180 



MANUAL OF GARDENING 



214. 



Leaves and fruits injured by fungi, chiefly apple- 

 scab. 



attack is commonly not so much in the individual leaves as in 

 the stems, the sources of food supply being thereby cut off from 

 the foliage. The symptoms of this class of ^^^ 

 diseases are general weakening of plant when 

 the disease affects the plant as a 

 whole or when it attacks large 

 branches; or sometimes the 

 .leaves shrivel and die about 

 the edges or in large irregu- 

 lar discolored spots, but 

 without the dis- 

 tinct pustular 

 marks of the 

 parasitic fungi. 

 There is a gen- 

 eral tendency 

 for the foliage on plants affected with such diseases to shrivel 

 and to hang on the stem for a time. One of the best illus- 

 trations of this type of disease is the pear-blight. Sometimes 

 the plant gives rise to abnormal growths, as in the "willow 

 shoots" of peaches affected with yellows (Fig. 215). 



Another class of diseases are the root-galls. They are of 

 various kinds. The root-gall of raspberries, crown-gall of 

 peaches, apples, and other trees, is the most popularly recognized 

 of this class of troubles (Fig. 216). It has long been known as 

 a disease of nursery stock. Many states have laws against 

 the sale of trees showing this disease. Its cause was unknown, 

 until in 1907 Smith and Townsend, of the Bureau of Plant 

 Industry, United States Department of Agriculture, undertook 

 an investigation. They proved that it is a bacterial disease 

 (caused by Bacterium tumefaciens) ; but just how the bacteria 

 gain entrance to the root is not known. The same bacterium 

 may cause galls on the stems of other plants, as, for example, 

 on certain of the daisies. The "hairy-root" of apples, and 



