362 THE FORAGE AND FIBER CROPS IN AMERICA 



3. Sore shin or damping off (Rhizoctonia) . 



4. Anthracnose x (Colletotrichum gossypii South.). 



5. Root rot (Osonium Sp.). 



LEAVES 



6. Rust, black rust or mosaic disease (Macrosporium nigricantium Atk.). 



7. Red rust (Tetranychus telarius). 



8. Leaf blight (Cercospora gossypina Cke.). 



9. Cotton mildew (Ramularia areola Atk.). 



458. ROOT KNOT is due to the same nematode worm that causes the root 

 knot on cowpeas. (319) Red rust is due to a minute mite resembling 

 the so-called red spider of greenhouses, while the other diseases above men- 

 tioned are due to mycelium-bearing fungi. In addition to these diseases there 

 are the angular leaf spot, and the cotton-boll rot, the causes for which have 

 not been determined, although probably bacterial. The shedding of bolls is 

 also believed to be due to causes other than insect attacks. The most 

 destructive diseases are the cotton wilt, root knot, black rust, and anthracnose. 

 Sore shin or damping off destroys many plants, but owing to the habit of heavy 

 seeding and subsequent thinning it does not ordinarily produce serious loss. 

 No specific remedies have been found for any of these diseases. 



459. COTTON WILT. "The wilt is very distinct from any other disease 

 of cotton, so that there need be no difficulty in its identification. It usually 

 makes its first appearance in the spring about the last of May, when the plants 

 are 6 to 8 inches high. It appears in well-defined areas, which enlarge if 

 cotton is planted on the same land again. The first outward indication of its 

 presence is a dwarfed growth and unhealthy appearance of the plants. The 

 leaves turn yellow between the veins, their margins shrivel up, and some 

 plants wilt and die at once. In other plants the progress of the disease is 

 often slow, and many of them live the entire summer and die late in the 

 season. On cutting across the stem of a diseased plant, the woody part will 

 be found to be stained brown wherever the disease is present. In the absence 

 of microscopic examinations, this brown discoloration of the internal tissue is 

 the best ocular evidence of the presence of the wilt disease. 



"Plants may partially recover from a severe attack of the wilt disease by 

 the development of strong lateral branches near the ground. Such plants 

 may be distinguished by their dwarfed and bushy appearance, and by the 

 tendency of their branches to lie prostrate on the ground." 2 



The fungus does its damage by entering the smaller roots, and subsequently 

 by its growth, filling up the water ducts with its mycelium, and thus cutting 

 off the supply of plant food. The wilt disease of okra is supposed to be due 

 to the same fungus, but that of cowpea is supposed to be slightly different. 

 The infection is known to remain in the soil for four years, and probably 

 longer, hence rotation of crops is of little avail after a field has become 



1 Also seriously attacks bolls. 



2 U. S. Dept. Agr., Div. Veg. Phys. and Path. Bui. No. 27 (1900), p. 6. 



