184 THE CHIEF SOUTHERN AGRICULTURAL CROPS 



Alabama Experiment Station. The literature of the 

 subject will be found in the bulletins of that institution 

 and in the publications on plant pathology issued by 

 the Department of Agriculture. 



Only the more important of them will be considered 

 here. 



The term " rust " or " blight " is applied to a widely 

 occurring diseased condition of cotton, the most con- 

 spicuous feature of which is the spotting and prema- 

 ture falling of the leaves. These spotted leaves are 

 found to be infested by any of three or four different 

 species of fungi. These are all, however, what are 

 known as facultative parasites ; that is, they are only 

 able to invade leaves that have been weakened by 

 some other cause, and cannot, like the true rusts, 

 attack them when growing vigorously. The disease 

 is an obscure one and its exact nature has not yet 

 been satisfactorily determined. It is usually worse 

 on thin, sandy lands, and is especially serious when 

 periods of excessive rainfall are followed by a sharp 

 drought. It may, however, be induced by any un- 

 favorable conditions that suddenly arrest growth. 

 The facts seem to be that while leaves are young 

 and thrifty they have the power of repelling the 

 attacks of these fungi, but as they approach maturity 

 or have their vitality suddenly checked they are no 

 longer able to do so. With the development of the 

 fungi the leaves soon turn yellow and fall off, thus 

 preventing the maturity of all the later part of the 

 crop. In badly rusted fields the yield will be reduced 

 to a half or a fourth of the normal. That this 

 disease is primarily connected with the nutrition of 



