l8 9i.J Black Rttst of Cotton. 63 



nate spores in favorable weather the leaf is soon covered with 

 a mass of spores giving" a blackened appearance to the leaves. 



My correspondents in Alabama use the term ' 'black rust" 

 when the disease progresses very rapidly and the develop- 

 ment of the hyph«ne of Cercospora and setae of Colletotrichium, 

 or the Macrosporium and Alternaria spores, is very profuse 

 causing the leaves to appear black. When the disease pro- 

 gresses more slowly, being checked by unfavorable weather, 

 oris in the first stages, the term "red rust" is used. In such 

 cases the Macrosporium or Alternaria has extended centrifu- 

 gally the spots attacked by the Cercospora, increasing their 

 size, causing them to become more nearly circular, and mark- 

 ing the spots with concentric lines. Also the edges of the 

 leaf are dead and dried, and curled either below or upward, 

 being favorite places for the attack of either the Cercospora 

 or Colletotrichium. The body of the leaf is still green, paled 

 by different shades of a dull yellow or dull purple. 



In some cases in the early stages of the disease the Col- 

 letotrichium severely attacks the upper part of the stem of the 

 plant and petioles of the leaves giving the stems a dark color 



from the internal changes, to the leaves a scalded appearance 

 and causing them to shrivel and dry up much as if frost- 

 bitten. 



Sometimes the development of Cercospora may be so great 

 and the attack of the other fungi so tardy as to give the ap- 

 pearance of "black rust" produced by it alone. Specimens 

 of this kind were received from one of my correspondents at 

 Eutaw, Ala. The conditions for the development of Cercos- 

 pora were so favorable that from one-fourth to three-fourths 

 of the leaf surface was covered with a dense mass of the dark 

 brown hyphae, the remaining portion of the leaf being yellow- 

 ish with numerous small points of attack. The hyphre and 

 conidia in such cases are very long, often five to eight times 

 as long as described by Cooke. Specimens collected at a 

 later date at this place gave an abundance of the Macrospo- 

 rium and Alternaria. 



Where other fungi, as Colletotrichium, Macrosporium and 

 Alternaria are abundant, it is often very difficult to find the 

 Cercospora on the leaf. By placing the leaves, freshly gath- 

 ered, in moist chambers for ten or twelve hours I. have never 

 failed to get an abundance of Cercospora, even on the smallest, 

 uppermost leaves of the plant. Sometimes the Macrosporium 



