Parts of a yam tuber: head Activity 3.5
Unit 3: Tuber Crops 3.15 middle tail section (See figures 2.2, 2.3, 2.4) Figure 3.3: Dioscorea spp. - parts of the yam plant
Unit 3: Tuber Crops 3.16 Figure 3.4: Dioscorea spp. - parts of the yam plant
Unit 3: Tuber Crops 3.17 Figure 3.5: Dioscorea esculenta - parts of the sweet yam plant Dormancy : At harvest yam tubers are dormant (1-6 months) depending on the cultivar. Corms have well preformed buds. Tubers do not. Tubers only form buds if the corm is removed or damaged and the dormancy broken. Tubers also have apical dominance. Sprouts will only come from the head. The other parts will only sprout if the head is removed. Aerial tubers or bulbils grow in axil of leaves like in D. bulbifera .
Unit 3: Tuber Crops 3.18 3.2.3 Propagation T y p e s o f p l a n t i n g m a t e r i a l s : Small whole tubers are the best planting material, provided they are from healthy plants. The second best planting material is the second harvest from 'milked' yams. Pieces of large tubers are also good for planting material. Head pieces are better planting material than middle and tail pieces. Cut pieces should be dusted with ash, well washed sand, or fungicides and then dried in a shady place for 2-3 days. Aerial tubers are the best planting materials for D. bulbifera. Size of planting materials: Large planting materials produce large total yields, but they do not necessarily give higher net yields. Net Yield = Total yield - planting material To produce high net yields, sett weighing 150-500 grams are recommended. Farmers normally plant larger setts because under adverse conditions, large setts sprout better than small ones. Health of planting material: Only tubers from healthy plants should be used as planting setts. Therefore you should select planting material in the field so you know from which plant each tuber was harvested. This is why it is very important to mark the diseased plants. The most important diseases are viruses and nematodes. Symptoms of virus: vein banding; vein clearing; mottling and leaf distortion; and internal brown spotting in the tuber. *for leaf curling and distortion check leaf underside for insects. Symptoms of nematodes: vertical cracks; tuber skin flakes off exposing black-coloured dry rot; and tubers become soft and wrinkled and feel spongy.