Exobasidium myrtilli

Exobasidium myrtilli is a basidiomycete fungus belonging to the family Exobasidiaceae. This species produces distinctive reddish, swollen leaf galls on bilberry plants, making it visually recognizable in the field. The infection causes infected leaves to become enlarged, discolored, and often bright red or pink—a striking departure from the normal green foliage of its host plant.

The fungus is a plant pathogen that specializes in bilberry and related Vaccinium species. Rather than forming the typical fruiting bodies seen in many fungi, Exobasidium myrtilli produces basidia directly on the surface of galled leaves, where spores are released into the air. iNaturalist data indicates the species has been documented across much of northern and central Europe, with recent observations from Switzerland, the United Kingdom, Sweden, and other regions.

Identification and Appearance

Exobasidium myrtilli does not produce conventional mushroom fruiting bodies. Instead, the fungus manifests as localized swellings and color changes on bilberry leaves and occasionally shoots. Infected leaves become markedly enlarged and distorted, typically turning bright red, pink, or pale cream-colored, contrasting sharply with healthy green foliage on the same plant.

The infected tissue becomes soft and somewhat waxy in appearance. On the undersurface of galled leaves, the fungus produces a fine, powdery layer of basidia and basidiospores—this layer may appear whitish or pale yellowish under magnification. The galls are usually confined to individual leaves or small sections of shoots, though multiple leaves on a single plant may be affected simultaneously.

Microscopic examination reveals the characteristic basidia of the genus Exobasidium, which are thin-walled and produce spores directly on the leaf surface. The presence of this pathogenic galling, combined with the host plant identity (bilberry, Vaccinium myrtillus), confirms identification in the field.

Life Cycle and Growth

The life cycle of Exobasidium myrtilli is intimately tied to the phenology of its bilberry host. The fungus overwinters in infected plant tissue or as spores in the soil. Infection occurs in spring when new leaves emerge, typically as temperatures warm and moisture levels increase. The fungus penetrates young, developing leaves and causes the characteristic hypertrophic response—the leaf tissue expands abnormally as the fungus colonizes it.

Throughout the growing season, infected leaves remain swollen and discolored. By early to mid-summer, the basidia mature on the leaf surface and release basidiospores. iNaturalist data indicates peak observation of galled leaves occurs from June through August in most of Europe, with some records extending into September. Once spores are dispersed, the galled leaf tissue gradually deteriorates and may fall from the plant.

The fungus completes its reproductive cycle before autumn, and the infected plant tissues either abscise or persist as overwintering inoculum. Reinfection of new leaves occurs in the following spring, maintaining the fungal population on susceptible bilberry plants.

Distribution and Habitat

GBIF records show Exobasidium myrtilli is widely distributed across Europe, with a total of 720 documented occurrences. The species has been recorded in the United Kingdom, Switzerland, Sweden, Spain, Norway, the Netherlands, Austria, Finland, Denmark, Poland, Germany, the Russian Federation, France, Ireland, and the Åland Islands. In North America, the fungus has been documented in the United States, with records from California.

The fungus occurs wherever its host plant, bilberry (Vaccinium myrtillus), grows. This includes heathlands, moorlands, coniferous and mixed forests, and montane habitats. In Scandinavia and northern regions, it is found in boreal forest understories and open heath communities. In the Alps and other mountain ranges, it occurs from lower elevations up to subalpine zones. The species shows no particular preference for soil type, growing on bilberry plants across acidic, nutrient-poor substrates typical of its host’s range.

Recent observations from 2024 and 2025 document the fungus in active distribution across its known range, with sightings in Switzerland, the United Kingdom, Sweden, Norway, Spain, and Germany. The widespread and consistent presence of Exobasidium myrtilli indicates it is well-established throughout its range and likely to persist wherever bilberry plants occur.

Ecological Role

Exobasidium myrtilli functions as a biotrophic pathogen, deriving nutrients from living bilberry tissue without immediately killing the host plant. The galls represent a form of pathogenic overdevelopment—the fungus stimulates abnormal cell division and expansion in the leaf, creating a nutrient-rich microhabitat for its own reproduction. While the infected leaves are rendered non-functional and eventually lost, the host plant typically survives and produces new uninfected foliage in subsequent growing seasons.

