TROPICAL YAMS AND THEIR POTENTIAL, PART 3 



21 



aavantageous to use it, especially 

 at heavy rates (5 to 50 metric tons 

 per hectare) . 



Staking 



As a rule, stakes for climbing 

 guarantee better yields. However, 

 the gain in yield must be balanced 

 against the cost of the staking sys- 

 tem. In most locations it is profit- 

 able to stake, and some inexpensive 

 staking method should be sought. 

 When necessary, however, the 

 wing-stemmed yam can be made 

 to produce without staking. 



Staking plants with banana, 

 corn, pigeon peas, and other crop 

 plants has not proved beneficial. 

 The support plants compete for 

 nutrients with the yam crop and 

 usually reduce yield considerably. 



The staking system used should 

 rely on local materials. The stakes 

 should be about 2 meters tall. 

 Taller stakes drastically increase 

 installation costs, are difficult to 

 manage, and may not increase 

 yield sufficiently to justify their 

 cost. Individual stakes are pre- 

 ferred over systems of strings or 

 wires, which encourage the vines 

 to become tangled. 



One of the best staking systems 

 was developed in Trinidad (30). 

 Twelve-foot teak poles are placed 

 in holes every 30 to 50 feet in a 

 row and supported by guy wires. 

 The poles are strung with 12-gage 

 galvanized wire 6 to 8 feet off the 

 ground. Two rows of yams are 

 planted, one on each side. When 

 the vines are sufficiently devel- 

 oped they are tied with strings, 



which are passed over the wire. 

 Sufficient space is left between 

 rows of poles to provide access to 

 the vines. In a variation of the 

 technique a lower wire is also used, 

 and strings are passed between the 

 two wires. Details of this well 

 thought-out technique merit study. 



It is not unusual to find the 

 wing-stemmed yam growing with- 

 out support (fig. 9) . On Barbados, 

 it is normally grown between crops 

 of sugarcane. Before the cane is 

 planted, yams are planted on the 

 ridges and usually given some 

 fertilizer. They soon cover the 

 ground. Under this condition 

 weeds are somewhat difficult to 

 control, and the vines do not reach 

 their maximum development. With 

 careful attention to details, stake- 

 less culture can yield up to about 

 10 metric tons per hectare, about 

 one-third of the yield of staked 

 yams under optimum conditions. 



Pests and Diseases 



Dioscorea alata foliage is usual- 

 ly free from serious insect pests. 

 Susceptibility is closely related to 

 variety. The work of leaf-cutting 

 insects (fig. 10) detracts from the 

 appearance of vines but does little 

 damage. Mites, mealybugs, scale 

 insects, and caterpillars are some- 

 times seen. The best review of in- 

 sect pests is Coursey's (9). 



On the other hand, various spe- 

 cies of beetles can severely damage 

 the tubers in the ground or after 

 harvest. The worst of these, 

 Heteroligus meles (Billb.), the 



