1. Weevils
In most parts of the tropics and subtropics, one of the most serious problems in storage is the sweet potato weevil (SPW) (Cylasformicarius Fabricius)and two closely relates species (C. puncticollis Boheman,and C. brunneus) (Sutherland, 1986).Cylas puncticollis is confined to several countries in Africa. However, C. formicarius is more cosmopolitan, being distributed all over the sweet potato growing regions (Smit and Matenogo, 1995; Uritani, 1998). The insect feeds on the vines of the growing crop and migrates down to the roots and infests those roots near the soil surface. Moreover, phenolics of unspecified chemical composition increased significantly for up to 14 days in roots fed upon by adult weevil or larvae (Padmaja and Rajamma, 1982). This coincided with the development of an unpleasant turpentine odour and a bitter taste in the roots. The bitter taste was due to production of phytoalexins such as ipomeamarone and ipomeamaranol (Uritani, 1998). Regardless of the toxicity of this compound, the unpleasant odour makes sweet potato less acceptable for human consumption.
Control Measures
Integrated pest management (IPM) programme and some pre-harvest cultural practices are mostly followed in various sweet potato growing countries for controlling weevils. These include the use of terminal vine cuttings as planting material, hilling up of soil, sex pheromone traps, irrigation, dipping planting materials in insecticide solution and early harvest (Smit, 1997). Weevil population starts between 2 and 3rd month of the crop and increases rapidly with age of the crop. Although, it attacks all parts of the crop, the main damage is done to the roots by the larvae both in the field and during storage. The pest can breed successfully inside the roots with repeated cycles if there is sufficient food available.
The root damage is influenced by season, method of planting, soil type, temperature, rainfall, soil moisture, time of harvest and also cropping pattern (Palaniswami et al., 1990). In India, up to 79% of produce may have weevil infestation (Pillai 1994). In farmer‘s fields in India, root damage was greater in upland (4-50%) than in lowland (0-22%) conditions because of better soil moisture in the latter. The weevil population and root damage was comparatively lower during rainy season or irrigated conditions and higher during summer season or rainfed
conditions (Palaniswami et al., 1990). When soil moisture is high, the weevil cannot get access to the roots easily through moist soil. Sweet potato planted during rainy season showed lowest weevil incidence (10.9 to 36.5%) whereas those planted during dry season had 47.9 to 87.4% weevil incidence. Dry spell throughout the cropping period results in depletion of soil moisture which favours weevil activity. Similarly, delayed harvest also increases weevil infestation. Therefore, weevil free, better quality sweet potatoes can be produced through adequate irrigation and early harvest (Pillai, 1994).
In the case of Cylas puncticollis survival of all the life-stages of sweet potato weevil was greater between 24 and 31oC and lower at 18 or 16oC (Nteletsana et al., 2001). Therefore, weevils can be effectively controlled if the storage temperature was lowered to 16 -18oC because it is not able to accumulate enough thermal units required to complete development.
Furthermore, roots immersed in hot water at 52 – 62 oC for 10 min or 42 oC for 30 min could kill all larvae and adult weevils (CIP, 1997; Das, 1998). Ali et al. (1991) reported partial control of weevils using a repellant water trap baited with synthetic pheromone and storage in dry sand mixed with tobacco leaf powder. Sweet potato roots heaped on the floor and covered with a 2-cm thick layer of sand and rice husks were either free from infestation or had negligible infestation after three months of storage (Smit, 1997).
Gamma radiation between 150-200 Gy had no effect on surface injury or storage decay when roots were evaluated after one month storage at 13 oC and 90% R.H (Sharp, 1995).
During storage, weight loss by irradiated roots was 0.5 to 3.3% more than that of non- treated ones. In some instances, this affected root firmness. An irradiation dose of150-165 Gy has been recommended for sterilizing female weevils to reduce reproduction and further multiplication and life time (Hallman, 2001; Follett 2006). All stages of the sweet potato weevil may be found in marketed roots. The adult is invariably the stage of insects which requires the highest radiation dose to control (Hallman, 2000). Vapour heat treatment kill SPW pupae in roots (Shimabukuro et al. 1997). Delate and Brecht (1989) and Delate et al. (1990) conducted studies on controlled atmosphere for the control of weevil in stored sweet potatoes. They identified that adults of sweet potato weevil or sweet potato roots infested with immature stages of the pest were exposed to controlled atmospheres containing low oxygen (O2) and increased carbon dioxide (CO2) with a balance nitrogen (N2) for up to 10 days at 25 and 300C. Adults were killed within 4-8 days when exposed to 8% O2 + 40-60% CO2 at 300C.
