TROPICAL YAMS AND THEIR POTENTIAL, PART 3 



25 



PN-4448 



Figure 12. — Mosaic disease of D. alata leaves. 



plications become expensive. 

 Therefore, good varieties of D. 

 alata susceptible to leaf spot are 

 often grown only in areas fairly 

 free of the disease or where 

 weather conditions are not con- 

 ducive to its full development. 



In addition to leaf spot, mosaic 

 (fig. 12) diseases of the leaves 

 often cause severe losses in plant- 

 ings of the wing-stemmed yam. 

 Leaf symptoms include dwarfing, 

 narrowing, crinkling, and other 

 distortions of shape; mottling; and 

 vein banding. Such symptoms 

 vary not only among plants of a 

 single clone but also among the 

 branches and the leaves of a single 

 plant. In most cases the symptoms 



have been attributed to virus, but 

 the range in type and intensity of 

 the symptoms suggests that more 

 than one virus is involved. In a few 

 cases the presence of a virus has 

 been verified. 



The virus diseases of D. alata 

 have not been well studied, but 

 certain generalizations based on 

 experience can be made. Once a 

 virus disease occurs in a planting, 

 it tends to extend itself rapidly, 

 and new plantings tend to be more 

 diseased than previous plantings. 

 The tubers of heavily infected 

 plants are smaller than those of 

 lightly or apparently uninfected 

 plants. When planted, they pro- 

 duce heavily infected plants. Tu- 



