Penicillium digitatum (green mould)

Different cultivars of clementine oranges differ in their susceptibility to P. digitatum (Farih et al., 1995). Certain rootstocks confer increased resistance to fruit decay on grafted trees (McDonald and Wutscher, 1974).Postharvest resistance correlates with production near wounds of phenolic compounds, such as benzoic acid derivatives, and phytoalexins, such as scoparone (6,7-dimethoxycourmarin) and scopoletin, by the fruit tissue (Rodov et at., 1994; Bonnas et al., 1995; Lattanzio et al., 1995) and to the production of chitinase and beta-1,3-glucanase by tissue cultures (Niedz et al., 1994).

Minimizing fruit injury and the efficient removal of infected fruit are the most effective ways to control this disease. Disinfectants can be used to clean equipment in packing and storage facilities (Brown and Eckert, 1988). Harvest following rain is discouraged because wet fruit are more prone to injury. Spore accumulation is prevented in orchards and factories by the removal of fallen or rotting fruit; efficient packing houses systematically segregate spoiled fruit, effectively reducing the disease (Snowdon, 1990). The disease develops most rapidly at temperatures near 24°C and fruit is thus usually refrigerated during storage (Brown and Eckert, 1988). Postharvest washing at 40-50°C in detergent or weak alkali solutions, usually including some fungicides (see Chemical Control), reduces decay. Individual wrapping of fruit in wax paper, or packaging in trays, prevents spread from infected to uninfected fruit during transport or storage. Ultraviolet light and higher temperatures induce formation of the phytoalexin scoparone, which promotes healing of wounded tissues (Pitt and Hocking, 1997). Ripening or degreening of fruit at 30°C and 95% RH promotes healing of wounds before fungal colonization can occur (Snowdon, 1990). Gamma irradiation of grapefruits with 0.3 kGy reduced decay, but higher levels may cause peel damage (Miller and McDonald, 1996). Storage at 5°C in a high carbon dioxide atmosphere reduces rot (Amarjit-Singh et al., 1992). Inclusion of a pouch of KOH in polyethylene wrapped fruit reduces rot (Piga et al., 1997).

Postharvest treatment of fruit with fungicides should be completed within 24 hours of harvest (Snowdon, 1990). Fungicidal dips and sprays, primarily thiabendazole, benomyl, carbendazim, imazalil, thiophanate-methyl, sorbic acid, guazatine or sodium o-phenylphenate (SOPP) are used to control the disease (Brown and Eckert, 1988; Pitt and Hocking, 1997). Imazalil and prochloraz inhibit sporulation, while benomyl and thiabendazole are more effective protectants (Lateef et al., 1994). Waxes, alone or incorporating fungicides, are often applied to fruit to reduce decay (Waks et al., 1985).Tolerance of fungicides has been reported to varying degrees in most countries (Bus et al., 1991; Pitt and Hocking, 1997) and may be more frequent in populations in citrus packing houses and storage areas than in wild populations (Gutter et al., 1981). Multiple tolerance of benzimidazole fungicides is frequently noted (Dave et al., 1980; Wild, 1983). A mutation in the beta-tubulin gene (a protein involved in mitosis) confers tolerance (Koenraadt et al., 1992). Some strains with fungicide tolerance revert to a susceptible phenotype when the selective pressure of the fungicide is removed (Jimenez et al., 1985).Experiments with volatile solvents have shown that vapours of ethanol and to a less extent acetaldehyde reduce infection by P. digitatum (Yuen et al., 1995).Spraying fruit with gibberellic acid has frequently been used to enhance the appearance and storage life of fruits (Bevington, 1973).

Sprays for fruit-wounding insects are often applied in groves (Snowdon, 1990). Hot water immersion to control fruit flies (such as Bactrocera tryoni) also reduces infection by P. digitatum (Williams et al., 1994).

Several yeasts and bacteria have been demonstrated to have antagonistic abilities against P. digitatum, but few are applied commercially because of the high inoculum loads required (up to 1000 million spores/ml) (Pitt and Hocking, 1997). The yeast antagonists are assumed to operate by inducing phytoalexins (Rodov et al., 1994) or by competition for nutrients (Droby et al., 1989); no antibiotics are known (Cheah et al., 1995). The mechanism of biocontrol for Bacillus spp. is assumed to be antibiotic production (Rodov et al., 1994). Some strains of Pseudomonas cepacea, while being antagonistic towards P. digitatum, also retard wound healing in orange fruits (Huang et al., 1991). A biocontrol product based on a formulation of Pseudomonas syringae strain ESC-10 is now commercially available under the trademarked name BIO-SAVE 1000.Various natural products have been tested for their effectiveness at controlling P. digitatum. An 'essential oil' from Thymus capitatus had similar effectiveness in vitro to thiabendazole (Arras et al., 1994); gel from Aloa barbadensis (=A. vera) slowed and reduced infection of inoculated grapefruit (Saks and Barkai-Golan, 1995). Similar results have been obtained with extracts from a variety of herbs.