3. | Yam | Root-knot nematodes (Meloidogyne spp.) | D. rotundata was the mostsusceptible and heavily affected among all tested species. D. alata was the least affected, and D. cayenensis was intermediately affected yam species. | Use of nematicides could resolve the problem. | Mudiope et al. (2012)
4. | Yam | Nematode (Scutellonema bradys) and Meloidogyne spp. | D. alata (4.73 %) and D. rotundata (3.35 %) were most affected. | Effective use of nematicides, crop rotation, varietal resistance, biological control and several other disease management strategies. | Coyne et al. (2006)
5. | Cassava | Large grain borer (Prostephanus truncatus) | Field infestation and storage losses. | Conventional methods and novel approaches of loss assessment need to be developed. | Boxall (2002)
6.3 Losses Observed during Various Stages at the Time of Marketing
Many forms of post-harvest losses occur during transportation from farmers’ fields to the market and during storage. These losses, which have also been reported under controlled environmental conditions, tend to be high in tropical developing countries where handling and storage technologies are either simple (Woolfe, 1992) or are non-existent. Shrivelled, broken, cut, skinned roots of tubers is a common occurrence due to careless handling at the farm, during transportation, at the market and in the hands of the consumer (Tomlins et al., 2000; UNIDO, 2004). These injuries lead to a decline in quality, invasion by spoilage microorganisms (Jenkins, 1982) and other forms of post-harvest deterioration. The various losses that occur in general varied in case of roots and tubers. The factors that should be kept in mind to avoid such losses are summarized in Table 6.5.
Table 6.5 Factors to be taken into consideration at different stages to avoid losses in roots and tubers
1 Market demand
2 Cultivation
3 Harvesting and field handling
4 Packaging and storage
5 Transportation
6 Market handling (Possible storage)
7 Sales to consumers, wholesalers or agents
8 Perishability of the produce
Besides the fact that roots and tubers normally dry out due to exposure to low humidity conditions during transit and marketing (Rees et al., 2003), the rate of water loss is higher in injured produce compared to sound produce. Physiological deterioration, that is invisible to the naked eye, also takes place at a high rate, because of the prevailing high temperature conditions in the tropics (Wills et al., 1998).
In tropical developing countries, roots and tubers are traditionally handled, stored and marketed almost exclusively under ambient tropical climatic conditions of temperature and relative humidity, as this may be the only option available for many producers and traders (Snowdon, 1990). In addition, the roots and tubers are commonly handled roughly, leading to severe breaking, cutting and skinning injury among others, as they are harvested, loaded and unloaded, transported and marketed (Rees et al., 2003). In Tanzania, for example, where sweet potato roots are traditionally transported in woven polypropylene sacks weighing between 100–200 kg., root quality declines with transportation between the production areas and the port where the sacks are loaded and unloaded for further distribution (Tomlins et al., 2000). Since the ambient temperature conditions in the tropical environment favour microbial activity and metabolic rate, both pathological and physiological losses are usually high (Jenkins, 1982; Osunde, 2008), leading to high loss of quality and marketability of produce.
6.4 Methods employed for Storage of Roots and Tubers
Appropriate storage can help to reduce post-harvest losses by slowing down respiration rate, minimizing moisture loss and reducing microbial activity. To overcome the problem of post-harvest deterioration of roots and tubers, growers commonly avoid harvesting until the produce is needed for consumption (Oladele and Arowojolu, 2003), a practice commonly referred to as “in-ground storage”.It is an effective method in that cassava roots can only be stored for 24–72 hours after harvesting (FAO, 1995; Ravindran, 1995), whereas mature roots can remain in the ground for up to 2 years (UNIFEM, 1993). Sweet potato roots and yam tubers are also harvested “piece-meal”, where mature roots are harvested when needed for consumption leaving others, usually the immature small ones, to continue bulking (Bashaasha et al., 1995; Knoth, 1993). From an economic point of view, in-ground storage is feasible because no costs are incurred in erecting a storage facility, but large areas of land may become unavailable for further cropping, and this may decrease economic output of the land resource. The other disadvantage of this method is that the roots and tubers become vulnerable to attack by rodents, insects and microorganisms. During in-ground storage, pest attack, starch depletion, fibrous development and crack formation on the surface occurs, as reported in sweet potato and cassava (IITA, 1990; Pillai, 1991). Cassava roots also become more fibrous since they develop lignifications and there is a decline in their starch content (IITA, 1990). Even under other traditional storage methods, long-term storage is not possible without loss of quality due to excessive weight loss, dry matter depletion, rotting, insect damage (Pillai, 1991) and sprouting, especially in yam and sweet potato (Janssens, 2001). During the dry season, the ground may dry out excessively and increase mechanical injuries to roots and tubers during periodic harvesting.
Curing facilitates toughening of the skin of high water content produce by promoting wound periderm development over injury sites, and reducing the risk of post-harvest
infection, decay and excessive water loss, hence recognized as one of the most effective ways of increasing storage life (Padmata, 1990; Clark, 1992). Recommended curing involves exposure of produce to conditions of high temperatures and high relative humidity for 4–7 days (Van Oirschot et al., 2006), but in many developing countries, it is difficult to follow these practices because they involve high initial costs (Ray et al., 2010). Roots and tubers have the ability to heal their skin wounds when held at relatively high temperatures and humidities for a few days after harvest, whilst at the same time there is a general strengthening of the skin. The conditions required for curing of roots and tubers are presented in Table 6.6.
Table 6.6 Conditions required for curing of roots and tubers
Crop | Temperature(°C) | Relative humidity (%) | Duration(days) | Reference
Sweet Potato | 30–32 | 85–90 | 4–7 | Booth (1974)
Yam | 32–40 | 90-100 | 4–7 | Booth (1974)
Cassava | 30–40 | 90–95 | 2–5 | Kitinoja and Kader (2003)
However, research has shown that a simple solar curing technique involving the use of polythene to raise temperature and relative humidity can be used successfully to reduce post-harvest losses in sweet potatoes (Kihurani, 2011). To overcome problems associated with severe handling of sweet potato roots, Tomlins et al. (2000) recommended change of packaging materials, introduction of a curing stage, educating handlers and redesigning loading and unloading areas, which should help to reduce injuries. This is because proper handling, and the resultant reduction of injuries, is viewed as a strategic practice with the potential to reduce the risk of excessive water loss, infection and decay (FAO, 1981), thereby extending shelf life of roots and tubers (Jenkins, 1982; Rees et al., 2003).