Blueberry leaf rust fungus spreads as far north as Oulu – how to identify it and help researchers

A destructive fungal disease affecting blueberry plants has spread widely across Finland, reaching as far north as Oulu, with researchers now calling on the public to report sightings to track its expansion.

The blueberry leaf rust (Naemacyclus vaccinii) can reduce wild blueberry yields by up to 20 percent and devastate cultivated crops by over 50 percent, according to observations from last year’s unusually severe outbreak. Biologist Jouni Issakainen of the Finnish Environment Institute documented cases across Southwest Finland and noted that the fungus thrives in damp, shaded blueberry patches.

Researchers from the Natural Resources Institute Finland (Luke) and the University of Helsinki are urging citizens to monitor blueberry thickets from late spring onward. “We’re particularly interested in reports of otherwise healthy plants with sudden wilting, blackened shoots, or dry brown spots on leaves—changes that can occur within days,” Issakainen said.

The fungus spreads via spores capable of leaping up to 20 centimeters—unusually high for plant pathogens. Infected leaves develop lace-like holes as the disease consumes the central vein, turning it black. While harmless to humans, the fungus may also target lingonberries and low-growing grasses, though its impact on these species remains under study.

To assist research, the public can submit photos and observations via the laji.fi platform, noting whether infected leaves appear only near the ground or higher up. Issakainen demonstrated a simple DIY “spore trap” using a damp chamber to capture jumping spores for study.

Wet, foggy springs and summers appear to fuel outbreaks. Last year’s epidemic followed an exceptionally damp season, though dry conditions this spring could suppress its spread. “This fungus has always been present but overlooked,” Issakainen noted. “Climate change may amplify its cycles—more humid summers could mean more severe infestations.”

How to identify blueberry leaf rust

Leaves and shoots: Infected plants show patches where most leaves have dropped, with few or no berries. Shoots turn black and brittle, resembling burnt matchsticks, while lower stems remain green. Surviving shoot tips may appear pale yellow-green.

Leaf spots: Brown, papery dry patches (about 1 cm wide) cover the leaves. Similar spots may appear on nearby lingonberries or ground-level plants.

Spore structures: A hand lens reveals tiny (0.5 mm) brown spikes on the underside of spotted leaves—these are the fungus’s spore-producing bodies.

Sightings are most likely in shaded, damp blueberry habitats where symptoms appeared last year. Searching a few days after rain increases detection chances.