3i8 DISEASES OF CROP-PLANT^, 



I 

 limit of penetration. The spots are caused *"by the fungus 

 Cercospora vagince Kniger, the progressive stages in the develop- 

 ment of which are represented in the series just described. 

 The mycelium of the fungus grows in the tissue of the leaf-sheaths, 

 to which it gains access by way of the stomata, and by passing 

 from one to the next penetrates towards the middle of the shoot. 

 The brown central portion of the oldest spots marks the tissue 

 finally killed by the fungus. Upon it is found a brown external 

 mycelium with short upright branches, which at their tips 

 produce the elongated narrow conidia. These readily and quickly 

 infect other leaf-sheaths to which they may happen to be borne. 



The damage done by the fimgus is inappreciable. It finds 

 its nourishment in the tissues lying between the vascular bundles, 

 and the latter are little if at all affected until a rather late stage 

 in the development of the spot. 



The newer leaf-sheaths become infected so early from the 

 spots on the older ones that stripping would usually be rather 

 useless. The use of top-plants undoubtedly helps to carry over 

 the fungus, since traces of the infection may usually be seen upon 

 them. Unless the fungus can be shown to have more importance 

 than is usually attributed to it its presence does not afford a 

 reason for the discouragement of the use of such plants. 



Ring-Spot. 

 The leaf disease known as ring-spot was investigated by 

 Breda de Haan about the year 1893 in Java. It occurs in 

 India, the Hawaiian Islands and other Eastern locaHties. 

 In the West Indies it has been definitely recorded from Cuba, 

 San Domingo, Porto Rico, Antigua, Barbados, Trinidad, British 

 Guiana and Surinam, and probably occurs in all the islands. 



Causation. 



The disease is caused by a parasitic Ascomycete LeptosphcBria 

 Sacchari Br. d. H. It has two spore forms. The conidia are 

 curved bodies consisting of a central cell with one or two smaller 

 cells at each end. They are capable of infecting leaves upon 

 which they come. The ascospores are formed later in small 

 perithecia. which appear as black dots in the substance of the 

 leaf near its upper surface ; the spores are four-celled, the 

 terminal cells with thinner walls. 



Symptoms and Incidence 



The spots are generally most abundant towards the outer 

 end of the older leaves, giving rise when abundant to a general 

 rusty appearance. The spot starts as a purplish fleck on the 

 surface, which as the fungus grows expands outwards and dries 

 up in the middle, so that it comes to consist of a small brown or 

 yellowish patch enclosed by a purple or brownish band, with 



