Chapter 1 Microbial deterioration of cassava: organisms involved

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Earlier publications on the subject of the deterioration of cassava simply state that cassava roots will not store well, have a short storage life, will not keep for more than a few days and are highly perishable (Rickard and Coursey, 1981 ) without giving any indication of the nature or even the symptoms of the deterioration processes involved. Other publications refer loosely to "rots" or "decay", giving the impression that the deterioration is essentially due to microbiological infection.

The number of different species of fungi and bacteria isolated from roots stored under different conditions shows that post-harvest decay of cassava is a complex matter, involving more than a single initial organism. Two distinct specific types of rot have been described by Majumder (1955), a dry rot occurring under aerobic conditions and caused by an unidentified Rhizopus sp. and a soft rot which developed under anaerobic conditions caused by a Bacillus sp. Under West African conditions, Affran (1968) and Doku (1969) have suggested an association between post-harvest decay and preharvest infection of the roots with white thread disease, Rigidoporus lignosis (Klotzsch) Imasaki.

A more detailed investigation (Ekundayo and Daniel, 1973) indicated that soft rot of cassava roots was caused by a complex of fungi; Lasiodiplodia theobromae (Pat.) Griff. et Maubl., Aspergillus nigervan Tieghem, Aspergillus flavus Link, Cylindrocarpon candidum (Link) Wollenw and Trichoderma harizianum Rifia, the first organism being the most important. Although these workers clearly associated the decay with invasion through wounds, they concentrated on the later stages of decay rather than on the initiation of postharvest deterioration. Wegmann ( 1970), who also worked mainly with material that was in an advanced stage of deterioration, isolated A. niger together with "Cylindrium cladostrinum " (presumably C. clandestrinium (Corda) Saccardo) and unidentified Penicillium and Cladosporium spp. Studies by Burton (1970) on cassava shipped from Puerto Rico to the United States indicated that, while Diplodia manihotis (Sacc.) was the most serious market disease, a number of other fungal pathogens were also isolated, including species of Fusarium, Mucor, Phomopsis, Rhizopus and Trichoderma spp.

Booth (1976), in a more detailed study on the deterioration of cassava, isolated from the surfaces of cassava roots various species of Pythium, Mucor, Rhizopus, Penicillium, Aspergillus, Fusarium, Cladosporium, Glomerella, Gloeosporium, Rhizoctonia, Bacillus, Xanthomonas, Erwinia, Agrobacterium and many saprophytic bacteria. However, Booth was consistently unable to isolate any specific microorganism from the advancing margins of deterioration in the flesh of the rots. It was therefore concluded that the earlier stages of postharvest deterioration, manifest as discoloration of the vascular tissue, were not inherently the results of attack by pathogens and that the later stages were essentially the decay of already moribund tissue caused by a wide variety of saprophytes.

In a later study by Noon and Booth ( 1977) a number of microorganisms, both fungi and bacteria, were isolated from severely decayed cassava roots. The pathogenicity of the organisms was tested by inoculating freshly harvested, surface-sterilized roots. Vascular streaking developed in the roots throughout the 14-days storage period under tropical ambient conditions (25C). Within four days of harvest, over 50 percent of the roots showed symptoms of vascular streaking. Some of the isolated microorganisms proved to be pathogenic when introduced into healthy cassava roots, notably Botryodiplodia theobromae Pat. and to a lesser extent Aspergillus flavus Link, Trichoderma harizianum Rifia and Fusarium solani (Mart.) (Table5). In some cases inoculated roots developed symptoms of vascular streaking (Figure 3), but there was no evidence that this was associated with the introduced organisms. In these cases the inoculated organisms could not be recovered from the advancing fronts of discoloration, although they could be recovered from the margins of the grossly necrotic areas. In other cases rotting was caused by the inoculated pathogen, but no vascular streaking occurred. The findings of Noon and Booth, which concluded that vascular streaking is a physiological process, were substantiated by a detailed cytochemical study of the development of vascular streaking using light and electron microscopic techniques. Rickard, Marriott and Gahan ( 1979) were unable to detect any signs of microbial infection during the early stages of vascular discoloration and, following results obtained using Phycomycetes, Taniguchi and Data (1984) also concluded that there was no direct relationship between vascular streaking in cassava roots and microbial decay.

TABLE 5 Microorganisms isolated from damaged cassava roots

Organism Disease
Bacillus sp. Minor wet rot
Post-harvest secondary deterioration
Corynebacterium manihot Root fermentation
Armillanella (armillana) mellea Young root necrosis
Minor dry rot
Aspergillus spp. Post-harvest secondary deterioration
Circinella sp. Post-harvest decay
Clitocybe tabescens Root rot
Cylindrocarpon candidum Post-harvest deterioration
Diplodia manihotis Root rot
Erwinia sp. Minor wet rot
Young root necrosis
Fusarium spp. Minor wet rot
Ganoderma pseudoferrum Red root rot
Geotricum candida Root fermentation
Helicobasidium compactum Minor dry rot
Lasiodiplodia theobromae Post-harvest secondary deterioration
Mucor sp. Post-harvest decay
Penicillium spp. Post-harvest decay
Phaeolus manihotis Root rot
Phytophthora spp. Young root necrosis
Wet rot
Pythium sp. Young root necrosis
Minor wet rot
Rhizoctonia sp. Root rot
Rhizopus spp. Post-harvest secondary deterioration
Rigidoporous (Fomes lignosis) White root
Rosellinia spp. Black rot
Scleroinia sp. Young root necrosis
Sclerotium rolfsli Young root necrosis
Minor dry rot
Sphaceloma manihoticola Minor root rot
Sphacrostilbe repens Root rot
Syncephalastrum sp. Post-harvest decay
Trichoderma sp. Post-harvest deterioration
Xanthomonas manihotis Cassava bacteria blight and minor dry rot
Unknown Frog skin disease

On the basis of his observations, Booth (1976) made a clear distinction between primary deterioration of stored cassava roots, considered to be an endogenous physiological process (Figure 4), and secondary deterioration. Microbial activity is the most common cause of secondary deterioration although fermentation or root tissue softening can also occur. Primary deterioration is the initial and major cause of the loss of acceptability, while secondary deterioration can become more important later. On occasion secondary deterioration may be the initial cause of loss and in these instances symptoms of vascular streaking frequently occur ahead of the rots. Pre- and post-harvest root rot diseases of cassava have been reviewed by Booth (1978) and are summarized in Table 6.

TABLE 6 Mean distance1 of tissue decay from points of inoculation of healthy roots with icroorganisms isolated from deteriorated cassava roots2

Microorganism Mean distance of tissue decay (mm)3
Aspergillus flavus LK ex Fr 5
Bacterial isolate 1 1
Bacterial isolate 2 2
Botryodiplodia theobromae Pat 26
Fusarium solani (Mart) Sacc 7
Mucor sp 2
Penicillium sp, isolate 1 1
Penicillium sp, isolate 2 1
Rhizopus sp 1
Trichoderma harizianum Rifai 10

Source: Booth 1976.
Notes:
1 Distance measured 14 days after inoculation.
2 Microorganism isolated 14 days after harvest of roots
3 Mean for two experiments. each consisting of four replicates

Secondary deterioration occurs when pathogens penetrate through wounds and bruises inflicted during harvesting and handling. Storage at high humidity encourages fungal rotting but is also necessary for effective wound healing (see Chapter Two). The use of a microbial protectant is therefore often required with preservation methods that are favourable to root curing, such as storage in plastic bags (see Introduction).


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