PLANT DISEASES 453 



fungicide, like Bordeaux mixture, can be used on very 

 young plants to reduce the spread of infection during a 

 growing season ; but the more serious trouble is in the soil, 

 from infected tubers. It was in connection with this im- 

 portant disease that the first attempts were made to develop 

 resistant races, and many have been obtained. The trouble 

 has been that resistant races do not continue to be resistant, 

 and in a few seasons they are no more resistant than other 

 races. Also, the resistant races have not proved to be re- 

 sistant in all localities. 



In the case of the Fusarium wilts, as of cotton, the cultiva- 

 tion of resistant races began seven or eight years ago, and 

 in four or five years success was attained. Several resistant 

 races of cotton, and also of cowpea, were secured. As in 

 the case of resistant races of potato, however, the resistant 

 races of cotton have not always retained this character in 

 all localities. 



The discovery and development of a race of wheat resist- 

 ant to rust have been described (p. 355). 



The cultivation of races resistant to disease is very new 

 work, but it promises to be the method by which we shall 

 finally eliminate all the diseases of cultivated plants. 



151. Suggestions for work. Probably no work can be 

 done with plant diseases except in learning to recognize some 

 common diseases. As many cultivated plants as possible, 

 including street trees, should be examined for diseases, 

 especially for spotted leaves, wilts, galls, black knot, and 

 cankers. All wild plants are subject to disease, and these 

 might be used to extend the observations. 



In addition to this, specimens showing the usual diseases 

 of the common cultivated plants can probably be obtained 

 by any school from its state Agricultural Experiment Station. 

 These will serve as valuable guides to the recognition of these 

 diseases among the plants cultivated in the neighborhood of 

 the school. 

 30 



