resulting from Meloidogyne incognita has been observed in Mar- 

 tinique (22). In Jamaica, Pratylenchus coffeae is the main para- 

 sitic nematode of the D. trifida tuber. D. trifida seems to be 

 resistant to the common yam nematode Scutellonema bradys (19). 

 Roots can suffer from Meloidogyne, Helicotylenchus, Pratylen- 

 chus, and Rotylenchus (19, 23). Rarely, a foliage nematode, 

 Aphelenchoides besseyi (the ''white tip" rice nematode), is seen. 

 Necrosis can cause death of the plant (23). No resistance against 

 nematodes seems to exist in tested material. However, variation in 

 tolerance is likely. Cultivation of cushcush after vegetable crops 

 must be undertaken cautiously. Most nematicides are phytotoxic to 

 yams. 



At planting time, white mealy bugs are often found around 

 the base of the growing stem and at the lower nodes. When num- 

 erous, they can kill the stem or occasionally an entire plant. On the 

 adult plant, mealy bugs are found around the collar or upper part 

 of the tubers. Pseudococcidae (Planococcus citri, Planococcus gos- 

 sypii, Planococcus citricus) are the most injurious; some Diaspidi- 

 dae (Gonaspidiotus hartii) are also found. Clean or treated seed 

 pieces at planting give good control of the disease. It must be noted 

 that the world parasite of the yam, Diaspididae (Aspidiotus 

 destructor) has not been found on D. trifida (31). 



Harvest and Yields 



In conventional plantings following normal cycles the harvest 

 is made 10 or 11 months after planting when the foliage of the 

 plant dies back. Special care must be taken when digging the 

 tubers to avoid damaging the tender surface. Mechanized harvest 

 has been studied in Guadeloupe and appears promising with 

 varieties bred for this purpose. 



Yields of D. trifida have tended to be low, compared to those 

 of other yam species, one of the reasons this species has been 

 neglected. Under conditions of a viral disease, yields can reach as 

 low as 1 to 2 tons per hectare. Yields of 15 tons per hectare can be 

 considered good for this species. Two years of cultivation of new 

 hybrids in 20 locations in Guadeloupe resulted in yields from 4.6 to 

 37 tons per hectare. Weeding, staking, and appropriate soil-water 

 relations, in addition to soil fertility, are the factors that most 

 influence yields. 



On an experimental basis, yields in Guadeloupe have reached 

 55 tons per hectare, a yield comparable to the best yields of other 

 yam species. But current yields represent only one-half to two- 

 thirds of that which would be expected under similar conditions 

 from D. alata. The main reasons for this are probably the narrower 

 ecological adaptation of D. trifida and its need for a more plentiful 



19 



