26 



AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE 



bers from symptomless plants may 

 be symptomless but nevertheless 

 virus infected. Occasionally, symp- 

 tomless plants grow from the 

 tubers of lightly infected plants, 

 and thus selection of seed stock 

 with low virus symptomolgy is pos- 

 sible. 



Regular rogueing appears to be 

 the best protection against mosaic 

 disease. Treatment of infected 

 tubers by hot air over long periods 

 has also rid them of disease. In 

 yams grown for medicinal pur- 

 poses, similar symptoms have been 

 shown to be carried by aphids. 

 Control of insect pests appears to 

 be an effective way of reducing 

 spread of the disease. 



Rotting of tubers in storage is 

 common and accounts for most 

 losses after harvest. Rotting is 

 often associated with wounds in- 

 flicted at harvest and can be re- 

 duced by careful harvest. A curing 

 period of about 1 week in warm, 

 dry conditions leads to the forma- 

 tion of a cap of dry cells that pro- 

 tect wounds against fungal 

 entrance. Afterwards, a cool tem- 

 perature and moderate humidity 

 are required for maximum storage 

 life. Rotting is also associated with 

 insect and nematode lesions and 

 with a history of poor drainage 

 around the tubers. 



Many organisms are associated 

 with storage rots. Soft rot is caused 

 principally by Botryodiplodia theo- 

 hromea. Indoleacetic acid solutions 

 applied to the yam tubers before 

 storage are useful in suppressing 

 the disease. Nematodes are asso- 



ciated with dry rot before and 

 after storage. The most important 

 organisms in the rotted tissue are 

 Rhizoctonia solani and Fusarium 

 oxysporum. From other rots the 

 organisms Penicillium oxalicum 

 and Aspergillus niger have been 

 isolated. A common treatment to 

 avoid rot is to dust the tuber, es- 

 pecially its cut surfaces, with wood 

 ashes or other alkaline substances. 

 Avoiding damage at harvest is the 

 best preventative measure. 



A brown spot found within the 

 living tuber of D. alata affects 

 food and keeping quality (10). The 

 brown nodules are concentrated 

 near the crown end of the tuber. 

 Plants grown from tubers with 

 this condition show virus symp- 

 toms of the foliage, and bundles of 

 filamentous particles can be seen 

 in the tissues with the electron 

 microscope. The virus appears to 

 be of the class found in cacao swol- 

 len shoot and may thus be trans- 

 mitted by mealybugs as well as 

 other insect vectors. 



Because the virus diseases of 

 yams are not well understood, in- 

 troduction of varieties from one 

 region to another is hazardous, 

 even though it appears that virus 

 is universal in its culture. Unless 

 needed for study purposes, virus- 

 infected plants should be rooted 

 out of plantings as soon as detected. 

 Large tubers from healthy mother 

 plants should be selected as seed 

 material. In this fashion it should 

 be possible to reduce virus occur- 

 rence in commercial fields to a 

 tolerable level. 



