Alternaria rot is a fungal disease of citrus fruit that affects mainly navel oranges and lemons.
Identification
Fruit infected with A. citri frequently turn a mature color prematurely and develop dark decay. The rotted tissue is generally softer on lemons than on oranges. On navel oranges, the disease is also called black rot because it causes dark brown to black firm blotches at the stylar (bottom) end of fruit or in the navel. If you cut infected fruit in half from top to bottom, you can see the rot extending into the core and flesh.
Life cycle
Alternaria fungi are spread by airborne spores. Fog, high humidity, and rain favor spore release. Spores are produced on older lesions on fruit, old leaves, and wilted twigs.
There are several strains of Alternaria citri. The strain on mandarin causing brown spots has been referred to as A. alternata pv. citri.
There are toxin-producing strains of A. alternata that cause other diseases of citrus, but these have not been reported in California. The strains that cause Alternaria fruit rot do not produce a toxin.
Damage
Alternaria citri causes a dark decay on and inside of citrus fruit. This makes fruit inedible or unappetizing. Some infections that occur in the grove may not develop into disease until after harvest. Much of the fruit decay from Alternaria rot develops during storage.
Solutions
Provide trees with a good growing environment and appropriate cultural care. This favors healthy, good quality fruit that are more resistant to Alternaria rot than damaged or stressed fruit. Inspect fruit regularly and remove and dispose of those that changed color prematurely or have decay. Periodically inspect fruit during storage, and dispose of any that develop disease.
Adapted from Integrated Pest Management for Citrus and Pest Management Guidelines: Citrus, University of California Statewide Integrated Pest Management Program (UC IPM).