When baked, irradiated sweet potato roots were sweeter than non-irradiated roots but they were not preferred due to the darker appearance (McGuire and Sharp, 1995). The entemopathogenic fungi, Metarrhizium anisopliae and Beauveria bassiana were found to be effective in controlling weevils. However, in the drier regions of Africa, B. bassiana had limited potential for weevil control (Alcazar et al., 1997). In the traditional agricultural systems in the tropics and sub-tropics where inputs are low, the use of weevil resistant varieties is the most economic way of effectively controlling weevils. Several attempts have been made during the past five decades to find resistance to this pest with limited success (Collins et al., 1991; Yasuda, 1997). Significant variations in the amounts of triterpenoid boehmeryl acetate (a known ovipositional stimulant for the pest) were found before and after harvest, with season and between cultivars which are resistant and susceptible to SPW (Son et al., 1991).
Mass trapping of male weevils by sex pheromone trap (Figure 1) is one of the most effective IPM practiced in many sweet potato-growing countries. Its female -produced sex pheromone was identified as (Z) –3- dodecenyl (E)-2- butenoate. Control programs involving
mass trapping of male weevils have been used with limited success in many Asian countries (Ray and Ravi, 2005). The intensity of trapping used varied from 4 traps/ha to 100 traps/ha.
Similar results were obtained using pheromone traps against C. puncticollis and C. brunneus (Pillai et al., 1993; Smit, 1997; Braun and van de Fliert, 1999).
Figure 1. Sex pheromone trap used for capturing male sweet potato weevils at CTCRI farm at Bhubaneswar (Source: Ray and Ravi, 2005).
2. Other Pests
Apart from sweet potato weevils, there are nearly 80 species of arthropods infesting sweet potato in storage. The three most important are coffee bean weevils (Araeceras fasciculatus), black fungus beetle (Alphithobius laevigatus) and ground beetle (Gonocephalm sweet potato.) (Talekar, 1987). The first two pests damage the roots completely and cause decay while the ground beetle feeds on the periderm by making irregular galleries. Infestation by A. fasciculatus is generally associated with stored roots, which are soft, or in a state of decay. Treating sweet potato chips with 2-3 % salt prior to drying controls damage by A.
fasciculatus in dried chips in storage.
Diseases
Pre harvest and post harvest infections by pathogenic microorganisms (mostly fungi and to a lesser extent bacteria) are serious causes of post harvest loss of sweet potato roots (Snowdon, 1991). The relative importance of the major pathogens can differ considerably between localities and with environmental conditions (Ray, and Byju, 2003).Different post harvest diseases (Table 2) are described below in brief.
1. Black Rot
Black rot, caused by Ceratocystis fimbriata, has been a problem wherever sweet potato is intensively grown. Most references of this rot are from USA, New Zealand and Japan (Clark, 2001), although the disease has virtually been eliminated by use of thiabendazole fungicides on seed roots and by cutting transplants above the soil line.Nevertheless, it is still considered as an important post harvest disease in other tropical, and sub-tropical regions such as Papua
New Guinea, Haiti and Peru and Vietnam. However, the rot has not been reported from sweet potato growing Asian countries like Bangladesh, China, India, Nepal and Pakistan (Ray and Edison, 2005). The characteristic symptoms of disease are sunken circular lesions, which are initially brown and latter greenish black,and the cooked roots taste bitter. Associated with lesions are minute black bodies (perithecia) with long necks. Infection occurs through wounds or even in healthy roots, and favoured by wet soil, and warm temperatures.
Table 2. Microorganisms associated with sweet potato rots in the tropics (Ray and Ravi, 2005; modified)
Types of rot Causative
organism Symptoms Pre-disposing
factors Avoidance/control measures Black rot Ceratocystis
filmbriata
Sunken circular lesions initially brown and later greenish black.
