PLANT DISEASES. 259 



five years will greatly reduce the trouble. Old stalks 

 should be removed from the field at the end of the 

 season. Varieties of cotton show a marked difference 

 in natural susceptibility to the disease, and by selection 

 the resistance can be increased. A satisfactory degree 

 of resistance has thus been developed in several varie- 

 ties. 



Cotton Black Rust. This disease is caused pri- 

 marily by unfavorable soil conditions and not by any 

 parasitic fungus. Affected plants have a yellowish 

 appearance, the leaves drop off, the later formed bolls 

 open poorly, and the lint yield is inferior in quantity 

 and quality. All plants in a particular area show these 

 effects. Rust occurs on thin soils from which the 

 humus has been exhausted by continued cultivation of 

 cotton, in soils that are deficient in potash, and in soils 

 that lack drainage either because of their low situation, 

 or because of their poor physical condition. The 

 remedy for black rust is soil improvement. Humus 

 can be restored and its physical condition improved by 

 fertilizing with stable manure, or by adopting a rota- 

 tion that will bring in a leguminous crop that may be 

 plowed under. Drainage of low places will be neces- 

 sary. A deficiency in potash can be met by applying 

 a proper commercial fertilizer. 



Shedding of Cotton Bolls and Squares. Shedding 

 is a physiological disease which occurs generally when 

 there is a sudden check after a period of rapid growth, 

 as when a season of showery weather is followed by 

 dry, hot days. It is an adjustment to the new, less 

 favorable conditions. The plant is not able to develop 

 all of the fruitage begun under the more favorable con- 

 ditions; and it eliminates the excess of bolls by shed- 

 ding, giving the remaining ones a better chance to ma- 

 ture perfectly. Since the shedding depends so largely 

 on weather conditions, little can be done to prevent it 

 except to adopt such cultural practices as will tend to 

 make the growth of the crop uniform. Cultivation 



