Mucor Rot

Mucor rot occurs on both apples and pears. Although it can cause significant losses of fruit, Mucor rot is generally not a major problem, particularly when good harvest management and water-sanitation practices at packing are implemented. 

Symptoms:
Mucor rot originates primarily from infection of wounds on the skin of fruit. Stem-end Mucor rot is also seen on d’Anjou pears (Fig. 7). The decayed area appears light brown to brown with a sharp margin. The decayed tissue is very soft and juicy and can be readily separated from the healthy tissue. Gray mycelium with dark sporangia may appear on the decayed area. Mucor rot fruit has a sweet odor. Without the signs of the pathogen present on decayed fruit, Mucor rot can be mistaken as blue mold, particularly in the early stage of symptom development (see the comparison in Table 7).

Causal Organism:
Mucor rot is caused primarily by Mucor piriformis E. Fischer. 

Occurrence:
Mucor piriformis is a soilborne pathogen and survives in the orchard soil. The pathogen enters packing facilities through infested soils or organic debris adhering to field bins. Drench solutions and dump-tank water are the primary source of inoculum for fruit infection at drenching and packing.  Mucor piriformis may infect the stem, calyx or wounds on the skin of fruit.

Control:
Orchard sanitation to remove fallen fruit on the orchard floor helps reduce inoculum levels in the soil. Good harvest management is important for control of Mucor rot. Field bins should be positioned on the orchard floor to minimize contamination of the undersides of bins by infested soil or organic debris. Fruit that fall to the ground during harvest can be contaminated by infested soil and should not be put back in fruit bins.

Photo Plate: Mucor Rot
Fig. 7. Symptoms and signs of Mucor rot (Mucor piriformis) on apples and pears.

mucor-golden-side-200

A: Mucor rot on a Golden Delicious fruit; decayed tissue very soft, juicy, often with a sweet odor; lesion with a sharp margin

Mucor-stem-end-200

B: Advanced stage of Mucor rot originating from stem infection on a d’Anjou pear fruit; gray mycelium with dark sporangia

Mucor-anjou-sporulate-200

C: Advanced stage of Mucor rot on a d’Anjou pear fruit; gray mycelium with dark sporangia.

Disease Comparison:
Table 7. Comparison between Mucor rot and blue mold

Mucor rot
Texture: very soft, juicy
Color of decayed area: light brown to brown
Signs of pathogen: gray mycelium with dark sporangia
Color of internal flesh: light brown to brown
Odor: sweet

Blue mold
Texture: soft, watery
Color of decayed area: light tan to dark brown
Signs of pathogen: white mycelium, blue or blue-green spore masses
Color of internal flesh: brown
Odor: earthy, musty

References:
Michailides, T. J., and Spotts, R. A. 1990. Postharvest diseases of pome and stone fruits caused by Mucor piriformis in the Pacific Northwest and California. Plant Dis. 74:537-543.

Spotts, R. A. 1990. Mucor rot. Pages 57-58 in: Compendium of Apple and Pear Diseases. A. L. Jones and H. S. Aldwinckle (ed.). American Phytopathological Society Press, St. Paul, MN.