Quality deterioration of fresh produce fungal and bacterial pathogens

The most important microorganisms causing post-harvest wastage of fresh produce are fungi. This is particularly true for fruits, where the relatively acid conditions tend to suppress bacterial growth. Vegetables with a higher pH can, however, suffer high losses from bacterial infections. The most important pathogens of fruits and vegetables are described by a number of authors (Beattie et al., 1989; Coates et al, 1995; Dennis, 1983; Snowdon, 1990; 1991). The majority of pathogens rely on damaged tissues to obtain entry into fresh produce (wounds or sites of physiological injury). For example, the Penicillium species which cause blue and green mould infections of citrus and other fruit crops are classic wound pathogens, incapable of invading an undamaged fruit. An intact, fresh commodity is resistant to the majority of potential pathogens. The physical barrier of the skin and the presence of antimicrobial compounds in the skin and flesh are sufficient protection.

Some pathogens can gain entry through natural openings such as stomata and lenticels. Bacteria may use this penetration route. The most common group of bacteria causing significant reductions in shelf-life is the soft rotting species of the genus Erwinia. Under suitable conditions of warmth and the presence of free water, the bacteria can readily colonise produce such as potatoes through the lenticels. They produce large quantities of extracellular enzymes which rapidly macerate the tissues. Sometimes, soft rots are accompanied by the growth of saprophytic bacteria which give rise to highly unpleasant off-odours (Lund, 1983).

Only a small number of fungal pathogens are capable of direct penetration of the undamaged skin of the produce. On the whole, these latter pathogens are particularly problematic owing to the fact that they may infect produce before harvest but remain quiescent in the tissues until conditions become favourable for growth. This phenomenon is largely seen in fruits, where initial pathogen development and subsequent quiescence occurs in the unripe fruit. As the fruit ripens, quiescence is broken and the pathogen colonises the fruit tissues (Swinburne, 1983). Colletotrichum gloeosporioides is a common pathogen showing this behaviour on a number of tropical fruits such as mango and papaya. Typical symptoms on ripe fruits are sunken, lens-shaped lesions, which may develop salmon-coloured sporing structures. Colletotrichum musae causes similar symptoms on bananas. Botrytis cinerea may also show quiescent behaviour on certain fruits, for example, in strawberries, fungal spores contaminate the flowers, germinate and the hyphae grow into the developing fruit where they remain symptomless until the fruit is fully ripe. The subsequent disease development can be extremely rapid and the whole fruit is completely colonised and covered with a grey, sporulating mycelium within a few days at 20°C.

Skin diseases may remain superficial but cause large market losses owing to the blemished appearance of the produce. The potato industry has a major problem with a number of skin diseases, such as black scurf (Rhizoctonia solani), black dot (Colletotrichum coccodes), silver scurf (Helminthosporium solani) and common scab (Streptomyces scabies) which can spread rapidly on the tubers after the temperature rises in retail outlets (Snowdon, 1991).

On the whole, fungal and bacterial infections are stimulated under high humidity conditions and in particular in the presence of free water. Pathogens of fruits and vegetables are variable with respect to their ability to grow and reproduce at different temperatures; however, most will grow between 6 and 35°C. Some will survive and even grow slowly at temperatures as low as 1°C, for example, B. cinerea. The incidence of particular pathogen species is thus affected by both pre-harvest and post-harvest conditions. So, for example, B. cinerea is particularly important on produce grown in cool temperate climates, whereas infections caused by Botryodiplodia theobromae or Aspergillus niger tend to cause serious losses in warm regions.

Certain pathogens can impact heavily on the fresh produce processing industry: for example, the presence of just a few citrus fruits infected with Alternaria rot in a consignment can result in off-flavoured juice (Patrick and Hill, 1959). The presence of certain cell wall degrading enzymes from infecting pathogens, for example Rhizopus spp., can cause continuing softening of canned products even after the fungus has been killed during the sterilisation process (Harper et al., 1972).

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