Yellow foam, black jelly and saddle fungi: strange forest finds

Yellow foam on a rotting stump is not a fungus but the slime mould Fuligo; black jelly on a branch is the jelly fungus Exidia; a “saddle” on a stem is a Helvella, and a brown “cup” in the leaf litter is a Peziza. All of these are common inhabitants of European forests, they are just rarely covered in mushroom guides. Nothing on this list is worth eating, but it is all fascinating to look at.

Below are our own finds from forest walks in 2025–2026, each with an explanation of what it is and what to look out for.

Yellow foam on a stump: the slime mould Fuligo

Our brightest find of May 2026 was a lemon-yellow foam spilled across a rotting trunk; it is in the photo at the top of the article. It looks as if someone had cracked an egg or spilled construction foam. Its English folk name says exactly that: “scrambled egg slime”.

This is Fuligo (Fuligo septica), a slime mould, or myxomycete. And most importantly: it is not a fungus. Slime moulds were once classed as fungi, but they are a separate group of organisms, closer to amoebae. While a slime mould is in its active phase, it is one big living cell with many nuclei that slowly crawls over wood and litter, eating bacteria, yeasts, spores and other tiny particles.

Then the yellow mass sets, darkens and becomes a brittle crust, with a dark powder of spores forming inside. So if you come back to the same spot a few days later, the “foam” may look completely different. Fuligo does no harm to the forest and does not infect plants. The only caution for people: Fuligo spores can be an allergen. In American studies from the 1970s–80s, some people with asthma or allergic rhinitis had a positive skin test to an extract of these spores. This does not mean the slime mould in the forest is dangerous, but you should not crumble the dry crust and breathe in its dust.

Black jelly on a branch: Exidia

This find looks like melted black rubber or a pile of glossy brain folds. In fact it is a true fungus — the jelly fungus Exidia. In English, such fungi are called witches' butter.

We found it in June 2026 on a rotting branch among fallen oak leaves: a black jelly-like mass coating the wood.

The secret of Exidia is water. In dry weather it shrivels to a thin black crust that is easy to overlook, and after rain it swells up and glistens again. It grows on dead branches of broadleaf trees, especially oak, and breaks down the wood. Exidia is not considered poisonous, but it has no value as food either: some field guides list it as inedible, others as edible but tasteless and too “empty”. So this is a find for the eyes, not for the basket.

The saddle fungus: Helvella

Helvellas are fungi whose cap looks like a saddle, a lobe or a crumpled handkerchief on a stem. They are relatives of morels and false morels: their spores ripen on the surface of the cap, not in gills or tubes.

We found our first Helvella at the end of May 2025 in an open forest among horsetails. A dark brown, almost black, smooth saddle-shaped cap and a thick, smooth white stem. Most likely this is Helvella leucopus, but the exact species cannot be determined from a photo.

The second one, found the same day, is different: a black-brown lobed cap, a paler grey underside and a grey stem cut by lengthwise grooves and pits. This is also a Helvella, close to Helvella lacunosa, but we do not identify it to species.

Are helvellas edible? In short: raw, definitely not, and it is better not to eat them at all. According to reference books on poisonous mushrooms, raw helvellas cause stomach upset. Popular literature often says they contain the same toxin as the false morel, gyromitrin. But there are no reliable analyses to confirm this: gyromitrin has been looked for with modern methods mainly in false morels and their relatives, and the researchers themselves, in 2023, advised re-examining old reports of it in other cup fungi. In other words, gyromitrin in helvellas is not proven, but not ruled out either: their chemistry has simply barely been studied. Some species, such as the white saddle, are still listed in field guides as conditionally edible after thorough boiling, but telling one Helvella species from another is not easy. For us, these fungi are for photography, not for the frying pan.

The cup fungus: Peziza

Among fallen oak leaves in May 2026 we found a large brown, wavy cup with no stem and no gills. This is Peziza, a cup fungus. From above it looks like someone's lost brown bowl.

From the side you can see that the cup is paler outside than inside, and its rim is thin and wavy. The spores ripen on the inner surface. If you gently touch a ripe cup, it may “exhale” a barely visible puff of spores: thousands of microscopic sacs, the asci, burst at once, and the wind carries the spores away.

There are over a hundred species of Peziza, and even a mycologist often tells them apart only under a microscope. Nothing definite is known about the edibility of most of them, and some, such as the bay cup, cause poisoning when raw or undercooked. So a Peziza is also a trophy for the camera only.

Roof tiles on a trunk: bracket fungi

On a mossy trunk we saw dozens of thin grey-white caps growing one above another, like tiles on a roof. Look closely: each cap has thin concentric bands of different shades.

This is a wood-decaying bracket fungus, most likely of the genus Trametes. There are several species that look alike, so we are not naming a specific one. Their caps are thin, leathery and tough, and underneath there are no gills but tiny pores. Such fungi break down dead wood and do important work in the forest, but they are too tough to eat.

There are edible bracket fungi too, for example chicken of the woods, but they look completely different. More about them in the article edible bracket fungi.

Not moss and not a fungus: lichens

Two more finds that are often mistaken for moss or mould. The first is an old stump completely covered in a greenish powdery coating, like velvet. This is what some powdery lichens look like, although ordinary algae also produce a similar green coating, so you cannot be sure from a photo.

A lichen is not a plant, not a moss and not simply a fungus, but a partnership between a fungus and an alga (or a cyanobacterium). The fungus provides a home and protection, and the alga makes food for both in the light. Many lichens, especially the shrubby and leafy ones on tree bark, are very sensitive to polluted air, above all to sulphur dioxide. So a variety of lichens on tree trunks is a good sign of clean air.

The second find is thin grey “matchsticks” sticking up out of cushions of moss. We photographed them in October 2026. This is the lichen Cladonia.

The famous “reindeer moss” belongs to the same genus: shrubby grey cladonias that carpet dry pine forests on sand. The name is misleading: it is not a moss but a lichen. Reindeer moss grows very slowly, about 4–5 millimetres a year, so a trampled or torn carpet takes on average some twenty years to recover, and longer still to reach full maturity.

What to do with a strange find

• Photograph it from several sides: from above, from the side, from below, together with its base and whatever it is growing on.
• Note the circumstances: what it grows on (soil, rotting wood, living bark), which trees are nearby, which month it is.
• Do not taste or take into the kitchen anything you cannot identify with certainty. Folk “tests” such as the silver spoon do not work — see our article myths about recognising poisonous mushrooms.
• Leave it where it is. Slime moulds, jelly fungi, bracket fungi and lichens are part of the forest's economy, which breaks down wood and builds soil.

More about the surprises the forest has in store in the article the biggest and most amazing mushrooms of Europe.