24 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE 



areas, and in most areas, rotting fections generally occur late in the 



and loss of quality are inevitable growing season, when the yams 



results of infestation. Since the are switching from the production 



biology of some of the species is of vegetation to the production of 



complex, no easy solutions can be tubers. At this stage, aided by 



expected. rains, the disease can spread rapid- 



The most serious foliage disease ly, affecting much of the foliage, 

 of the wing-stemmed yam is leaf The vines wilt in severe cases, 

 spot (fig. 11) . The disease appears The most effective control of 

 as very small brown or blackish leaf spot is the use of resistant 

 spots on the leaves and stem. The varieties. Many have been found, 

 spots enlarge, the uninfected leaf but most of these do not have de- 

 tissue tends to yellow, and even- sirable agronomic characters. Se- 

 tually the leaves die, but may hang lection of varieties from a broader 

 on the vine for long periods. Pre- base of germ plasm appears 

 mature dieback of leaves and stems promising, however. When accept- 

 drastically reduces yield. The dis- able resistant varieties are not 

 ease is generally attributed to a available, the disease can be con- 

 species of Colletotrichum, but trolled by the use of copper-based 

 other fungi might be involved, per- fungicide. In addition, zineb and 

 haps in secondary roles. ferbam sprays at 10-day intervals 



Although leaf spot can be found have been reported to be useful, 



on the leaves of almost all D. alata If rains are frequent, control is 



varieties at all times, severe in- seldom effective, and repeated ap- 



PN-4447 



Figure 11. — Leaf -spot disease of D. alata leaves. (Two-fifths actual size.) 



