Fungus Diseases of Plants 149 



barberry bushes. A number of fungi known as "rusts" 

 have more than one host-plant. The yellow rust of 

 apple leaves is the same fungus that produces the so- 

 called cedar apples on cedar trees. 



218. Not All Fungi Cause Disease. Some fungi are very 

 useful, like the little bacteria that gather the free nitro- 

 gen of the air for beans and clover plants; the yeast, 

 used in making bread, and in making wines and beers. 

 Some fungi are quite large, as the mushrooms and puff- 

 balls. Certain kinds are highly esteemed as table deli- 

 cacies, and are cultivated. Some species of mushrooms 

 should not be eaten because they are poisonous. 



219. Preventing Fungus Diseases. There is no cure 

 for the fungus diseases in plants. Prevention is the only 

 safeguard against loss from parasitic fungi. This is 

 accomplished in four ways: 



(a) Treating the Seeds with substances that destroy 

 the disease-causing germs, as scab in potatoes, smut in 

 oats and wheat. 



(b) Using Resistant Varieties. Not all plants are 

 equally subject to the attacks of parasitic fungi. Some 

 varieties are much less injured than others. (Fig. 86.) 

 Many varieties of cultivated plants owe their value to 

 their power to resist disease. 



(c) Sanitation. When crops are subject to a particu- 

 lar disease, all the dead parts, trash and litter that 

 harbor the spores, should be gathered up and burned. 



(d) By Using Fungicides. Fungi are poisoned by ex- 

 tremely small amounts of copper salts, or sulphur in 

 some cases, while green plants are not affected by small 

 amounts. Preparations of copper salts in water are, 

 therefore, used to spray plants to protect them from 

 attacks of fungi. A compound of copper sulphate (blue 



