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.