Fungal Spores: types of fungal spores

Chaetomium spores
Chaetomium spores.

Fungal spores are microscopic reproductive structures in fungi. Their function is similar to that of seeds in plants. Fungi, which include moulds, yeasts, mushrooms and toadstools are an important component of the ecosystem. Fungi cause damage worth billions of dollars annually by causing diseases of animals and crops. While they also cause damage through biodeterioration and their spores pollute indoor environment, they help in the recycling of minerals and carbon by the decomposition of organic debris and waste. Every year trillions of tons of garbage are recycled worldwide by fungi

Alternaria spores
Spores of Alternaria

Fungi used to be classified as plants. However, at the cell level, fungi are more like animals than they are like plants. Other characteristics that distinguish fungi from plants is that fungi cannot synthesize their own food like plants do and they have no roots.

Fungi spread either by forming reproductive spores that are carried on wind and rain or by growing and extending their hyphae. Hyphae are thread-like, strands of cells and grow as new cells form at the tips. A network of branching hyphae is called mycelium (plural mycelia).

Fungal spore release

Cladosporium spores.

Fungal spores vary greatly in size, shape, color and method of release. Spores produced on raised structures are passively released by air movement. Many of these spores are delicately attached such that slight air movements or any disturbance of the substrate causes them to become airborne. The concentration of dry airborne spores e.g., Cladosporium can increase at the start of rainfall. Rapid air movement in advance of splashes and vibration can blow or tap spores into the air.

Spores of some fungi are released seasonally rather than throughout the year. Seasonal release of fungal plant pathogen spores coincides with a particular growth stage of the host plant. Ascospores and basidiospores are forcibly discharged into the air after wetting by rain or dew. The water creates pressure to force the ascospores from the ascii.

Airborne fungal spores in houses

Stachybotrys spores
Stachybotrys spores.

Several fungal spore types are frequently detected in the indoor environment. Since most of these spores originate from outdoors their counts are at levels generally lower than those detected in the outdoor samples. Some spores such as those of Penicillium/Aspergillus group are the most prevalent types indoors, usually exceeding the absolute levels and relative percentages of these spores outdoors. Cladosporium, Alternaria, Ascospores, and Basidiospores are the most prevalent spore types outdoors. Moisture-indicator fungi such as Chaetomium, Stachybotrys, and Ulocladium species are very common in water damaged buildings and very rarely are they found in outdoor air samples.

Health effects of fungal spores

In spring, summer and fall in temperate climates outdoor spore counts may be extremely high. Inhalation of airborne fungal spores has been implicated in several ailments in humans. Airborne fungal spores contain allergens which can trigger a range of respiratory issues in humans such as allergies, asthma, and pathogenic infections of the respiratory tract. It is estimated that around 20% of the general population is atopic and easily get symptoms from normal fungal spores concentrations. Symptoms may range from mild allergies to severe asthma. A number of fungal spore types share similar allergens which means that those who are allergic to moulds are likely to react to multiple types of spores. Due to the fact that the different fungi vary in the release times of their spores, some people can therefore suffer most of the year.

Airborne fungal spores are also important agents of plant disease, and the means for dissemination of many common saprotrophic (saprophytic) fungi.

Cladosporium spores
Cladosporium spores.

The most well known allergy and asthma causing fungal spore types worldwide are Alternaria, Cladosporium, and Aspergillus/Penicillium. Allergic reactions to each spore type differ between persons and the allergens vary in the severity of the allergic reaction they induce. More people are allergic to Alternaria than Cladosporium, for example, even though the latter is much more common in the air. Alternaria also produces more strongly positive reactions while Cladosporium tends to produce a milder allergic reaction. However, Cladosporium, and in particular Cladosporium herbarum, is often the major contributor to air-spora and due to its high concentrations is therefore a major cause of inhalant allergy and allergic asthma in humans.

Outdoor fungal spore seasons

Fungal spores are released and distributed in a variety of ways and have specific climatic requirements. Some types favour warm, dry weather and are wind-dispersed and the numbers of these can be very high in the outdoor air. Examples of these ‘dry air’ spores include Cladosporium, Alternaria, Epicoccum, Drechslera, Pithomyces, and Curvularia, and smut spores. Other types need certain temperature and moisture thresholds to be reached before they are released, so a warm, humid period can also produce a high spore risk. Examples of ‘wet air’ spores include basidiospores and ascospores which are often abundant on mild, humid nights in Summer.

Seasonal outdoor fungal spore summary

Fungal spores are mainly seasonal in their release but many types can be found in the air all year round typically peaking in the summer or late fall.

Spring: Fungal spores are generally low in number but the risk can rise to moderate in the late Spring if conditions are favourable.

Trichoderma spores
Trichoderma spores.

Summer to early fall: The spore risk starts to rise in mid-June with the increase in temperatures and in late June/early July the risk starts to peak and continues until late September. Typically, during this season, there will be the ‘dry air’ spore types of Alternaria & Cladosporium and some types of Penicillium and Aspergillus and others by day and then the ‘wet air’ spores that respond to the dew during the night. Therefore, there are a wide range of types that can affect people during each 24 hour period and many of the types have shared allergens that cross-react. In addition, rainfall will help the production of spores which are then released later after the rain (if it’s heavy) or during light showers or drizzly rain. Very windy weather tends to dilute the spores in the air and unusually cold, dry weather will lead to a lower spore risk.

Mid-fall: many of the main allergenic types are going into decline but there are still plenty of spores around and the risk can be moderate or even high in October on warmer, humid days. In November, the risk decreases to low by the end of the month unless it is very mild and damp. The lowest risk is on dry, cold frosty days.

Winter: Winter can be okay for some sufferers of fungal spores but for those sensitive to Penicillium and Aspergillus there will be some risk, particularly in January and early February.

Habitats and seasonality of the main allergenic spore types North America


Habitat / Substrates: Outdoors it is grows on many substrates including soil, seeds and plants. Some species of Alternaria are also important crop pathogens. For example, A. brassicae causes leaf spot on brassicas and A. solani causes early blight of potatoes. Indoor substrates include foodstuffs, carpets, textiles and window frames. Mould colonies are usually black or grey.

Season: Main season July to September but may occur at other times of year and can occur in the winter indoors.
Allergenicity: High.


Aspergillus or Penicillium spores
Aspergillus/penicillium spores

Habitat / Substrates: There are many different species of Aspergillus and Penicillium and they have a very wide range of substrates and tolerance. Examples: A.flavus grows on corn and peanuts, A. fumigatus grows in composts, mild to warm soils and on cereals, P. chrysogenum is found widely in nature, indoors it is found on food and is also the type used to produce Penicillin and P.expansum is a crop pathogen, causing post-harvest rot in apples.

Season: Throughout the year, with a small peak in August/September and the highest peak January/February.
Allergenicity: High for some types, particularly A. fumigatus and P. chrysogenum. A. fumigatus is major cause of aspergillosis (farmer’s lung).


Cladosporium spores
Cladosporium spores.

Habitat / Substrates: Outdoors on a wide range of plant substrates and in soil. Indoors, on paint and textiles. The mould colonies are generally black or olive-brown to brown.

Season: April to November and peaking in July and August. Occurs in low concentrations at all other times of year.
Allergenicity: High.


Habitat / Substrates: Outdoors, widespread on soil and plant debris. Indoor substrates include paper and textiles.

epicoccum spores
Epicoccum spores

Season: Main season August and September but may occur at other times of year in low concentrations.
Allergenicity: High.

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