Disease Resistance Yet To Be Introduced

Throughout the entire Coastal Plain cigarette tobacco-producing region of Virginia, North Carolina, South Carolina, Georgia, and Florida, root knot and nematode root rot diseases are prevalent. They cause the roots of the tobacco plant to swell and decay. Affected crops are reduced in size and ripen prematurely. The search for resistance to these diseases has involved the testing of hundreds of seed collections. Out of these, T. I. 706, a collection from Central America, proved to be the best. From it, selections that resist both root knot and nematode root rot were obtained. T. I. 706, aside from its resistance, was a poor type and much work has been required to combine this resistance with Suitable characteristics of growth and quality. Lines have been developed that have the full root-disease resistance of the T. 1. 706 parent and that grow and cure like flue-cured tobacco. Further improvement is necessary before we will have commercial varieties, but these are definitely in prospect. When produced they will solve a disease problem that each year affects some 600,000 acres of tobacco.

Wildfire is a disease that is extremely destructive in Pennsylvania, middle Tennessee and Kentucky, and in some other areas. Satisfactory resistance to wildfire has not been found in any variety of N. tabacum. Some wild species of Nicotiana, however, are immune to wildfire. One such species is N. longiflora. After extensive efforts, successful crosses were made between cultivated tobacco and N. longiflora. As the result of this cross, and subsequent work, plants have been obtained that look and grow like cultivated tobacco, but are immune to wildfire.

Blue mold is a disease that has caused tobacco growers much grief since it became widespread in this country in 1932. Plant breeders have tried hard to find resistance to it, but without success in collections. Again, certain of the wild relatives of the cultivated tobacco were immune to this disease. One of them, N. debneyi, was successfully crossed with tobacco. This cross was even more difficult than the one previously described because the N. debneyi chromosomes—those small units of heredity--were so different from the tobacco chromosomes that the breeders' problem was greatly complicated. Most of the hybrid seedlings died, but a few survived, matured, and produced a few seed. To facilitate their work, plant breeders built a special temperature and light controlled chamber in which plants were tested for blue mold resistance during the heat of the summer, instead of merely during the cooler months, when the disease was normally active. Progress is being made, and blue-mold-resistant lines have been obtained that at least look much like cultivated tobacco and cross with it readily.

THE AUTHOR

E. E. Clayton is a pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering, where he does research on tobacco. Dr. Clayton is a graduate of the University of Wisconsin.