DISEASES OF ROOT CROPS 337 



bursts to expose the pustules of the fungus, which produce masses 

 of white spores. 



Young stems are to some extent attacked, and S. F. Ashby 

 has found in one district of Jamaica the potatoes themselves bear- 

 ing rounded mammillated galls which may exceed two inches in 

 diameter, and are occupied by a stout non-septate mycelimn 

 with spherical haustoria bearing oospores characteristic of Albugo. 

 The galls soon decay and spoil the appearance and keeping qualities 

 of the potatoes. 



The wild Ipomcea biloba, a common plant on sandy beaches, is 

 often infested with a fungus assumed to be the same as the one 

 on the sweet potato. 



A true rust, Coleosporiiim Ipomoece Burr, is reported common 

 in Porto Rico, but has not so far been noticed in the Lesser An- 

 tilles. 



Yam. 



Wilt. 



One or more wilt diseases of which the nature is as yet obscure 

 occur in Antigua and Barbados and probably in other islands. 

 Under certain conditions, in Barbados usually dry weather, the 

 yam vines become black-spotted on the leaves and in sections 

 of the stem and dry off prematurely in much the same way as 

 they normally do at the end of the season. Several fungi are 

 recorded in other countries, associated with black spotting of 

 yam foliage, but careful and repeated examinations have not 

 revealed any such on the Barbados material. The indications 

 are somewhat more in keeping with a root disease. 



Tuber Rot. 



A rot of the growing tubers has several times been reported 

 from Antigua in which broviTiing and blackening of the tissues 

 starts in spots and patches from the outside and leads to the 

 production of large cavities containing pulpy remains of the 

 broken-down tissues, and infested with fungi, bacteria, and 

 scavenging insects. The decay apparently ceases to make pro- 

 gress after the yams are dug and stored, the rotted tissues being 

 then found enclosed by layers of wound cork. Material showing 

 the active condition of the disease has not yet been obtained, 

 and infection experiments with cultures of the fungi found have 

 had negative results. 



Leaf Blotch. 



A Cercospora leaf-spot is common on the yam in Trinidad, 

 taking the form of large black blotches on the leaves. 



