PLANT DISEASES 



119 



Two well-defined examples of abnormal swellings are the 

 black knot on the plum or the cheny and the crown gall on 

 nursery stock. The sudden wilting of a plant is usually 

 indicative of a bacterial trouble, which attacks the plant 

 suddenly and cuts off the water supply by clogging up the 

 water-carrying ducts or canals. In addition to the symptoms 

 of unhealthy plants already mentioned several more exist 

 which can easily be detected where the plant is carefully 

 studied. 



Fig. 53. — A diseased fruit. 



Fungous Diseases. — These diseases are caused by small, 

 microscopic, parasitic plants living upon the tissue and 

 sapping the vitality from the host. Some fungous diseases 

 spread to all parts of the plants, such as the leaf spot of the 

 apple, while others are local and only affect a small portion, 

 as the black knot on the plum or the cherry. The greatest 

 damage to our cultivated plants may be attributed to the 

 fungous diseases, because they are more numerous and affect 

 the root, stem, leaves, and fruits. The potato scab and the 

 apple scab are good examples of fungous diseases. 



Control Measures for Diseases. — There are various means 

 at our command for the control of many of the diseases that 

 affect our cultivated plants. It is apparent, however, that 

 different methods must be adopted for the different diseases 

 on the plants just the same as different remedies must be 

 used for the control of diseases in the human system. 



