Some authorities consider yellow yams to be a variety of D. rotundata. The edible portion is the tuber, which is borne singly and can weigh up to 10 kg. But more usually are within the range 500–2 kg. They have brown skin and yellow flesh.
As far as harvesting there is no clear dormancy period, but the vines die back about 12 months after planting and then regrowth. At this time the vines may be cut off and the tubers may be harvested and dug from the ground. Jamaica is a major producer and there they are harvested by digging up the tuber while the vine is still attached.
The top of the tuber, called the head end, is cut off with the vine attached and replanted in the same place. This means that there is a large cut surface on the harvested tuber that is easily infected by microorganisms.
Pest and disease control
A common way of controlling postharvest diseases is to dip the tubers in a fungicide. However, dichloran treatment was found actually to increase rotting. In Jamaica, benzimidazole fungicides were used for many years. However, during the commercial application, a rot caused by infection with Penicillium sclerotigenum was frequently observed on the treated tubers.
In in vitro tests, this organism was found to be tolerant to benomyl. This tolerance was confirmed in vivo tests, but the organism was highly susceptible to the fungicide imazalil. However, the use of benomyl or imazalil is now not permitted in many importing countries and an alternative method of disease control is needed.
Field infestation of yellow yam tubers with parasitic nematodes was shown to increase when they were stored in tropical ambient conditions, resulting in areas of necrotic tissues. However, when they were stored at 13°C there was no increase in nematode population in the tubers and no increase in necrosis.
Exposure of the tubers to 35–40°C and 95–100% r.h. for 24 hours initiates the curing process and controls storage rots. This treatment could well replace chemical fungicides for postharvest disease control. See the section on D. rotundata for details of the process.
In ventilated storage in ambient conditions of 24–31°C and 52–68% r.h., tubers lost 41% in weight after 4 months. The storage period was too long and should probably be confined to a maximum of 1 month. Refrigerated storage recommendations are as follows:
• 13°C and 95% r.h. for less than 4 months with 29% weight loss and internal necrosis.
• 16°C and 80% r.h. for 60 days (Tindall 1983).
At tropical temperatures, tubers sprouted about 4 weeks after harvesting. The most popular variety of yellow yam in Jamaica is called Roundleaf and that began to sprout about 90 days after harvest compared with the variety Common that sprouted after about 50 days. This study was at ambient temperatures of 25–34°C and 64–92% r.h.
A proprietary potato sprout suppressant containing CIPC and IPC in a dust formulation had no effect on sprouting. Low-temperature storage can control sprouting. No sprouting occurred on any tubers at 13°C during 5 months of storage, but there was a chilling injury at temperatures below 15°C when stored for over 1 month, so this method was not appropriate.