Associated with lesions are minute black bodies (perithecia) with long necks, appeared to naked eye as dark bristles
Wet soil, humid and warm temperature, contamination in seed roots
Crop rotation, careful handling of roots, heat treatment for no more than 24 h and curing at 35oC for 2 to 10 days.
Cultivation of resistant varieties.
Java black rot Botryodiplodia theobroame
Infected tissues are at first yellowish brown and fairly firm, later darkening to black.
After some weeks, affected roots become mummified and skin is pimpled with minute black bodies (pycnidia)
Wounding during harvesting and handling
Curing and subsequent storage at a temperature between 13-16oC;
cultivation of resistant varieties
Fusarium rot Fusarium spp. Type of decay is variable. End rot is characterized by a dry decay at one or both ends of fleshy roots.
Infected tissues shrivel, forming cavities filled with white molds
Wounding during harvest and handling, infected roots used as seed, infestation by weevils
Minimizing injury during harvesting and handling, curing, cultivation of resistant varieties
Charcoal rot Macrophomina phascolina
Infected roots show three zones- the advancing edges of the lesion is pale brown and spongy, intermediate zone is reddish brown and firm and the older part is almost black (micro sclerotia)
Wounding Minimizing injury during harvesting and handling, and curing
Table 2. (Continued) Types of rot Causative
organism Symptoms Pre-disposing
factors Avoidance/contr
ol measures Rhizopus rot Rhizopus spp. Decay beings at one end
and under humid conditions, roots shrivel, become soft and watery and the skin ruptures.
The mold spreads causing next of decay.
Wounds during post harvest handling, R.H.
(75-85%), high temperature (< 35oC)
Careful
handling, curing, cultivation of resistant varieties
Sclerotium rot Sclerotium rolfsii Circular lesions, sometimes internal tissues becoming water- soaked yet firm later hand and stringly
Wounds during post harvest handling, warm moist conditions (R.H. 75,-85%;
temperature
<35oC)
Careful handling curing
Spongy rot Cochliobolus lunatus (Curvularia lunata)
Infected roots swollen
and spongy Wounding, warm
and humid environment
Careful handling, curing
Rhizoctonia rot
Rhizoctonia solani
Pale brown spot on skin,
tend to shrivel Wounding, warm and humid environment
Careful handling, curing Gliomastix rot Gliomastix
novae-zelandiae
Lesions appear as brown
corky tissue Wounding, warm
and humid environment
Careful handling, curing Foot rot Plenodomus
destruen
Lesions appear as brown
corky tissue Wounding, warm
and humid environment
Careful handling, curing Bacterial rot Erwinia
crhrysanthemi
Roots become soft and watery similar to Rhizopus rot but differ by the absence of mycelia
Hot, humid
weather Cultivation of Resistant variety
2. Java Black Rot
Java black rot is caused by Botryodiplodia theobromae, the most prominent storage disease in tropical and sub tropical regions such as Bangladesh, India, Philippines, Nigeria, Ghanaand the sub-tropical zone of USA (Ray and Punithalingam, 1995; Sowley and Oduro, 2002; Ray and Edison, 2005). The rot usually spreads from the proximal end of the root or from other wound sites. The infected tissues are initially yellowish brown, and latter become black. After 6- 8 weeks of storage, the affected roots show dark patches externally, within which develop numerous pycnidia, and internally the tissues turn yellow, and latter black.
Finally, the rotted roots become shriveled, brittle and mummified (Figure 2). Wounding is the most important pre-disposing factor for Botryodiplodia infection. The optimum temperature and R.H. for growth of B. theobromae are 25-350 C and 85-90 %, respectively.
A
B
C
Figure 2. Characteristics of java black rot in sweet potato roots caused by Botryodiplodia theobroame (a) Cut sections of healthy and infected roots (b) Infected roots showing drk coloured pycnidia. (c) Mummified roots (Source: Ray and Ravi, 2005).