The fungus contributes to the natural regulation of bilberry populations and may influence the competitive dynamics of plant communities where bilberry is dominant. The production of copious basidiospores in summer means the fungus disperses widely and can infect multiple plants within a stand. In this sense, Exobasidium myrtilli represents a significant selective pressure on bilberry, though its impact is generally non-lethal and the host populations persist despite chronic infection.

Edibility and Uses

Little is documented about the edibility of Exobasidium myrtilli. The fungus does not form conventional fruiting bodies and is not harvested for consumption. Infected bilberry leaves are not toxic to humans, but they are unsuitable for use and should be avoided when harvesting bilberries for food.

The primary significance of this species is as a plant pathogen affecting bilberry crops and wild populations. In agricultural contexts where bilberries are cultivated, Exobasidium myrtilli infection can reduce fruit yield by damaging foliage and diverting plant resources to the infected tissues. No specific culinary or medicinal uses for the fungus itself are documented.

Similar Species

Exobasidium myrtilli is the primary gall-causing Exobasidium species on bilberry in Europe. Other Exobasidium species occur on different Vaccinium hosts or in different geographic regions. Exobasidium vaccinii, which infects cranberry and other Vaccinium species, produces similar galling but occurs on different host plants and in different regions. The identification of Exobasidium myrtilli relies heavily on the host plant identity—it is found exclusively on bilberry—and the characteristic bright red or pink galling of leaves in summer.

Confusion with other leaf diseases of bilberry is unlikely given the distinctive appearance of Exobasidium galls. Rust fungi and other pathogens produce different symptoms (pustules, discolorations) and do not cause the pronounced swelling and distortion characteristic of Exobasidium infection.

Conservation

No formal conservation status has been assigned to Exobasidium myrtilli. The fungus is not threatened and shows no signs of decline across its range. With 720 documented occurrences and ongoing observations in multiple countries, the species is clearly widespread and stable.

The fungus is likely to persist indefinitely as long as bilberry plants remain present across its range. Climate change and habitat loss affecting bilberry populations could indirectly impact Exobasidium myrtilli, but no direct threats to the fungus are currently recognized.

Sources and References

• GBIF – Distribution records and occurrence data (720 total occurrences across Europe and North America)
• iNaturalist – Observation data and photographs documenting recent sightings from 2014–2025
• Wikidata – Taxonomic classification and structured species data
• EOL TraitBank – Trait data and ecological information for Exobasidiaceae
• Wikipedia – General information on basidiomycetes and plant pathogens
• Taxonomy follows Index Fungorum and standard mycological nomenclature

Fun Facts

• The bright red or pink coloration of infected bilberry leaves is so striking that galled leaves can be spotted from a distance, making Exobasidium myrtilli one of the most visually obvious fungal infections in northern European forests and heathlands.
• Unlike most fungi that form fruiting bodies with a defined lifespan, Exobasidium myrtilli produces basidia continuously across the surface of infected leaves throughout the summer, releasing spores over an extended period.
• The fungus induces the host plant to overproduce cells in the infected leaf, creating a hypertrophied tissue that is soft and swollen—a manipulation of plant development that benefits the fungus’s reproduction at the expense of the leaf’s normal function.
• iNaturalist records show the fungus has been documented by citizen scientists and professional mycologists across at least 16 European countries, making it one of the well-studied Exobasidium species in the Northern Hemisphere.
• Bilberry plants infected with Exobasidium myrtilli often produce new, uninfected leaves after shedding the galled ones, allowing the plant to recover and continue photosynthesis despite the pathogen’s presence.
• The fungus appears to have a preference for cool climates, with the highest concentration of records in Scandinavia, the Alps, and other northern and montane regions where bilberry thrives.
• Recent observations from 2025 document the fungus actively infecting bilberry plants in summer across Switzerland, the UK, Sweden, and Spain, confirming its continued prevalence and seasonal predictability.

Range Map

• Countries: United Kingdom of Great Britain and Northern Ireland, Switzerland, Sweden, Spain, Norway, United States of America, Netherlands, Austria, Finland, Denmark, Poland, Germany, Russian Federation, France, Ireland
• Recorded occurrences: 720
• Georeferenced points available: 270