285 



tnrbed, and there is no break in the continuity of the surface 

 tilni on the soil particles below the few inches which ha\e 

 been stirred. The few inches of soil which have been stiritd 

 thus act as a mulch. 



" (Jharacters of the disease. — In the normal and usual pro- 

 gress of the disease there first appears a peculiar yellowing of 

 the leaf, which gives it a checkered or mosaic appearance. 

 The yellow color appears in small areas, and bears a definite 

 relation to the venation of the leaf, being bounded by veinlets 

 which subtend areas more or less rectangular in outline. The 

 green color is found along the larger and intermediate veins. 

 The portions of the mesonhyll lying along the veins, being 

 near the channels for the distribution of the nutriment, re- 

 ceive a better supply of moisture and assimilative material 

 than the areas farther away, and those along the smaller and 

 terminal ramification of the vascular channels at a time when 

 the supply is being cut short because of unfavorable condi- 

 tions of the soil. They are thus enabled to hold the green 

 color and continue the activities of the leaf for a longer pe- 

 riod, while the angular areas most remote from the sources of 

 supply are the first to feel the loss, and the deficient nutrition 

 is manifested by the yellow color of the parts. 



"During the first stages of the disease this color mny be- 

 come very pronounced, but later it may be marred by the ap- 

 pearance of discolored spots produced by the growth of fungus 

 organisms in the tissues, weakened by the failing nutrition of 

 the plant. Soon, however, there appear minute brownish 

 spots in the yellowish areas, which increase in size centrifu- 

 gally, assuming a circular outline and marked by concentric 

 rings. The concentric rings are probably due to the periodic 

 growth of the fungus threads within the tissues, the period- 

 icity being produced by variations in the temperature. The 

 first fungus, which in most cases appears following the mo- 

 saic condition of the leaf, is Macrosporium nigricantivm Atk. 

 As the leaf thus becomes in a badly diseased condition, the 

 Macrosporium is likely to be soon followed by an Alternaria.* 

 The black hypha? and spores of these two fungi soon give a 

 black appearance to nearly the entire leaf, from which the 

 disease takes the name of "black rust." These are not, how- 

 ever, the only fungi which are found as accompaniments of 

 the later stages of the disease. Colletotrichum gossypii South- 



*This may be Alternaria tenuis Nees, which Gasparrini found 

 with other molds as an accompaniment of the disease of cotton in 

 Italy known as Pelagra. (See Gasparrini, Observationi sopra una 

 malattia del cotone, etc. Inst. D'Incoraggiamento. Napoli, 1865.) 



