What actually makes a mushroom poisonous?
A mushroom is poisonous when it carries a chemical that harms living tissue. In the Bashkortostan study, the researchers tested several mushroom species against a culture of Paramecium caudatum, a single-celled organism used as a simple stand-in for living cells [1]. The species tested included the death cap (Amanita phalloides), the fly agaric (Amanita muscaria), and Hygrophoropsis aurantiaca, with two edible species used as controls [1].
The results separated the mushrooms by the kind of toxin they contain. Species with toxic peptides, the class of poisons behind phalloid syndrome, showed a strong toxic effect on the test cells [1]. Toxins that cause cholinergic syndrome also produced a toxic effect, while the toxins behind atropine and hallucinatory syndromes were only slightly toxic in this test [1]. In plain terms, the poison's chemical identity predicts how hard it hits, at least in this single-celled test system.
Different toxins, different kinds of harm
The word "poisonous" hides a lot of variety. The same study groups mushroom toxins into distinct syndromes, meaning recognizable patterns of symptoms caused by different poisons [1]. Peptide toxins that cause phalloid syndrome are the most damaging in the cell test, while the toxins behind atropine and hallucinatory syndromes were far milder there [1].
That ranking comes from a Paramecium test, not from people, so it describes how these toxins affect a simple cell rather than the full course of human illness. The study's stated use is etiological diagnosis in toxicology, meaning working out which toxin caused a poisoning case [1]. So the practical value of this work is telling poisons apart, not explaining why the mushroom makes them.
Why do the fungi make these toxins?
This is where the supplied evidence runs out. The Bashkortostan paper focuses on toxicology, characterizing toxins and the syndromes they cause, and does not examine the reasons fungi produce them [1]. The older guides to edible and poisonous mushrooms are about telling the two apart and avoiding the poisonous ones, not about the biology behind toxin production [2][3].
The modern identification papers have the same gap. One uses computer vision to classify poisonous fungi from images, reaching 78.4% accuracy on a private test set [4]. Another trains a deep neural network to separate edible from poisonous mushrooms and reports 83% accuracy [5]. Both are about recognizing toxic species, not about why the toxins exist. So the honest answer to "why" is that the supplied papers show what the poisons are and how they act, but not the evolutionary or ecological reason the fungi make them.
How can you tell a poisonous mushroom from an edible one?
You cannot rely on a simple visual rule from these papers. The identification research treats the problem as genuinely hard: the dataset is large and imbalanced, differences between species are subtle, and individuals of the same species vary a lot [4]. That is why automated systems are being built, and they are good but not perfect, with accuracies of 78.4% [4] and 83% [5] in the reported tests.
Those numbers mean the systems misclassify a meaningful share of mushrooms, so they are aids rather than guarantees. The older guides took the same practical aim, helping readers select edible fungi and avoid poisonous ones [2][3]. Across all the supplied papers, the reliable message is that toxicity comes from specific toxins with specific effects [1], and that identifying a poisonous mushroom is a separate, still-imperfect skill [4][5].
Sources used in this answer
FEATURES OF THE TOXIC EFFECT OF SOME MACROMYCETE FUNGI OF THE BIRSKY DISTRICT OF THE REPUBLIC OF BASHKORTOSTAN
Testing mushrooms from the Birsky district against Paramecium caudatum showed that peptide toxins causing phalloid syndrome had a strong toxic effect, cholinergic-syndrome toxins were also toxic, and atropine and hallucinatory-syndrome toxins were only slightly toxic.
… fungi: toadstools, mushrooms, fungi, edible and poisonous: how to select and cook the edible: how to distinguish and avoid the poisonous, with full botanic …
An early guide focuses on distinguishing edible from poisonous fungi and on selecting and cooking edible species, without explaining why fungi produce toxins.
Studies of American Fungi: Mushrooms, Edible, Poisonous, etc
A 1911 study of American fungi identifies edible and poisonous mushrooms and how to tell them apart, but does not address the reasons behind toxin production.
Fine-Grained Classification for Poisonous Fungi Identification with Transfer Learning
A computer vision approach using ensemble classifier heads on self-supervised image embeddings achieved 78.4% accuracy and the best Track 3 score in the FungiCLEF 2024 poisonous-fungi identification task.
Deep Neural Networks for Accurate Identification of Edible and Poisonous Fungi
A seven-layer deep convolutional neural network classified edible versus poisonous fungi from images with 83% accuracy.