WSU Tree Fruit Research & Extension Center

Tree Fruit Market Diseases

Sunday, March 24, 2019

WSU-TFREC/Postharvest Information Network/Tree Fruit Market Diseases/blackrot

Physalospora obtusa (Schw.) Cke.

Occurrence and importance
Black rot, a disease of apples and other pome fruits, is found in most of the producing sections of the United States east of the Rocky Mountains. While the disease has been observed in the far west, its occurrence there is relatively rare. Black rot is more important in the Atlantic coastal states, where in some years it seriously damages trees and fruits. Summer and early fall varieties are more susceptible than the late-keeping ones. However, heavy losses have been reported on Delicious, Stayman Winesap, Rome Beauty, and Golden Delicious apples. This rot is of minor importance on the market.

The typical symptom on the market is a single, firm brown spot on any part of the apple. The affected surface may be marked with concentric zones of different shades of brown, especially if the fruit rotted on the tree (top photo). In advanced rots, which can involve the whole fruit, the skin is dark brown or even black and sometimes dotted with numerous small black fungal fruiting bodies called pycnidia. The presence of pycnidia and their random distribution help to distinguish black rot from most other apple rots.

In cold storage, black rot develops very slowly and does not usually exhibit pycnidia. The rotted flesh may be moist but still remain firm. After removal from cold storage, a soft mushy rot may develop in infected fruits.

Causal factors
The black rot fungus, Physalospora obtusa, attacks the leaves, wood, and fruits of apple. While immature fruits may be attacked, the disease is primarily a rot of ripe fruits. Infections may occur at insect injuries and wound sites. Calyx end infections may follow spray and frost injury. Core and calyx end rots may result from fungal invasion of the open calyx tubes in varieties such as Delicious. The disease develops very slowly in green or immature fruits, not making much progress until the sugar content increases as the fruits ripen. At temperatures above 50 °F, infected ripe fruits will rot rapidly. Black rot ordinarily does not spread from one fruit to another.

Control measures
Black rot should be controlled in the orchard. For fungicidal applications, follow the recommendations of the State agricultural experiment station or extension service. Prevention of insect and mechanical injuries to fruits in the orchard and the maintenance of temperatures below 40 °F in storage and in transit aid in reducing decay.

Black rot in orchard
Black rot in orchard

Black rot
Black rot

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