WSU Tree Fruit Research & Extension Center

Tree Fruit Market Diseases

Sunday, September 24, 2017

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



Alternaria tenuis auct.

Occurrence and importance
Alternaria rot, caused by Alternaria tenuis, may occur on apples from all producing sections. Fungus spores produced by the organism growing on dead or weakened plant tissues contaminate the fruits in the orchard. The amount of decay developing from this wound parasite on fruits in storage depends on the number of wounds and the vitality of the fruits. A. tenuis has been found growing and producing spores on the sepals of intact fruits in storage without causing rot.

Symptoms
Typical alternaria rot symptoms are more or less round, brown to black lesions, often centered around a skin break (top photo), or other weakened tissue. A few infections occur at the calyx and in the depression around the stem or in the wound where the stem is torn out in harvesting.

The decay progresses very slowly in cold storage. On firm ripe fruits, alternaria rot spots are often dry, firm, and shallow. In riper and older fruits, the surface of the spots usually becomes dark brown to black as the lesions enlarge (second photo). Advanced rots are spongy, and the affected flesh usually is streaked with black. When apples are removed from cold storage, restricted or arrested decays may become active to produce larger rots. A dark mold growth may develop in moist air above 50 °F.

Certain other rots may be confused with alternaria rot, but they can usually be differentiated by careful examination. Alternaria rot lesions of less than 1 inch in diameter, if brown, resemble side rot. Alternaria rot lesions, however, are usually firm with fairly tough skin, whereas tissues affected with side rot yield readily under pressure and the skin is tender.

Black rot is distinguished from alternaria rot by being much firmer. It may show alternating zones of light and dark brown. Eventually, black rot will produce black pimple-like fruiting bodies (pycnidia) on the surface.

Brown rot, rarely seen in storage or on the market, is usually not sunken. It may have black spots at the lenticels. Brown rot, in advanced states, becomes uniformly black and has a velvety sheen.

Bitter rot can be differentiated from alternaria rot by its lighter brown color of the decayed flesh and sometimes by the presence of spore masses. These spores, when they first issue, are wet and pink or cream-colored, and they may be in concentric rings.

Causal factors
The causal fungus, Alternaria tenuis, is a weak pathogen, which is unable to cause decay unless apples have been injured or weakened. Alternaria rot often develops by mid-season in cold storage on apples showing injuries such as delayed sunscald, bruising, or chemical injury. In apples held late in storage the rot may develop at skin checks, as in the York Imperial variety, or even at enlarged, ruptured lenticels over bruises. Alternaria infections frequently follow scald, soft scald, or Jonathan spot. The lesions following these physiological disorders are usually not sunken and often are without definite margins.

Control measures
Control of alternaria rot depends on careful handling during picking, washing, and packing to prevent physiological diseases and injuries that open the way for infection. Prompt storage and cooling of the fruits are essential. Warmer temperatures favor the development of the rot, therefore keeping apples at temperatures of 32° to 40 °F when moving them from storage to the consumer helps to prevent spoilage.

Alternaria rot, typical after wound
Alternaria rot, typical after wound

Alternaria rot after injury in controlled atmosphere storage
Alternaria rot after injury in controlled atmosphere storage

Alternaria rot leading to moldy core, Red Delicious
Alternaria rot leading to moldy core, Red Delicious

Alternaria rot leading to moldy core, Red Delicious
Alternaria rot leading to moldy core, Red Delicious

Alternaria rot, Gala
Alternaria rot, Gala

Alternaria rot, Golden Delicious
Alternaria rot, Golden Delicious

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