Alternaria

Scientific name

[Fungus] Alternaria alternata Fr. (Keissler)

Originally alternata was described as Alternaria citri Elli & Pierce or A. alternata pv. citri

Other common names

Alternaria brown spot (ABS), alternaria brown spot of mandarins, brown spot

Disease cycle

Alternaria is spread by airborne spores. Rain events or sudden changes in relative humidity also favor sporespore:
a.) small, walled reproductive body produced by non-flowing plants, algae, fungi and some protozoans; usually microscopic and resistant to desiccation and heat b.) dormant, nonreproductive body formed by certain unicellular organisms, especially bacteria, in response to harsh environmental conditions
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 inoculuminoculum:
a.) the pathogen or its part that causes infection of the host b.) small amount of a microorganism that is transferred to a substrate or a culture medium in order to propagate it
source. Even the leaf contributes to the inoculuminoculum:
a.) the pathogen or its part that causes infection of the host b.) small amount of a microorganism that is transferred to a substrate or a culture medium in order to propagate it
. Although the spores are airborne and carried by winds, alternaria brown spot is often spread among groves on nursery stock transported by humans.

Symptoms

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 necroticnecrotic:
a cell, group of cells, or tissue that has suffered a rapid destruction and quick death; typically localized and may be caused by injury, radiation, chemicals, toxic substances, or microbiological interactions
areas which can involve large portions of the leaf, especially on highly susceptible cultivars like 'Minneola'. A fungal toxin is produced that can cause necrosis and chlorosischlorosis:
an abnormal yellowing or whitening in green plant tissue due to insufficient chlorophyll production
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.

Host range

Dancy tangerine hybrids ['Minneola' tangelo ('Honeybell'), 'Orlando' tangelo, 'Sunburst', 'Nova,' and 'Lee'] and 'Murcott' ('Honey' tangerine) frequently have fruit infections of alternaria brown spot. Grapefruit and lemons may also be affected.

Distribution

Has been reported from Argentina, Australia, Brazil, Columbia, Greece, Iran, Israel, Italy, Peru, Spain, South Africa, Turkey, the United States, and Uruguay. The disease is likely found in other countries that produce susceptible cultivars.

Easily confused with

Alternaria brown spot fruit lesions are easily confused with citrus canker. Alternaria is extremely common on certain cultivars of tangerines while citrus canker is uncommon on tangerine cultivars.