PLANT DISEASES 



297 



385. Sanitation and quarantine. Other important means of 

 control of plant diseases besides pruning, crop rotation, spraying, 

 seed treatment, and growing of disease-resistant varieties are sani- 

 tation and quarantine. For example, if a district in which pear 

 blight has been prevalent is inspected every year and kept clean 

 by rigid pruning and burning all diseased fruit, leaves, twigs, 

 and branches, the disease may be controlled more easily from 

 year to year. Similarly, if 

 wheat seed free from stink- 

 ing smut is planted in a 

 community and no smut is 

 carried into that community 

 by machinery, sacks, or im- 

 plements which have been 

 used in handling smutty 

 wheat, such a community 

 may remain free from stink- 

 ing smut. 



Diseases prevalent in 

 one country or state and 

 not in others may often be 

 prevented from getting a 

 foothold in disease-free re- 

 gions by quarantine against 

 the importation of diseased 

 plants. Such quarantine 

 can now be declared by the 

 United States through the 

 Federal Horticultural Board and has already been employed 

 against several diseases, among which the blister rust of the 

 white pine, a very destructive disease in European nurseries, 

 and the potato wart, a dangerous disease of potatoes which is 

 prevalent in the British Isles and on the continent of Europe, 

 are striking examples. 



FIG. 147. Chestnut almost killed by 

 blight 



The fungus-producing chestnut blight girdled 



this tree just above the first limb. All upper 



limbs have been killed. (After Metcalf) 



