fully with a shielded spray nozzle. When plants are grown without 

 stakes, less weed control is possible. 



Insects and diseases 



A serious disease of yams believed to be caused by one or more 

 viruses (30) debilitates and destroys D- trifida plants. An irregu- 

 lar mosaic of the leaves is followed by stunting and distortion that 

 arrest growth of foliage and reduce or eliminate the tuber. This 

 disease seems to occur throughout the Tropics and may have been 

 introduced by another species of yam. Species differ in suscepti- 

 bility, from least affected to worst, as follows: D. hulbifera, D, 

 esculenta, D. alata, D. cayenensis, and D. trifida. In Puerto Rico, 

 Rajania cordata L., a species with yamlike tuber, seems to be a 

 reservoir of the disease in the wild, and it has been associated with 

 outbreaks of the ''virus." 



Cushcush exhibits a well-defined mosaic pattern that can be 

 associated with distorted, shortened, and aborted stems and leaves. 

 In contrast to the viral leaf disease in D. esculenta and D. alata, 

 the D. trifida symptoms have never been associated with internal 

 tuber brown spot (24')- Nevertheless, it seems that in all virused 

 yams a flexuous long rod of 770 nanometers is found and can be 

 attributed to the potato Y-virus family (30), Migliori, who con- 

 firmed this finding, has also observed "pinwheel" structures in 

 virused D, trifida plants (29), He also observed, though rarely, 

 30-nanometer particles attributed to a type of cucumber mosaic 

 virus (CMV). Until now, apical meristem culture, which could 

 cure virused plants, has not succeeded. New efforts are in progress, 

 based on stem axillary tissue culture as used by Mapes and Urata 

 (25). 



If varietal differences are observed in extensiveness and se- 

 verity of symptoms, no clones can be said to be free from virus. 

 Variation in severity seen in seedling progenies has to be con- 

 firmed. The virus is apparently not transmitted by seed; therefore, 

 controlled infection of seedlings is needed to test for resistance. 

 The growing and selection of hybrids isolated from contamination 

 are needed to produce virus-free varieties. 



Although roguing is sufficient to maintain plants virus free in 

 less susceptible species, avoidance of the disease is recommended 

 in the case of D. trifida- Complete destruction of infested plant- 

 ings may be necessary to keep the disease from spreading. Seed 

 material should never be taken from a diseased planting. 



Nematodes can destroy cultures of D. trifida. In tubers, ne- 

 crotic lesions are confined to epidermal and peridermal paren- 

 chyma tissue, but the ground parenchyma is not attacked, at least 

 in the field (12). Cracks and pitting may be seen, and in heavy 

 attacks (30,000 larvae per 100 grams of soil), gall formation 



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