Symptoms and Signs
Initial infections of Rhizopus fruit rot appear as discolored, water-soaked spots on fruit. These lesions enlarge rapidly, aided by enzymatic breakdown that leaves the berry limp, brown, and whose contents leak out onto the bed . Under conditions of high relative humidity, the berry rapidly becomes covered with a coat of white mycelium and sporangiophores. The sporangiophores develop black, spherical sporangia, each containing thousands of spores. When disrupted, these sporulating berries release a cloud containing millions of spores.
Rhizopus and mucor fruit rots closely resemble each other but can be differentiated in the field by examining the fungal growth with a hand lens. Look for the tiny, dark brown to black, spherical structures on the ends of the white fungal strands. These black spheres are the spore-bearing structures, or sporangia. For Rhizopus the sporangia appear dry, while the Mucor sporangia are wet or sticky looking due to a viscous liquid film.
Comments on the Disease
The fungus is an excellent saprophyte that lives on and helps break down decaying organic matter. It invades strawberries through wounds and secretes enzymes that degrade and kill the tissue ahead of the actual fungal growth. The fungus is especially active during the warmer months in late summer and survives cold periods as mycelium or spores on organic debris. Spores are airborne. The pathogen has a wide host range and is prevalent worldwide.
Field sanitation is extremely important. Handle fruit with care at all times. Remove all ripe fruit from the field at harvest. Be sure when fruit is being picked that the entire fruit is removed from the stem, not leaving behind the fleshy receptacle of the fruit as it can serve as a site for invasion by fungus.
Rhizopus stops growing at temperatures below 46° to 50°F (8° to 10°C), so rapid postharvest cooling of fruit is essential for disease control. Field sanitation also is extremely important: do not leave discarded plant refuse or berries in the furrows and be sure to remove all ripe fruit from the field.
Organically Acceptable Methods
Use sanitation, cultivar selection, and rapid postharvest cooling.
Fungicide applications are not currently recommended.