170 PLANT PATHOLOGY 



pear-scab, pear leaf-blight on the pear and quince, leaf-blight of the 

 plum and cherry, and the mildew and leaf-blight of the potato. 

 Many other diseases are capable of prevention by spraying with this 

 valuable fungicide. Curl-leaf of the peach is preventable by spraying 

 with Bordeaux Mixture or the simple solution of copper sulphate 

 shortly before the buds swell in early spring. The lime sulphur mix- 

 ture is about equally successful applied at the same time. There are 

 still some troublesome problems in the application of Bordeaux 

 Mixture and other fungicides to growing plants. For some unex- 

 plainable cause, under certain climatic conditions, apples and pears 

 are russeted or even deformed by the absorption of the poisonous 

 copper through the cuticle. How to prevent this and to make 

 Bordeaux Mixture that will be uniformly safe in spraying these 

 fruits is an unsolved problem. Peaches and Japanese plums when 

 sprayed in foliage, with Bordeaux Mixture or any other fungicide 

 which has been tried, will, under most conditions, suffer severely. In 

 most cases the fungicide kills round spots, which drop out of the 

 leaves, producing a shot-hole effect, and at various intervals, from 

 a few days to a month or two, all leaves touched by the fungicide 

 fall to the ground. This defoliation results, of course, injuriously 

 to the growth of the tree and the fruit. How to find a fungicide 

 that will prevent the brown rot of the peach and plum and not in- 

 jure the foliage is one of our most serious problems. 



Disinfection methods. Certain diseases of plants are carried over 

 mainly by means of spores or mycelium of the fungus which cling to 

 seeds, tubers, and cuttings, etc. Where these can be disinfected and 

 all the parasites destroyed, successful crops can frequently be grown 

 in infested localities. One of the first treatments of this type was 

 the dipping of wheat infested with smut in a solution of copper 

 sulphate. Jensen found that hot water could be used for the loose 

 smut of oats and other smuts successfully. The germination of the 

 seed was even benefited by the treatment. Recently formalin has 

 come into use for the same purpose, at first by the expensive method 

 of dipping, and later by simply sprinkling it over the grain. Potato- 

 scab was found by Arthur and Bolley to be preventable by dipping 

 in a weak solution of mercuric chloride. Later they substituted 

 formalin as a preferable disinfectant. The black rot of the sweet 

 potato is preventable to a large extent by keeping the houses and 

 hotbeds annually disinfected with copper solutions or formalin, and 

 by selecting for planting only the sound tubers free from disease. 



Eradication. No more puzzling disease as to its cause exists than 

 the so-called "yellows" of the peach. There are two other serious 

 diseases of the peach and plum of a somewhat similar nature, namely, 

 the peach rosette of the Southern States, and the "little peach" of 

 New York and Michigan. We are thus justified from their similarity 



