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S. bradys causes a characteristic disease of yam (Dioscorea spp.) tubers known as 'dry rot disease'. Dry rot of yams occurs in the outer 1 to 2 cm of tubers directly associated with S. bradys. The initial stage of dry rot consists of cream and light-yellow lesions below the outer skin of the tuber. There are no external symptoms at this stage. As the disease progresses it spreads into the tuber, normally to a maximum depth of 2 cm but sometimes deeper. In these later stages of dry rot, infected tissues first become light brown and then turn dark brown to black. External cracks appear in the skin of the tubers and parts can flake off exposing patches of dark brown, dry rot tissues. The most severe symptoms of dry rot are seen in mature tubers especially during storage when it is often associated with general decay of tubers. No foliar symptoms have been observed on yams growing in soil infested with S. bradys (after Jatala and Bridge, 1990).
Management of S. bradys can be achieved by one or more of the following measures: (1) controlling nematodes in field soil by cultural or chemical means (2) use of planting material that is naturally free of nematodes, or treatment of seed material (tubers and setts with yams) prior to planting to reduce or eliminate nematodes from propagative material, and (3) in the case of yams, treatment of tubers after harvesting to prevent storage losses (Jatala and Bridge, 1990).
In Field Soil
In Cuba, keeping fallow land free of all host plants is a suggested means of reducing damage by S. bradys to yams (Decker et al., 1967) but this is unlikely to be economic or practical in most situations. Yams are frequently intercropped, sometimes with as many as five other crops (Coursey, 1967). Control of weed hosts and the exclusion of other crop hosts of S. bradys from around yams will help to reduce nematode damage. Soil populations of S. bradys will be reduced if a non-host or poor host crops, such as groundnut, chilli pepper, tobacco, Indian spinach (Beta vulgaris var. benghalensis), cotton, maize or sorghum are grown prior to yams (Adesiyan, 1976).
Application of chemical nematicides has, at best, only produced moderate yield increases and control of S. bradys (Anon., 1964; Ayala and Acosta, 1971) and information on the economics of this means of control is lacking for large scale use.
Resistance to S. bradys in yams has yet to be confirmed and all the main food yams (D. alata, D. bulbifera, D. cayenensis, D. esculenta, D. rotundata) are susceptible to damage. However, resistance could prove to be the most practical and economic means of managing S. bradys if found in commercially acceptable cultivars.
Clean Planting Material
In yams, using nematode-free planting material is a practical and economic means of preventing damage by S. bradys and also their dissemination. Seed tubers showing symptoms of dry rot (cracking and flaking) should not be used for planting. The presence of dry rot in tubers without external symptoms can be determined by scraping away sections of tuber skin, or by the use of tuber pieces rather than whole tubers enabling the grower to examine for dry rot symptoms before planting.
Bulbils or aerial tubers of the yam D. bulbifera and some forms of D. alata, which are used for propagation will be completely free of nematodes. A number of yams, such as D. alata, D. rotundata and D. dumentorum, can be produced from vine cuttings (Coursey, 1967). Even true seed can be used for propagating D. rotundata (Sadik and Okereke, 1975). Although these methods of propagation are not a practical means of producing ware tubers, they can be used to produce nematode-free seed tubers (Jatala and Bridge, 1990). The use of 'microsetts' or 'minisetts' cut from mature tubers (International Institute of Tropical Agriculture, 1984) can provide clean planting material if the mother seed yams selected are free of nematodes.
Some traditional practices in Africa, such as the use of wood ash on yam tubers or mixing cow dung in yam mounds before planting are reported to decrease nematode numbers (Adesiyan and Adeniji, 1976).
Hot water treatment can reduce or eliminate S. bradys from yam tubers. The expense of heating equipment, and the difficulties of maintaining constant temperatures, are the main prohibitive factors against its large scale use. However, it is feasible for small scale operations and for establishing nematode-free planting material. Most studies have shown that a water temperature of 50-55°C for up to 40 min gives the best control of S. bradys without damaging tubers (Jatala and Bridge, 1990).
All information on the economic importance of S. bradys is derived from research on yams (Dioscorea spp.). The primary importance of S. bradys on yams is in the direct damage it causes to the tubers resulting in dry rot disease (Jatala and Bridge, 1990). The nematodes produce a marked reduction in the quality, marketable value and edible portions of tubers, and these reductions are more severe in yams that have been stored. Weight differences between healthy and diseased tubers harvested from the field have been estimated to be 20 to 30% in the Côte d'Ivoire (Smit in Bridge, 1982) and 0 to 29% in Nigeria (Wood et al., 1980). Weight reduction due to moisture loss is more likely to occur in late harvested tubers left in dry soil (Bridge, 1982). Water loss from tubers continues during storage and is significantly greater in tubers infected with S. bradys compared with healthy tubers (Adesiyan et al., 1975b). Nearly 47% of all yam tubers on sale in Nigerian markets were infested with S. bradys (Bridge, 1973) and both dry rot and wet rot diseases of tubers have been observed in all Nigerian yam barns and markets sampled (Adesiyan and Odihirin, 1977).