VEGETABLE PATHOLOGY AN ECONOMIC SCIENCE 171 



in calling this type of disease the "Yellows Group." Peach yellows 

 has been demonstrated repeatedly to be readily controllable by 

 promptly digging up the diseased trees and burning them as soon 

 as they appear in the orchards. Thus, while the yellows is one of 

 the most obscure diseases as to its cause, it is one of the cheapest 

 diseases to control. A careful inspection of the orchard two or three 

 times during the season, especially at the ripening of the fruit, and 

 the digging out of an occasional tree for the benefit of the rest of 

 the orchard, is vastly cheaper than the thorough and repeated spray- 

 ings necessary to prevent the average fungus disease. Rosette is 

 pretty certainly preventable by the same means. The writer is 

 now engaged, with his assistants, in testing the feasibility of this 

 treatment for the "little peach," with every prospect of success. 



Pear-blight, the bacterial disease which attacks the blossoms, 

 young shoots, and the bark of pomaceous fruit-trees, can be remedied, 

 or rather prevented, only by the eradication method. It is more 

 quickly contagious than the "Yellows Group" diseases, and the erad- 

 ication is more laborious and difficult and not quite so successful. 

 Black knot of the plum may be mentioned as another fungus disease 

 which can be controlled by the eradication method. Simply cutting 

 out the infected limbs in the fall or winter has been demonstrated 

 to control the disease. Even some of the leaf-spot diseases, such 

 as the leaf-spot of the violet, can be controlled by picking all the 

 infected leaves as soon as they appear. 



Breeding resistant varieties. Until recently our object has been to 

 discover some method of spraying or disinfecting the diseased plants, 

 or by destroying and thus losing them in the fight against disease. 

 At best all these methods are somewhat extravagant and wasteful. 

 By breeding varieties of plants which are immune to disease, or 

 which are resistant to a greater or less degree, we may even avoid the 

 necessity of treating the disease. No more desirable method can 

 be imagined of getting around a troublesome plant disease. In 

 fact, in cases of some root diseases, there may be no possibility of 

 using the other methods. For an example, Orton, in his work on the 

 root-rot of the cotton and cowpea, found resistant varieties which 

 would not take the disease, and grew a fine crop on infested soil 

 where common varieties were entirely destroyed. Swingle and Web- 

 ber found that by the use of the sour orange as a stock on which 

 to bud oranges, they were able to grow trees resistant to the root- 

 rot of the orange. Webber found that the Drake Star orange was 

 resistant to the Phytoptus rust on the orange. Bolley is breeding 

 varieties of flax resistant to the flax Fusarium disease. In case of 

 tobacco the mosaic disease appears to 'be susceptible of circum- 

 vention by selecting the plants free from the trouble. Mr. A. D. 

 Shamel has achieved a notable triumph in selecting tobacco in the 



