Tlie following paper was read by W. A. Orton: 



ON THE BREEDING OF DISEASE RESISTANT 

 VARIETIES* 



"W. A. Orton, Assistant Pathologist, Bureau of Plant Industry, United States 

 Department of Agriculture. 



In speaking on this subject I desire to confine myself principally to the 

 work now being carried on by the U. S. Department of Agriculture, and will, 

 therefore, not attempt to give any historical statements or references to other 

 work now being done on this problem. 



The experiments which are to be described were conducted in the South- 

 ern States during the past four years, mainly on the group of diseases known 

 as "wilt diseases," or Fusarium diseases. The most important of these is the 

 cotton wilt, and a brief description of it will apply very well to others of the 

 same class affecting cowpeas, watermelons, cabbages, tomatoes and other 

 plants, and will serve to make clear the conditions under which this plant 

 breeding work is being done. 



The cotton wilt is caused by a fungus, Neocosmospora vasinfecta Erw. 

 Sm., which gains entrance through the smaller roots from the soil and grows 

 upward through the water vessels, which it fills with its mycelium, thereby 

 shutting oflf the food supply and water supply of the plant. The symptoms 

 are usually a sudden wilting of the plant, at almost any age from youth to 

 maturity. When the progress of the disease is less rapid there is a slow dying 

 of the leaves, which turn yellow between the veins and dry up at the margins 

 before falling off. The woody portion of the stem is blackened; this latter 

 character furnishing the best means of distinguishing the disease. A micro- 

 scopic examination of the stem in cross section shows the fungus filling the 

 water vessels. The parasitism of this fungus has been proved by inocula- 

 tion experiments with pure cultures. It gains entrance to the plant from the 

 soil, where it appears to be capable of maintaining itself for an indefinite 

 number of years in the absence of its host plant, either as a saprophyte or by 

 means of resting spores. 



In the field it appears in scattered spots, which gradually enlarge and 

 spread and remain permanently infected. The amount of injury done varies 

 from little to the total destruction of each successive cotton crop planted on 

 the land. (PI. I.) The usual remedial measures have failed to give 

 relief. Rotation of crops is not a remedy, and extensive experiments with 



