selections resistant to wildfire also are resistant to blackfire. Collections of cultivated tobaccos from different places have been tested. Many levels of resistance were found. Investigations showed that this resistance, due to complex inheritance, was of doubtful value in a breeding program. Search through the species, on the other hand, revealed complete immunity in some. One of the best is N. longiflora, which was successfully crossed with tobacco. The immune reaction has been transferred into the tobacco genome. This immunity is a simple monogenic dominant and the immune reaction holds from the very earliest seedling stage in the plant bed until the end of the field season. Therefore, it provides complete protection.

A study of tobacco with N. longiflora genes to ascertain its immunity to wildfire has shown two effects, besides the disease reaction: A degree of extra seedling vigor and a tendency toward low nicotine content in the wildfire-resistant lines. The latter can be eliminated easily, but it is being conserved as a valuable asset because much tobacco today has a higher nicotine content than is desirable.

Brown spot is a minor disease because the common varieties have a considerable degree of resistance to it.

Resistance to mosaic was first reported in a cultivated tobacco, Ambalema, from Colombia. The virus invaded it but did not multiply very much, and the plants showed no symptoms of mosaic. Some 3o seed collections from different parts of Colombia and Venezuela had the Ambalema type of resistance. Some, including T 1. 448, had modifying genes that made them more resistant than Ambalema. resistance of Ambalema has not been satisfactory in combination with varieties grown in the United States. The resistant lines tended to be semi-drooping and susceptible to leaf scald.

A second type of mosaic resistance was developed by F. A. Holmes, of the Rockefeller Institute. It was obtained by crossing N. tabacum and N. glutinosa. It may be classed as immunity, for the virus cannot multiply in the plants with the glutinosa genes. It is a strange type of immunity. The tissues of the immune plants are so sensitive to the virus that leaf invasion results in a dead area; the so-called local lesion reaction. The death of the invaded tissues stops the infection. In ordinary tobacco the virus, once it enters, multiplies and spreads into every part of the plant. The glutinosa type of mosaic immunity is controlled by a single pair of genes. This immunity is readily incorporated into commercial varieties by backcrossing. (Backcrossing means an original cross, and then the hybrid plants are repeatedly crossed back to a specific variety of desired type.)

The tobacco grower, whether he be a grower of flue-cured tobacco in South Carolina or a grower of burley in Tennessee, needs better disease-resistant varieties.

They need to be more resistant in the case of a disease such as black shank, and resistant not to just one disease but to a whole group of diseases. Slowly but surely research is building a solid foundation of high-level resistance or immunity to all major tobacco diseases. Future varieties will be more and more resistant as these different types of resistance are perfected and combined. Out of this will evolve a new, improved type of tobacco that will reduce or eliminate many disease hazards that trouble the tobacco grower today.

Future varieties will be fundamentally different. When black root rot resistance was established and the first root rot resistant variety Havana 142 was distributed, nothing new had been added that did not already exist in the cultivated tobacco species. On the contrary, the transfer of mosaic immunity to cultivated tobacco from the wild species Nicotiana glutinosa and the transfer of wildfire-blackfire immunity from N. longiflora introduced into cultivated tobacco genes that had not existed previously in the species. Such transfers of desirable genes from distantly related wild plants means that the cultivated crops we grow are being steadily improved.

In the past, tobacco variety improvement was limited to the genes and characters found within the one cultivated species. In the future, desirable characters may be transferred from any of the 6o plant species that make up the genus Nicotiana.

Resistance depends on genes. In tobacco the genes for resistance may come from two sources: The many different types of cultivated tobacco and the wild plants that are related to tobacco. A vital difference exists in the quality of the resistance that comes from the two sources. The resistance obtained within the cultivated tobaccos was always a degree of resistance and never immunity. Also it was controlled by many genes. On the other hand, in the related Nicotiana species we found immunity; and in several instances the immune reaction has been demonstrated to be simply inherited. In a word, resistance within the cultivated species is easy to obtain, but hard to use, and it may not be adequate. Resistance in the related wild species is difficult to obtain but, once obtained, it is easy to use, and it may prove to be immunity and therefore completely adequate.

E. E. CLAYTON, principal pathologist, has been in charge of tobacco breeding and Pathology research for more than 20 years in the Department of Agriculture.