Rhizopus rot, caused by Rhizopus stolonifer, can be very destructive to harvested fruit. While it can develop in hail-injured or cracked fruit on the tree, it most commonly affects fruit in storage, during transit, and at the marketplace. Ripe fruit of peaches, nectarines, sweet cherries, and plums are most susceptible. Rhizopus fruit rot is usually of minor importance in the field but can cause important postharvest losses.

Symptoms

Rhizopus rot begins much like brown rot, as a small, brown, circular spot but with a detectable difference. The skin of Rhizopus rot-infected fruit slips readily from the underlying flesh, while the skin of brown rotted areas is tough and leathery. At normal temperatures, the small spots of Rhizopus rot enlarge rapidly and can cover the entire fruit in 24 to 48 hours. A white, fuzzy mold appears on the surface of infected fruits, spreading to nearby fruit and the walls of the container. By this time the fruit tends to "leak" and to smell like vinegar. Tiny, black, spherical fungal structures are produced on stalks above the white mold. Each of these sturctures contain thousands of spores that are released into the air. At this stage the mold looks mostly black.

Disease Cycle

Rhizopus rot occurs on all decaying vegetation, including ripe fruits and vegetables. When environmental conditions are not favorable, it produces thick-walled resistant spores that can withstand long periods of cold and drying. These are present on dead vegetation, in used fruit containers, and in packhouses and in storage. Thus, some type of spore of the Rhizopus rot fungus is always present where fruit is handled.

An injury through the skin of fruit must be present for the first infections to occur, and injuries as tiny as the prick of a pin are sufficient. In packed fruit or clustered ripe fruit on trees, the fungus can spread over the uninjured skin from an infected fruit nearby and eventually cause a rot. High temperatures and humidities favor the rapid growth of the fungus and decay of fruit.

Disease Management

Procedures that prevent wounding during harvest and packing is a prime factor in controlling Rhizopus fruit rot. Clean storage containers, clean shed and storage facilities, and hydrocooling with clean water will aid greatly in reducing fruit rot. Store fruit at or below 39 degrees F because the fungus does not grow below 40 degrees F.