INTERNATIONAL CONFERENCE ON PLANT BREEDING. 423 



The main object of crossing is to combine various individual 

 tendencies, which in later generations will be dissociated and recombined 

 in new proportions, giving the breeder a wider field for selection. The 

 plant-breeder, before making combinations, should select with great care 

 the individual plants which seem best adapted for his purpose. By careful 

 and intelligent breeding any peculiarity can be made permanent, and there 

 is no limit to the improvement of plants. The plant-breeder is an 

 explorer into the infinite, and the vast possibilities of plant breeding can 

 hardly be estimated. 



" Cultivation and care may help plants to do better work temporarily, 

 but by breeding, plants may be brought into existence which will do better 

 work always, in all places, and for all time." 



On the Breeding of Disease-resistent Varieties. 

 By W. A. Orton, of Washington. 



This paper describes experiments carried out by the United States 

 Department of Agriculture in the Southern States mainly on the group of 

 diseases known as " Wilt Diseases," affecting Cotton, Cow-peas, Water- 

 Melons, Cabbages, Tomatos, and other plants. The Cotton Wilt is caused 

 by a fungus, Neocosmospora vasinfecta, which gains entrance through the 

 smaller roots from infected soil, and grows upwards through the water- 

 vessels, shutting off the food supply of the plant. The usual remedial 

 measures, such as rotation of crops, fertilisers, and fungicides, were all 

 without effect in checking the disease on infected soils. It was, however, 

 observed that not all plants were equally attacked by the disease, and in 

 a field where nearly everything was killed a few plants would survive 

 and show no trace of the disease. Seed of these was saved from indi- 

 vidual plants, and their offspring all proved to be resistent to disease 

 when planted in infected soil, while the ordinary Cotton plants round 

 them nearly all died. This immunity was continued in the two following 

 generations, and in the latter case, out of fifteen acres planted on badly 

 infected soil, all resisted the disease except a few scattered plants ; while 

 the adjoining Cotton of another kind was much injured by the wilt. 



Seed of this resistent variety of cotton is being distributed by the 

 Department among the planters. 



In securing resistance to disease by simple selection other desirable 

 qualities have not been sacrificed, for in length, fineness, uniformity of 

 staple, and yield it is above the average. 



In the case of the Sensation Sea Island Cotton in the seventh genera- 

 tion, the wilt resistance is as marked as when first observed. 



In the case of the Cow-pea, the variety known as " Iron " is not only 

 resistent to the wilt disease, but also to the root-knot worm or nematode 

 (Heterodera radicicola). 



As this is a common pest with Tcmatos and other vegetables, it 

 seems likely that from individual resisting plants resistent races could be 

 easily formed. The precise reason of the resistance to infection in 

 individual plants is not yet known, but it appears to be physiological 

 rather than mechanical. 



These experiments are of great practical and biological significance. 



