142 



PLANT DISEASES 



eased trees are covered with numerous small roots which are 

 usually associated with small galls. There is no known treat- 

 ment for this disease. Diseased trees should not be planted. 



Galls or knots on the roots of plants may be due to other 

 causes. Among the most common are the galls on the roots of 

 apples and grapes which are caused by insects ; abnormal growths 



FIG. 88. Leaf spot disease of the pear. 



on the roots of cabbage and related plants due to slime moulds ; 

 and knots on many plants due to minute worms (nematodes). 

 These and many other diseases of the root systems of plants in- 

 terfere with nutrition, dwarf the plants, reduce the crop and 

 frequently cause death. 



The leaf diseases are mostly due to fungi which cause leaf 

 spots, leaf curls, blights, wilting, dying and droppings. One of 