3. Soft Rot
Soft rot in sweet potato is caused by Rhizopusstolonifer, R. oryzae and R. nigricans. The rot is widespread in all sweet potato growing countries of temperate as well as in tropical regions (Ray et al., 1997; Sowley and Oduro, 2002; Ray and Edison, 2005).Affected roots usually decay totally due to rapidly developing soft and watery rot. Dry atmospheric conditions do not favour decay, and the root tissue remains firm but shrinks whereas in humid conditions, the roots shrivel, becomes soft and watery, and at places where the skin ruptures there is copious development of coarse white mould bearing globular spore head (sporangia) (Figure 3). The sporangia are at first white but turn black as they mature, and the entire mycelium appears gray. Further colonization of the entire rot can occur within a few days and the mould spread to adjacent roots in a storage pile causing ‗nest of decay‘. Wounds predispose the roots to be attacked and the proximal end is especially susceptible to invasion because the natural presence of dead tissue at the site of wound caused at the time of harvest is advantageous to the fungus. The other pre–disposing factors for Rhizopus infection in sweet potato are R.H. (75-85%), chilling, and high temperature (< 350 C).
A
B
Figure 3. Characteristics of Rhizopus rot in sweet potato roots caused by Rhizopus oryzae. (a) Infected roots showing soft-rot (b) Cut section of the infected root (Source: Ray and Ravi, 2005).
4. Fusarium Rot
Fusarium species causes ―Fusarium root rot‖ in sweet potato. The common species found in sweet potato roots are F. solani, F. oxysporum, F. moniliforme and F. pallidoroseum (Sowley and Oduro, 2002; Ray and Edison, 2005). F. oxysporum and F. solani have been recorded on sweet potato in the USA, Brazil, China, India and Israel,while F. pallidoroseum is reported only from India (Ray and Misra, 1995). The type of decay is rather variable. End rot caused by F. oxysporum and F . pallidoroseum is characterized by a dry decay at one or both ends of the fleshy roots, and the lesions are brown with dark margins. Infected roots shrink, sometimes forming cavities filled with white mould (Figure 4). On the other hand, surface rot caused by F. solani appears as pale brown, circular lesions, and the decay remains shallow with white mould. Severe wounding caused by rough handling at harvest, and improper storage resulting in slow wound healing increase the incidence of this disease. Also, wet soil conditions favour the spread of the disease. The disease does not generally spread during storage except when additional wounding occur which provide new infection sites.
The pre-disposing factors for Fusarium infection are soil water deficit, mechanical injuries, and insect infestations.
A
B
Figure 4. Characteristics of Fusarium rot in sweet potato roots caused by Fusarium oxysporum. (a) Infected roots showing soft-rot (b) Cut section of the infected root (Source: Ray and Ravi, 2005).
5. Charcoal Rot
Macrophomina phaseolina causes ―charcoal rot‖ in sweet potato roots. The disease is wide spread in tropics (Jenkins, 1982), but is less severe as compared to Java black rot or Fusarium rot. Decay of harvested roots usually begins at the point of original attachment to the plant (proximal end) and spreads through the roots. Initially the infected roots show variously shaped, and sized pale brown discoloration of the surface with distinct margin from healthy tissues. Diseased areas remain firm, and dark brown; water loss causes these areas to shrink. Rotting root shows three distinct zones. The advancing edge has a pale brown and spongy tissue; the intermediate zone has reddish brown and firm tissue; the old rotted area has dark Grey to black, and firm tissue. The ‗charcoal‘ appearance results from thousands of minute micro-sclerotia that colonize the interior, but not the surface of the root. Infection occurs through injury. Optimum temperature for growth of the fungus is 31.5oC but it grows even at 42oC.
6. Sclerotium Rot
Sclerotium rolfsii causes two diseases of sweet potato: ‗Slerotial blight‘, which develops on sprouts, and mother roots in plant production bed, and ‗sclerotium rot‘ which develops circular spots on stored roots (Clark, 2001). The disease has been recorded from Bangladesh, Cuba,Jamaica, Israel, Mozambique and USA.The symptoms of decay may be variable. The rotting may appear as circular lesions of about one- cm diameter or may be invasive, and the root tissue, which initially remains water soaked but firms latter, becomes hard, and stringy.
