348 Diseases of Economic Plants 



treated plats, because these plants did not die until later 

 than those on the untreated plats, and therefore had greater 

 opportunity to mature their bolls." 



Root-knot (nematodes). — See p. 24. This knot is par- 

 ticularly destructive on cotton, not only because of the 

 direct injury it does, but also because root-knot greatly in- 

 creases the susceptibility of the cotton to wilt. All means to 

 hold the development of the nematodes in check should be 

 employed (pp. 24-25), particularly the use of nematode- 

 resistant varieties of cowpeas in the cotton rotation. 



Minor diseases 



Frosty-mildew (Seplocylindriuni areola (Atk.) P. & G.). — 

 This leaf-spot is limited sharply by the smaller veins, and 

 bears upon the lower surface numerous colorless spores upon 

 colorless hyphse, thus lending a frosted appearance. Seen 

 from above, the spots are light yellow or of a paler green than 

 the normal leaf tissue. Widespread, though not especially 

 destructive, it does not usually attract the attention of the 

 planter. 



Leaf-blight (MycosphoBrella gossypina (Atk.) Earle, Cer- 

 cospora). — The fungus which causes this disease was first 

 described from Carolina specimens in 1883, at which time its 

 presence was recognized in South Carolina, Georgia, and 

 Florida. The disease prevails in all cotton sections of the 

 United States, appearing first in damp localities upon the 

 lower leaves, and with the progress of the season, spotting 

 all of the leaves more or less. The leaf-spots appear at first 

 as small red dots, which finally, as they enlarge, bear brown 

 or white centers with a characteristic red Iwrder. A copious 

 development of dark hyphse upon the centers later gives 

 them a blackish hue, and numerous white spores give 

 these hyphse a white coating. The old diseased centers 

 of the spots are brittle and frequently break away, leaving 

 perforations. The disease is widespread but unimportant, 

 since it is largely limited to weakened tissues, especially 

 to spots produced by cotton mosaic. 



