Principal Postharvest Disease

LEMONS are susceptible to a number of postharvest fungal diseases. Some of the pathogens attack the fruit prior to harvest, but exist in a resting state until the conditions are right for infection. Others infect the fruit only after harvest.

Lemons should be harvested and handled gently to avoid bruising and skin injury, which greatly accelerates postharvest microbial decay. Reduction of postharvest decay is also achieved by the use of appropriate pre-harvest and postharvest fungicides, proper sanitation of the wash water, and prompt cooling to l2″C (54″F).

In addition, adequate ventilation during storage is necessary to remove ethylene, which increases the severity of many postharvest diseases. Small pads treated with the fungistat diphenyl (at the rate of 4.7 gml23 kg fruit) are beneficial in retarding decay development when placed inside export cartons. The main postharvest lemon diseases in Guyana are various moulds, black rot, brown rot, anthracnose, stem-end rot, sour rot, and cottony rot.

GREEN MOULD
Green mould, caused by the fungus Penicillium digitatum, is typically the worst postharvest disease of lemons. It attacks injured areas of the peel and first appears as a soft, watery, slightly discoloured spot on the rind. The spot enlarges to several centimetres in diameter within a day at ambient temperature, and the rot soon penetrates the juice vesicles.

White fungal growth appears on the fruit surface and, soon after, olive-green spores are produced. The sporulating area is surrounded by a broad zone of white fungal growth and an outer zone of the softened rind. The spores are easily dispersed if the fruit is moved. If the storage RH is low, the whole fruit shrinks to a wrinkled, dry mummy. If the RH is high, the fruit collapses into a soft, decomposing mass. This disease develops most rapidly at about 24″C (75″F).

It can be minimised by using good pre-harvest sanitation practices; careful harvesting and handling to avoid injuries to the peel; a postharvest dip or spray with a benzimidazole fungicide (i.e. 500 ppm benomyl, or 1000 ppm thiabendazole or imazalil); and holding the fruit at l2″C (54″F).

BLUE MOULD
Blue mould, caused by the fungus Penicillium italicum, is another common postharvest disease of lemons but is usually less prevalent than green mould. Early symptoms are similar to green mould. Diseased tissue becomes soft, watery, and slightly discoloured and is easily punctured. The lesions enlarge more slowly than those of the green mould.

A white, powdery fungal growth develops on the lesion surface, and blue spore mass forms, leaving only a narrow white fringe of fungal growth surrounding the lesion. A pronounced halo of water-soaked, faded tissue surrounds the lesion between the fringe of fungal growth and the sound tissue. The blue spores covering the fruit may become brownish-olive with age.

Healthy fruit in packed containers become soiled by spores shed from the diseased fruit. Unlike green mould, blue mould spreads in packed containers and results in nests or pockets of diseased fruit (Figure 6). Like green mould, blue mould develops most rapidly at about 24″C (75″F). However, blue mould grows better than green mould at cool temperatures and may predominate over green mould in yellow coloured lemons stored at 10″C (50″F).

Immediate cooling after packing significantly delays development of blue mould especially if the fruit storage temperature can be maintained at l2″C (54″F). Additional disease control recommendations are identical to those listed for green mould.