What is Alternaria?

Mary McMahon

Alternaria is a fungal genus with at least 50 species. It is quite ubiquitous in the environment, and it is one of the fungi which humans interact with most frequently, on a variety of levels. Alternaria species are a leading cause of crop blight, and they also cause allergies and infections in some people and animals. Mycologists, people who specialize in the study of fungi, estimate that eventually hundreds of species in this genus will be identified, with more being added to the list of known species all the time.

Alternaria can cause respiratory issues as well as other complications in those with a compromised immune system.
Alternaria can cause respiratory issues as well as other complications in those with a compromised immune system.

These fungi are asexual, reproducing by budding and creating club-shaped spores which join together in chains to facilitate the spread of the fungus. Alternaria species are generally gray to black in color, and their colonies have a downy to woolly texture. When disturbed, a colony can break up, spreading the fungus through the air in a black cloud. This is a common problem for farmers coping with crop infestations, as removal of the infected crop inevitably releases spores of the fungus into the air, contributing to its spread. The cloud of fungus can also contribute to the development of respiratory problems.

An estimated 20% of crop damage worldwide is caused by alternaria.
An estimated 20% of crop damage worldwide is caused by alternaria.

In the natural environment, Alternaria can be found anywhere where there is cellulose, as these fungi feed on cellulose. This includes decaying plant material and living crops. The fungus also grows in homes, and it can spread via waterways. Alternaria is a very hardy fungal species, going dormant during periods of prolonged drying and quickly reemerging as soon as a water source becomes available again. The colonies also grow very rapidly, allowing the fungus to take over an area very quickly.

Alternaria can cause asthma.
Alternaria can cause asthma.

For farmers and gardeners, Alternaria is a concern because it can cause plant blights. When it settles in plant, it classically causes a ring of dead tissue to emerge, and this ring slowly widens and eventually kills the plant or creates an array of unsightly spots. Controlling Alternaria can be difficult because it spreads so readily, and an estimated 20% of crop damage worldwide is caused by these busy fungi.

For animals, Alternaria can cause respiratory infections and allergies, especially if they are exposed for a prolonged period of time. Fungi in this genus appear to the contribute to the development of asthma in children, and they can cause coughing and respiratory infections. In people with compromised immune systems, the fungi can also cause more serious infections of the skin, mucus membranes, and internal organs.

Alternaria species are a leading cause of crop blight.
Alternaria species are a leading cause of crop blight.

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Discussion Comments


My mock orange trees developed alternaria. I think it liked to live in the manure compost that I used to fertilize it.

Though at first glance it looked kind of black, when I looked more closely I could see that it actually had a dark olive hue. It had long hairs that made a velvet patch.

When I tried to treat the trees, I discovered that alternaria is resistant to several fungicides. This pesky alternaria mold really made my plants ugly, and I will have to plant in a new spot further away next year to avoid reinfection.


My coworker decided to quit his job and move to Arizona to escape his problems with allergies and asthma. He had suffered from them all of his life, and like many people seeking refuge from these problems, he relocated to Tucson because of its reputation for clean air and easy breathing.

Though the desert climate may be free of many allergens, it is not free of alternaria. He did not know this when he moved, and he still experienced the same problems once he got there. While working in his yard, he discovered an old bed of untended flowers. When he saw the soft black spots, he knew that he had not escaped his problems after all.


Alternaria caused quite a problem for my friend and her daughter. They had threatened to move out of their rented house because they thought they had mold growing in the walls, but they found out the issue was much simpler than that.

After my friend complained to the landlady that her daughter’s severe allergies and asthma were brought on by house mold, she at first dismissed it. The girl’s eyes were purple, and she had to take breathing treatments every day. When my friend told the landlady she was moving out, she finally sent someone over to investigate.

The examiner found no mold in the house, but he did find a lot of it outdoors. It turns out that the girl had been laying outside in a pile of leaves and compost. It was the most comfortable part of her playhouse, and it just happened to be infected with alternaria.


While growing zinnia and dianthus in my flower garden, I experienced problems with alternaria. My poor garden was dotted with unsightly plants last year because of it.

My beautiful dianthus, normally lovely hot pink and white and the colorful star of the garden, became coated with black spots on the leaves and petals. I touched the alternaria accidentally, and its texture resembled felt or suede.

I did not know it until after I planted them, but some of my zinnia seed were infected with alternaria, and they grew very poorly. Their centers turned black, and they only developed one sparse ring of petals.

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