Alternaria Fungi - Classification, Habitat, Morphology & Pathogenesis

Alternaria fungi constitute a major group of plant pathogens. They belong to the phylum Deuteromycetes or Fungi imperfecti due to the lack of sexual stage in their life cycle. Thus, Alternaria species only reproduce asexually by conidiospores, and the process is called an anamorph state.

The genus Alternaria has nearly 299 species. They are ubiquitous. Alternaria species not only function as phytopathogens but also serve as common allergens that may infect humans as well as animals.

In humans and animals, the diseases or infections caused by Alternaria species are popularly known as Alternariosis and Alternariatoxicosis. This post mainly focuses on the classification, habitat, morphology, characteristics and reproduction cycle of the Alternaria.

Alternaria Classification

Kingdom: Fungi (represent a diverse group of spore-producing eukaryotic organisms including moulds, mushrooms, yeasts etc.)
Phylum: Deuteromycetes (represent a group of fungi that only reproduce asexually via germination of conidiospores and lack a sexual stage)
Class: Hyphomycetes (include fungi that hold conidia on separate conidiophores)
Order: Moniliales (include fungi that develop conidia directly on hyphae or free conidiophores)
Family: Dematiaceae (include fungi possessing darkish hyphae and conidia)
Genus: Alternaria (species of this genus possess lateral and transverse septa on the conidium)

Habitat

They are cosmopolitan in the distribution or ubiquitous (found in soil, air and plant residues). Alternaria species exist as saprophytes, endophytes and pathogens.

Saprophytic: The majority of members belonging to the genus Alternaria are saprophytes, which means they live in dead decaying organic matter.
Endophytic: Some species exist as endophytes, which means they inhabit different parts of crops, including foliage, seeds and fruits.
Pathogenic: A few species behave like opportunistic pathogens that cause infections in humans and animals.

Characteristics of Alternaria

One of the defining features of Alternaria species is that they produce phytotoxins and mycotoxins as secondary metabolites. Phytotoxins cause pathogenesis in plants, whereas mycotoxins cause pathogenesis in humans and animals. More than 70 mycotoxins of Alternaria have been introduced so far.
They are also considered necrotrophic fungi because they invade cell nutrients of plants, thereby causing necrosis in plant cells.
Among 299 species, Alternaria alternata is a predominant fungus that can grow in various substrates like food, clothes, goods etc. Alternaria species produce phytotoxins like alternariols, altenuene, tentoxin, and tenuazonic acid.
Human diseases associated with Alternaria fungi include cutaneous infection, paranasal sinusitis, pulmonary nodules, mycotic keratitis, osteomyelitis, peritonitis and asthma. Alternaria fungi cause leaf spots, leaf blight and rot diseases in different plant hosts.
Alternaria species show rapid growth, and they usually develop as grey to black woolly colonies with circular margins. They require an optimum temperature of 25-28 degrees Celsius and 5-7 days of incubation. The most common species of Alternaria include A. alternata, A. botrytis, A. leptinellae, A. oudenmansii and A. scirpinfestans.
Some strains of the genus Alternaria live as endophytes in different parts of the crops like leaf foliage, seeds and fruits. Thus, they are the significant phytopathogens causing pre and post-harvest losses, which deteriorate the crop’s quality and productivity. They can cause 20% of the agricultural loss.
Alternaria alternata and Alternaria infectoria produce DHN (Dihydroxynaphthalene) melanin pigment that plays a crucial role in imparting pathogenicity. Oxidative polymerization of phenolic precursors results in the production of DHN melanin that deposits in the conidia’s cell wall.

Morphology of Alternaria Fungi

We could study the structure of Alternaria fungi by knowing the features of the fungal mycelium, conidiophores and conidia.

Mycelium: Under macroscopic observation, the mycelium shows profuse growth. Initially, mycelium appears hyaline or colourless. Later, the fungal mycelium turns grey-brownish. Under microscopic observation, Alternaria species generally have a multicellular, coenocytic and irregularly branched mycelium. During the early stages, hyphae appear thin, narrow and colourless. But, the hypha becomes slightly thicker during later stages of fungal growth.

Conidiophores: They arise singly or in pairs and may appear long or short. Conidiophores of Alternaria species are geniculate with swollen apex. The colour of conidiophores usually ranges from pale olivaceous to olivaceous brown. We could visualize terminal scars under the microscope, which indicate a point of attachment of conidia. The length of conidiophores is 42.26 µm. The width is 4.29 µm.

Conidia: They exist in the form of chains. Ten or more conidia exist at each conidiophore tip. The conidia can be simple or branched. The shape of conidia can be obclavate, ellipsoidal and muriform. The conidial apex is tapered. Conidia possess 2-3 longitudinal septa and 2-10 transverse septa.

Pathogenesis in Plant Hosts

The spores produced by Alternaria species disseminate in the air and later fall on specific plant hosts. Spores start germinating by forming a short germ tube once they get enough moisture and a temperature between 31-35 degrees Celsius. The germ tube extends to form an erect and short conidiophore.

Mitosis division occurs in cells of conidiophores that develop reproductive structures (conidia) with a short beak. Conidia penetrate plant cells and invade all the nutrients by causing leaf necrosis and chlorosis. Besides leaf foliage, the spores of Alternaria fungi may also affect the fruit and lenticel. Infections are generally characterized by lesions, spots, blisters and rotting of leaves.

Pathogenesis in Human Hosts

The spores of Alternaria fungi enter the human body via small cuts, wounds or incisions. Primarily, the fungi cause cutaneous and subcutaneous infections. Cutaneous infections cause lesions mainly on the skin surface. Oppositely, subcutaneous infections typically affect the underlying layers of the human skin. Infections may lead to affect the other body parts like:

It colonizes the paranasal sinus by causing sinusitis.
It can also affect the cornea by causing oculomycosis.
Also, it affects the keratin tissues of nails by causing onychomycosis.

People with suppressed immunity may encounter infections caused by Alternaria species. Thus, Alternaria fungi cause opportunistic infections.

Conclusion

Therefore, we can conclude that species of the genus Alternaria only undergo the asexual saprophytic phase. They produce conidial spores in the form of a chain or in acropetal succession that detach during unfavourable conditions and live as a dormant spore until germination. In favourable conditions, conidia germinate by forming a germ tube.

Eventually, a short conidiophore develops that form a chain of conidia by undergoing mitosis. Thus, the reproductive cycle of the Alternaria species continues, and their free spores in the environment (air, soil, plant debris etc.) may infect the living system, including plants, animals and humans.