7. Spongy Rot
Spongy rot is caused by the fungus Cochliobolus lunatus (also Curvularia lunata). It has been reported from India (Ray and Misra, 1995). The infected roots are swollen and spongy, and the inside flesh turns brown to black.
8. Rhizoctonia Rot
This rot is caused by the fungus Rhizoctonia solani, and has been reported from India (Ray and Edison, 2005). Infected roots develop pale brown spots, and tend to shrivel.
Eventually, the entire root surface may be covered win brownish mould.
9. Gliomastix Rot
This rot is caused by the fungus Gliomastix novae – zelandiae. It has been reported in Egypt (Kararah et al., 1981). Lesions appear as irregular brown corky tissues usually slightly depressed. In a humid atmosphere, there is copious growth of black mould with abundant spores (conidia). Optimum temperature and R.H. for the disease development are 270 C and 84-100 % respectively.
10. Foot Rot
Foot rot is caused by Plenodomus destruen. It affects the roots in plant production beds, and in the field. It has been reported from USA (Clark, 2001) and Brazil (Rubin et al., 1994).
11. Pythium Rot
It is a major rot of sweet potato in Australia and USA, particularly in cool wet weather, and the causal agent is Pythium ultimum (Ray and Edison, 2005).Infected roots may become dry and crumbly, dark brown to dark gray in color or the decay may spread uniformly through the roots or may appear as a band of dark shrunken tissue which girdles the root.
12. Bacterial Rot
Erwinia chrysanthemicauses ‗Erwinia soft rot‘ of sweet potato roots in tropical as well as in temperate regions. Erwinia soft rot is similar to Rhizopus soft rot but primarily distinguished from the latter by the absence of mycelia. Roots infected with Erwinia develop black streaks in the vascular tissue and undergo a soft moist decay (Ray and Edison, 2005).
Biochemical Changes Associated with Fungal Rots
Microbial spoilage of sweet potato roots is manifested with changes in starch, total sugars, organic acids, enzymes, phenols, ethylene, and phytoalexin contents.
1. Changes in Starch, Total sugars, Protein and Organic Acids
One of the first parameter noticed following fungal decay of sweet potato is a decline in starch and ascorbic acid contents (Ray and Pati, 2001). The decline in starch content is either associated with concomitant increase (Acedo et al., 1996) or negligible variation (Ray and Pati, 2001) in total sugar. Like wise, the ascorbic acid contents of four sweet potato varieties was reported to decrease further following infection by B.theobromae or R. oryzae (Thompson, 1979; Ray and Pati, 2001).
Oxalic acid concentration was reported to increase significantly in microbial infected tissues (Faboya et al., 1983). The increase in oxalic acid was suggested to aid pathogen penetration by sequestering Ca or Mg in the middle lamella of cell walls, thereby increasing susceptibility of pectates to hydrolysis by cell wall degrading enzymes, i.e. cellulases and pectinases (Swain and Ray, 2007). Oxalic acid may also lower the pH of the tuber tissues to a level suitable for pathogenic enzyme degradative activity.
2. Proline and Carotenoids
There is no significant change in proline content between healthy and fungus (Rhizopus or Botryodiplodia) – infected sweet potato roots (Ray and Pati, 2001), although proline accumulation is considered as a measure of stress imposed on plant or plant parts (roots) due to adverse environments such as drought, temperature or microbial infection. On the contrary, roots infected with R. stolonifer contained only 24mg carotenoids /100 g (fwb) as compared with 50mg /100g in uninfected sweet potato roots (Thompson, 1979).
3. Enzyme Activities
In response to wounding of sweet potato by rotting fungi, many enzymes are induced.
The enzymes first activated belong to those in the phenyl propanoid pathway i.e.
phenylalanine ammonia–lyase( PAL) and trans-cinnamic acid 4-hydroxylase (Uritani, 1998).
Peroxidase and polyphenol oxidase activities were reported to subsequently increased (Arinze and Smith, 1982). Likewise, most of the rotting fungi i.e. B. theobroamae and R. oryzae produce cellulolytic and pectolytic enzymes in microbial cultures and infected tissues (Arinze