Alternaria brown spot (ABS)

Alternaria alternate

Alternaria is spread by airborne spores. Rain events or sudden changes in relative humidity also favor spore release. Spores are produced on older lesions formed on wilted twigs and mature leaves. Some spores come from fruit lesions but they are not a major inoculum source. Even the leaf contributes to the inoculum. Although the spores are airborne and carried by winds, alternaria brown spot is often spread among groves on nursery stock transported by humans.

Leaf - initial foliar lesions occur on young tissue as small brown to black spots that develop prominent yellow halos. Lesions expand into irregular or circular necrotic areas which can involve large portions of the leaf, especially on highly susceptible cultivarslike 'Minneola'. A fungal toxin is produced that can cause necrosis and chlorosis along the veins extending from lesions. Lesions are flat and visible on both sides on the leaf. Older lesions have a brittle paper-like texture in the middle of the lesions.

Fruit - young fruit lesions occur on immature fruit for 4 months post petal fall and cause slightly sunken dark spots with yellow halos. On more mature fruit, lesions can vary from small specks to large pockmarks. The fruit rind responds to infection by forming a barrier of corky tissue that erupts from the surface. In the later stages of the disease, the corky tissue can fall out, forming craters or pockmarks on the surface of the fruit. Early fruit drop is common, especially if infection has occurred shortly after petal fall. Alternaria fruit lesions can crack around the outer edge, giving a moat-like appearance. During the packing process, the tops of alternaria lesions may be lopped off, making visual identification difficult.

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