284 DISEASES OF CROP-PLANTS 



stages as " black rust," is sometimes met with in these islands, 

 on plants suffering from drought or poor nutrition, usually on 

 shallow or sandy soils. 



According to Atkinson, in the normal and usual progress of 

 the disease there first appears a peculiar yellowing of the leaf, 

 which gives it a checkered or mosaic appearance. The yellow 

 colour 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 colour is 

 found along the larger and intermediate veins. 



Up to this stage the affection is non-parasitic, and due to 

 imperfect nutrition. Later the weakened leaves are usually 

 infested with weakly parasitic fungi (Alternaria, Macrosporium, 

 and Cercospora ; see below) which give rise to the condition 

 known as black rust, in which leaf-spots are heavily developed. 



The treatment found to be successful in preventing the 

 development of yellow leaf blight on light soils in Alabama is the 

 manurial application of potash salts. 



Macrosporium Leaf-Spot. 



In the condition known as yellow leaf blight one of the fungi 

 commonly found on the browned margins and in zoned spots on 

 the failing leaves is a Macrosporium sp. In the United States, 

 G. F. Atkinson described the species thus occurring as M. nigri- 

 cantium, as follows : 



" Hyphse amphigenous, subfasciculate, or scattered, 50-140 x 

 6-7 micr., nodulose, septate, olive brown. Conidia 18-22x36-50 

 micr., strongly constricted about the middle, stoutly rostrate 

 at one side of the apex, smooth, transversely, longitudinally, and 

 obliquely septate, olive brown." 



Whether the Antillean form or forms are identical with this 

 has not been determined ; in some cases the agreement with the 

 description above is not very close. 



Alternaria Leaf-Spot. 



A species of Alternaria is common in these islands on leaves 

 which are failing or are damaged by disease. Similar records 

 have been made from time to time in the United States and the 

 fungus is generally regarded as probably assignable to Alternaria 

 tennis Nees., a species described from cotton in Italy. 



R. C. Faulwetter has recently made a study of Alternaria 

 leaf-spot as it occurs in S. Carolina, and his conclusions are as 

 follows : 



" The lesions are characterised by a pale green, then straw- 

 yellow and finally rusty brown colour, brittle papery texture and 

 irregular concentric ridged zonations. 



" The identity of the fungus causing the disease cannot be 



