Monilinia fructicola (Wint.) Honey
and M. laxa (Aderh. & Ruhl.) Honey
Occurrence and importance
Brown rot is a minor disease of apples and other pome fruits. It is most likely to be found along the eastern seaboard. However, it has been reported from the humid growing regions of the central and western States. Brown rot losses in storage are rare, and the disease is not common on the market.
Brown rot develops rapidly through wounds on apples at nonrefrigerated temperatures. Enlarged rots are soft but not mushy. Infection may occur around the stem or calyx or through serious wounds, cracks, or insect-feeding injuries. Typical decay spots are circular and medium brown during the early and medium stages of development. As the decayed area enlarges, small black spots about 1/8 inch across gradually develop at the lenticels (top photo). These spots may be grouped or scattered over the otherwise brown and rotted area. Eventually the entire fruit is decayed and under warm conditions turns black and develops a velvety sheen. In warm, moist conditions gray to tan fungal tufts develop, either in varying size patches or scattered over the decayed surface.
Brown rot and black rot may be confused before they develop characteristic symptoms. In general, however, brown rot is paler brown than black rot, and black rot is always firmer than brown rot. Black rot retains its firmness as the decay enlarges, and the decayed surface tends to form alternating zones of different shades of brown. In advanced stages, numerous black pustules (pycnidia) may develop on the surface of black rot decay. These hard, black pustules contain the asexual spores of the fungus that causes black rot.
The fungus, Monilinia fructicola, causes practically all of the brown rot in apples. In the far west, a closely related fungus, M. laxa, causes brown rot of pears and may also cause some brown rot of apples. Infection occurs only in injured tissues of fruits, which must be mature for the disease to make much progress. The disease is often found on damaged apples while they are still on the tree, especially in orchards where stone-fruit trees have been planted as fillers. Incipient infections may survive cold storage temperatures and develop active decay after the apples are removed to higher temperatures.
Brown rot is checked by low temperatures, more readily in incipient and very early stages than after it becomes well established in the fruits. Like most other apple rots, therefore, practical control after harvest can be accomplished by prompt storage and rapid cooling to the desired temperature. Recommendations of the State agricultural experiment station or extension service should be followed for control in the orchard.