172 PLANT PATHOLOGY 



root-rot districts of Connecticut and Florida. Carleton, by breeding 

 and selection, and by the introduction of resistant varieties of wheat 

 from Russia and other parts of the world, has made excellent pro- 

 gress in growing wheat free from the rust. In this case, it would not 

 pay to spray the wheat-fields, even if it were possible; so about the 

 only way to avoid the losses from this disease is by growing resistant 

 wheat. Of course resistance to a disease of parasitic nature is only 

 one phase of plant breeding and selection. General vigor, ability 

 to. withstand unfavorable climatic conditions, productiveness, and 

 special adaptability to certain soils and climates, or else wide range 

 of adaptability are parts of the object of the plant-breeder. 



This brings us to the fifth and last method of controlling disease. 



Cultural methods. Every grower of plants finds that with most 

 diseases, if he can grow strong, vigorous plants, they will be either 

 immune to the disease or they will suffer so slightly from it as to 

 grow successfully in spite of the trouble. By selecting proper soil and 

 other environmental conditions for exacting plants, by the intelli- 

 gent use of fertilizers and manures, the cultivator is able frequently 

 to grow his crops with but little loss from disease. 



In the arid regions of the United States the irrigated orchards and 

 vineyards are almost entirely free from the fungus diseases of humid 

 sections, such as black-rot and mildew of the grape, apple- and pear- 

 scab, bitter-rot of the apple, and the ordinary leaf-spots. In Virginia 

 the Newtown Pippin, when grown on the black, mountain loam soils. 

 on the higher slopes and coves of the Blue Ridge Mountains, is nearly 

 free from apple-scab and the fly-speck and smut fungus. It is also 

 partly immune to the bitter-rot fungus. In low situations, or on 

 the red clay hills at the foot of the Blue Ridge, it is extremely sus- 

 ceptible to all these troubles, so much so as to be commercially a 

 failure there. 



The selection of proper soils and localities for the peach, apple, 

 and pear is a matter of the utmost importance in the commercial 

 production of these fruits. This is true, of course, also of most of our 

 garden vegetables and other crops. By the use of fertilizers we can 

 frequently push plants into greater growth so as to enable them to 

 resist partly or wholly certain types of diseases. Peach-trees infested 

 by the root-rot, if heavily fertilized, will live and bear profitably 

 without any indication of the presence of the disease. This is espe- 

 cially true in the Lake region, where the root-rot is apparently slower 

 in its action than in the Southern States. The effect of fertilizers and 

 stable manures is even more prominent in case of certain insect 

 troubles than with the fungus diseases. A young peach-tree may 

 be so sick from the attacks. of the black peach aphis on the roots as 

 to have every leaf rolled up and yellowed by the disease, and yet a 

 bushel of stable manure placed around the tree in the winter and 



