yields decrease significantly. A viruslike particle has been found 

 associated with the so-called Puerto Rico yam mosaic (15). 



The mosaic disease of the yam tends to increase in incidence 

 in any given field or area of production. Studies in Puerto Rico 

 with sapogenin-bearing yams have shown that aphids may carry 

 the disease. In the absence of insect vectors, natural spread seems 

 to be slow. Nevertheless, continuous vegetative propagation with- 

 out selection appears to influence greatly the severity and spread 

 of the disease. 



The most important factor in control of mosaic disease is the 

 elimination of diseased plants. As soon as a new planting is well 

 established, diseased plants should be dug up and burned, es- 

 pecially if the field is going to be used as a source of planting 

 material. 



The second important factor in control of mosaic is the 

 selection of healthy seed tubers. It is a common practice in 

 propagating yams to use small tubers from the previous harvest. 

 Since small tubers may be the result of disease, their use tends 

 to increase disease incidence. Large, healthy, undamaged tubers 

 are the best sources of seed material. By selection of such tubers, 

 disease incidence has been reduced after two seasons. 



A second serious disease, especially in D. rotundata, is 

 anthracnose. Widely occurring, it varies strongly in incidence from 

 year to year, place to place, and among varieties. The disease is 

 stimulated by wet conditions, especially dew during the night, 

 and by high temperatures and sunlight during the day. The spores 

 are disseminated by wind and by acervuli on stems or tubers. 

 Seen as a black spot that enlarges quickly on leaf or stem, followed 

 by yellowing, wilting and leaf loss, and in severe cases by death 

 and dieback of the stem, the disease is caused by the fungus 

 Colletotrichum stoesporioides. Other fungi may become involved as 

 secondary pathogens. Zineb^ and ferbam*^^ applied as sprays each 

 10 days are partially effective in disease control. However, such 

 treatment is costly. More satisfactory results are obtained through 

 the use of resistant varieties. 



Nematodes, especially species of Prathylenchus , Meloidogyne, 

 and Scutellonema, are common pests of yams. Attacking the 

 tubers, they leave unsightly burrows or swellings that reduce the 

 usability of the tuber tissue. Furthermore, nematodes have been 

 associated with tuber decay, especially with dry rot, a serious 

 problem in harvested yams. To control nematodes, appropriate 

 soils are desirable, especially heavier clays that do not favor their 

 growth. Seed pieces can be cleared of most nematodes by hot 



^ Zinc ethylenebis[dithiocarbamate]. 

 ^' Ferric dimethyldithiocarbamate. 



25 



