418 SOUTHERN FIELD CROPS 



It is much more prevalent on sandy soils than on clay- 

 soil, and on poor than on fertile land. It usually occurs 

 in July, August, and September. 



The disease comes on with variable symptoms. When 

 the weather is dry, the leaves of the diseased plants usually 

 show at first a mottled yellowish color. After wet weather 

 there may be no yellowing but a sudden blackening, 

 dying, and falling of the fohage. 



No remedies can be employed after cotton-rust appears. 

 Prevention, instead of cure, is needed. Any treatment 

 of the soil and any application of fertilizers that promote 

 a healthy but not excessive growth of the cotton plant 

 increase its resistance to rust. 



On poor soils of any kind, the addition of vegetable 

 matter by proper rotation of crops is the most widely 

 applicable means of warding off rust. On very poor sandy 

 soils the application of potash usually enables the plant 

 to resist the disease and to retain the greater part of its 

 fohage until the crop is mature. For this purpose at least 

 80, and better 100 pounds of kainit per acre is advisable, 

 appHed in connection with the other fertihzers which may 

 be required on that particular soil. 



Where the unthrifty condition of cotton plants is caused 

 by poor drainage, ditching is usually a^means of decreasing 

 the amount of rust. 



388. Minor leaf diseases. — Other diseases of the leaves, 

 which are less destructive than cotton-rust, are angular 

 leaf -spot (Fig. 184), which appears earlier than rust; leaf 

 blight, in which the diseased areas show as small whitish 

 spots ; and cotton mildew, appearing on the under side of the 

 leaves. No remedies are in use for any of these diseases. 



