TROPICAL YAMS AND THEIR POTENTIAL, PART 3 



15 



disease resistance, since varieties 

 with low phenolics seem more 

 susceptible to diseases. External 

 tuber characteristics appear to be 

 unrelated to cooking quality. 



Varieties selected by the author 

 and their sources and character- 

 istics are given in table 2. All are 

 available for distribution.^ Other 

 superior varieties are expected to 

 come out of the author's program. 

 Tlorido' (fig. 6) is representative 

 of a very good D. alata variety, but 

 it is susceptible to leaf-spot dis- 

 ease. 'Gemelos' (fig. 7) usually 

 gives excellent yields and has very 

 high quality. The best variety for 

 kitchen and processing quality, 

 'Forastero' (fig. 8), bears some- 

 what irregular tubers, but its 

 superior cooking qualities com- 

 pensate for this defect. 



CULTURE 

 Environmental 



Requirements 



The growth cycle of the wing- 

 stemmed yam, a plant of shady 

 rain forests, is intimately tied to a 

 long rainy season followed by a 

 short dry season. In perhaps all 

 areas where this species has been 

 successful, the dry period corre- 

 sponds to the time of short days 

 and lower-than-normal tempera- 

 tures. When varieties are moved 

 from one hemisphere to another, ad- 

 justment difficulties are marked, 



3 Free tuber pieces distributed dur- 

 ing January and February upon request 

 to Mayagiiez Institute of Tropical Agri- 

 culture, Agricultural Research Service, 

 U.S. Department of Agriculture, Maya- 

 guez, P.R. 00708. 



and 2 years may be required to 

 change seasonal response. The fac- 

 tors controlling plant response are 

 not known, but are suspected to 

 include day length, temperature, 

 and water availability. 



When a collection of varieties is 

 assembled and grown in one loca- 

 tion, differences among their 

 growth cycles become obvious. 

 Some varieties mature consist- 

 ently early, while others are 

 consistently late. The tubers of 

 early-maturing varieties may also 

 germinate early. Thus, earliness is 

 not necessarily related to shorter 

 maturation time (which also var- 

 ies from 7 to 10 months) , but to a 

 season of dormancy unique for 

 each variety. 



In addition to differing in sea- 

 son of dormancy, varieties also 

 differ with respect to length of 

 dormancy. Natural dormancy is 

 closely related to storability. Vari- 

 eties with tubers that store poorly 

 are precisely those with short 

 periods of dormancy. 



Changing germination season, 

 time to maturation, and matura- 

 tion season is extremely difficult. 

 Planting earlier than normal, for 

 example, does not necessarily 

 stimulate earlier germination by 

 the same amount. On the other 

 hand, tubers germinate at the ap- 

 propriate season for the variety 

 even when conditions for growth 

 are unsuitable. This seasonal re- 

 sponse of D. alata is one of its 

 greatest limitations as a crop 

 plant. 



The wing-stemmed yam is 



