Mucor Rot of Pome Fruit		AG0166
W.S. Washington and R. Holmes, Knoxfield Updated: January 2006
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Mucor rot is a post-harvest disease of apples, pears and other fruit caused by the fungus Mucor piriformis and less often by other Mucor species. The disease is found in Australia, North America, South Africa and Europe, where it can cause serious losses, especially after longterm storage of fruit.

Symptoms

Mucor rot causes a light brown, soft, watery rot. It commonly occurs where the fruit skin has been damaged and also at the stem and calyx end of fruit. Where breaks in the fruit skin occur, white, whiskery fungal growth may appear, which is soon covered by black spore masses. The rot progresses rapidly at room temperature, and even at storage temperatures (0°C) the disease can cause extensive rotting over several months. Disease symptoms are superficially similar to those of rhizopus rot caused by Rhizopus stolonifer; however, mucor rot can develop at 0°C whereas rhizopus rot is restricted by temperatures below 4°C.

Economic importance

In Australia, mucor rot has only relatively recently been recognised as a serious problem. In some seasons considerable losses have occurred, especially in CA-stored fruit for both local and export markets. Spread within a packing shed can be rapid, as healthy fruit can be contaminated and infected during postharvest dipping, and during sorting and grading operations. M. piriformis can also be a major cause of core rot in Red Delicious apples.

Disease cycle

M. piriformis is a soil-borne fungus that is found mainly in the surface layers of orchard soil. It infects fallen fruit during and after harvest, and as a result orchard populations are highest in the months immediately following harvest. In the orchard the fungus may be spread by rain and irrigation splash, by bird and insect feeding, and by practices such as mowing which can spread infected pieces of fruit. M. piriformis enters the packing shed on soil adhering to fruit bins and machinery, and on fruit that was picked off the ground during harvest.

It may also survive between seasons on fruit storage bins. Inoculum can be spread during postharvest handling operations, for example, through drench and flotation solutions. During storage, rotted fruit break down and release juice which, in the case of pears, assists in secondary spread. Secondary spread during storage is less common in apples. Over-mature, late harvested and injured fruit are more susceptible to infections.

No fungicides are registered for the control of mucor rot. Control relies mainly on careful harvesting and hygiene in the orchard, storgage and packing facilities. Careful picking and handling of fruit to avoid injuries to the skin is most important. Rough areas in the grading and packing line, including dust and debris which could abrade fruit, must be eliminated. Clean all picking bins pre-season, and avoid soil buildup on their undersides during picking. Do not pick wet fruit, and avoid picking fruit that has fallen to the ground, as this may introduce soil and infected fruit directly into the fruit bin. Figure 2. Bins can be sanitised by pressure cleaning with hot water and other methods Avoid orchard soil being transferred to handling and storage areas on machinery tyres. Minimise the contamination of pre-storage drenches, dips or sprays by washing fruit and bins in fresh or sanitised water before further treatment. Bins can also be decontaminated with humidified hot air, hot water or steam before harvest. Minimise the contamination of flumes and dump tanks by appropriate filtration, water sanitation and regular changes of water. Rollers, brushes and belts must not become contaminated with spores or decayed fruit debris, and should be thoroughly cleaned as necessary. Washing of fruit in fresh or sanitised water after fruit with rots and other disorders have been-sorted out will help to minimise further contamination and infection of fruit.