Blue mold of apple is a common rot of stored apples and pears and is caused by the fungus Penicillium expansum. Other names for the disease are soft rot and Penicillium rot. Blue mold is a disease of ripe fruit and develops mostly on apples that are picked before they are mature. Firm fruit in the same container as decaying fruit may absorb a moldy odor and flavor.

Symptoms

Soft rot appears as soft, light-brown, watery areas that begin around injuries or lenticels on the fruit surface. Infected fruit have a characteristically moldy odor and flavor. When the relative humidity is high, grayish-blue masses of spores appear on the fruit surface. These spores are important in spreading the disease. Under favorable conditions, the entire fruit can rot in 2 weeks. The spores are very resistant to drying and can survive on the surface of packing and picking equipment. The spores can also build up in water used in dumping bulk boxes of fruit, postharvest drench solutions, and in water flumes used to float fruit onto packing lines.

Disease Cycle

Spores of the soft rot fungus are present almost everywhere and can survive long periods of unfavorable conditions. These spores survive from season to season on picking boxes, contaminated bins, and on storage walls. Injuries to fruit, especially during picking and handling operations, are the primary points of entry for the fungus. It is also easy for the fungus to invade lenticels of fruit that are overmature at harvest or fruit that have been held in storage too long.

Disease Management

Sanitation and harvesting and handling methods that minimize bruising and wounding are necessary to control blue mold. Disinfect contaminated bins and storage walls before reuse. Fruit should be harvested at optimum maturity. Remove decaying fruit daily from packing houses. It is also essential to move harvested fruit into cold storage as rapidly as possible. In many areas of the eastern United States, Penicillium has developed resistance to the benzimidazole fungicides, which are used in preharvest sprays and postharvest dips. As a result, registration for postharvest use on fruit crops have been withdrawn for most of these fungicides. Postharvest calcium treatments can aid in helping the fruit become more resistant to decay.