CROP DISEASES MAY CAUSE FAILURE 



257 



usually attacks corn that is grown continuously on the same land, 

 crop rotation is an effective means of control. Another complete 

 and effective means of controlling the corn root-aphis is thorough 

 stirring of the soil previous to planting, to disturb the ant colonies 

 and scatter and kill the aphids. 



Fig. 162. — Roots of tomato plant affected with root-knot. (Florida Station.) ' \-^ 



Crop Diseases May Cause Failure. — ^A great many diseases 

 attack plants. Many of them are caused by fungi and bacteria 

 which live in the soil; among which may be mentioned potato scab, 

 pea blight,^ wilt, root rot, cabbage yellows, brown rot in tomatoes, 

 black-leg in cabbage and cauliflower, and club root (Fig. 164), soft 

 rot and black rot, as in cabbage. Rotation of crops and avoiding 



2 Pea blight fungus stays in the soil so long as diseased tissue of the pea 

 plant remains in the soil. Potato scab on the potato may be treated, but 

 when soils become infested with the scab fungus other crops should be grown. 



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