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Alternaria is a genus of fungi. Alternaria species are known as major plant pathogens. They are also common allergens in humans, growing indoors and causing hay fever or hypersensitivity reactions that sometimes lead to asthma. They readily cause opportunistic infections in immunocompromised people such as AIDS patients.

There are 44 definite, well-known species, but there may be hundreds more. They are ubiquitous in the environment and are a natural part of fungal flora almost everywhere. They are normal agents of decay and decomposition. The spores are airborne and found in the soil and water, as well as indoors and on objects. The club-shaped spores are single or form long chains. They can grow thick colonies which are usually black or gray.

At least 20% of agricultural spoilage is caused by Alternaria species. Many human health disorders can be caused by these fungi, which grow on skin and mucous membranes, including on the eyeballs and within the respiratory tract. Allergies are common, but serious infections are rare, except in people with compromised immune systems. However, species of this fungal genus are often prolific producers of a variety of toxic compounds. The effects most of these compounds have on animal and plant health are not well known. The terms alternariosis and alternariatoxicosis are used for disorders in humans and animals caused by a fungus in this genus.

Not all Alternaria species are pests and pathogens; some have shown promise as biocontrol agents against invasive plant species.

Selected species:

• Alternaria alternata - causes upper respiratory infections in AIDS patients, asthma in people with sensitivity, and has been implicated in chronic rhinosinusitis. Causes early blight of potato and can infest other plants.
• Alternaria arborescens - causes stem canker of tomato
• Alternaria arbusti - causes leaf lesions on Asian pear
• Alternaria blumeae - causes lesions on Blumea aurita
• Alternaria brassicae - infests many vegetables and roses
• Alternaria brassicicola - grows on cole crops
• Alternaria brunsii - causing cumin bloosem blight

• Alternaria carotiincultae - causes leaf blight on carrot
• Alternaria conjuncta - grows on parsnip
• Alternaria dauci - grows on carrot
• Alternaria euphorbiicola - infests cole crops
• Alternaria gaisen - causes ringspot disease of pear
• Alternaria infectoria - infests wheat
• Alternaria japonica - infests cole crops
• Alternaria molesta - may cause skin lesions on porpoises
• Alternaria panax - causes ginseng blight
• Alternaria petroselini - causes parsley leaf blight
• Alternaria selini - causes parsley crown decay
• Alternaria solani - causes early blight in potatoes and tomatoes
• Alternaria smyrnii - infests alexanders and parsleys