Disease status

• Mucor rot commonly occurs at low levels in fruit from most orchards.
• However, in some seasons losses can be substantial in some fruit consignments.

Other hosts

• Most fruits are susceptible to Mucor.
• In the UK the rot occurs on pears, where losses are more significant and also on strawberries and raspberries.

Varietal susceptibility

• All apple varieties are susceptible.
• Previously the most significant losses due to Mucor rot have occurred on Bramley associated with fruit damage and contaminated drench tank water penetrating to the core via the open calyx.

Distribution

• The disease occurs worldwide wherever apples are grown.

Symptoms and recognition

Mucor rot can occasionally be seen on fallen fruit on the orchard floor prior to harvest, but is more common in the orchard on fallen fruits in the 1-2 months after harvest.  It is also common as a store rot.

• Mucor invades fruit through wounds or cracks anywhere on the fruit surface causing a pale to mid-brown, very soft wet rot.
• Rapid softening of the tissues occurs leading to a slimey disintegration of the flesh.
• Although the skin remains present it is very weak and collapses under the slightest pressure.
• On Bramley, and other open calyx varieties, it can also invade through the open calyx entering during the drenching operation.
• The apple tissue then rots internally leaving the peel parchment-like and intact and forming a fragile sack of almost liquid flesh.
• The Mucor fungus cannot penetrate the peel but emerges through any damaged area to produce a profuse growth of white coarse mycelium bearing black pin-shaped spore heads.
• Mucor rot can spread by contact but more usual are individual soft rotted fruits.

Other problems that may be confused with Mucor rot

• The soft watery rot and the distinctive coarse white mycelium with pin head-like black sporing bodies are characteristic and not easily confused with other rots.
• A closely related fungus – Rhizopus – causes similar rots on fruit and may be present in the orchard, but this fungus does not grow at storage temperatures, and therefore is not usually present in cold-stored fruit.

Disease cycle and epidemiology

Several species of Mucor (M. mucedo, M. racemosus and M. strictus) may be responsible for rotting in apples, but M. piriformis is the most common species.

• Mucor spores (sporangiospores) are soil-borne.  Most of these spores are located in the top 2 cm of the soil where they are associated with decaying organic matter such as fallen fruit.
• The spore populations vary over a season with the highest incidence 1-2 months after harvest, and a sharp decline during winter.
• Fallen fruit are infected by contact with infected soil or spores spread by rodents, birds and insects from rotting fruit.
• Rainfall dislodges the spores and they are washed into the soil.
• The spores are not wind-dispersed as they are embedded in a slimey matrix.
• Mowing rotting fruit may also serve to disperse spores.
• The fungus survives most successfully in cool, dry soil.
• Fallen fruit, low temperatures and a high moisture level encourages M. piriformis to increase in soil.
• Fruit becomes susceptible as it matures near harvest.
• Late-harvested, over mature fruit are more susceptible.
• Fruit usually becomes infected via wounds from soil contamination either directly at harvest or through the drench tank water becoming contaminated with spores from soil adhering to bulk bins.
• Consequently, risks are higher during wet harvests when bins and fruit are more likely to become mud-contaminated.
• The rot develops on contaminated fruit in store.

Cultural control

Successful prevention and control of Mucor rot are dependent on good crop handling and hygiene.

• Most important is prevention of soil contamination of bins by having grass alleyways on which to stand them during harvest.
• During wet harvests when risk of soil contamination is greater, avoid mud splash onto bins or hose off very muddy bins prior to drenching or storage to minimise soil contamination.
• Removing fallen fruit from the orchard after harvest may reduce the population of Mucor spores in the soil.

Biological control

• Biocontrol agents have not been developed.

Chemical control

• There are no fungicides effective against Mucor.
• Treatment of water with chlorine (calcium hypochlorite) has been used in the USA either alone or as a pre-wash prior to fungicide application, to reduce inoculum levels of Mucor present on fruit surfaces and in water in flotation tanks and hydro-coolers.
• Trials at East Malling have confirmed the effects of chlorine – see Penicillium rot.

Avoiding fungicide resistance

• No fungicides are effective against Mucor.