That black film on leaves is fungi too

Trichomerium is a genus of foliar epiphytic fungi that resemble sooty moulds and are commonly found on the surfaces of living leaves. Species in the genus are characterised by dark, intricately interwoven hyphae and by ascomata bearing setae.

Xiaoyan Deng's team described two new species forming sooty coating colonies from Guizhou Province, China: Trichomerium guiyangense and T. xishuiense. Both are morphologically distinct, and their identity was confirmed by multi-gene phylogenetic analysis based on two rDNA loci, LSU and ITS. Detailed illustrations and a phylogenetic tree accompany the taxonomic descriptions.

It is worth explaining what sooty moulds are, because they are among the most frequently seen and least understood things in a garden. The black, sooty film on leaves is not a disease of the leaf: the fungus does not enter the tissue and does not feed on the plant. It feeds on honeydew, the sugary excretion of aphids and other sap-sucking bugs feeding above. The film harms indirectly, by shading the surface and limiting photosynthesis.

A practical gardening conclusion follows: fighting the film without removing the aphids is fighting the symptom. The film disappears when the source of honeydew does.

Describing two new species in such a common group says something about the state of knowledge. Things visible to the naked eye on every tree in a city can be undescribed, because for decades they were treated as one undifferentiated "soot" and nobody checked how many species were hiding under it.

The affinities of the genus are surprising in themselves. Trichomerium belongs to the order Chaetothyriales, which also contains the so-called black yeasts — fungi with heavily melanised walls, known for tolerating desiccation, radiation and solvents.

Members of that group have been isolated from dishwashers, saunas, petrol stations and desert rock. The melanin that gives a sooty film its black colour is the same molecule that lets their relatives live in places most organisms cannot accept. The black deposit on a leaf and the fungus in a shower cubicle are close relatives.

What the paper establishes

Trichomerium guiyangense and T. xishuiense formed dark, sooty-looking coatings on living leaves in Guizhou. The T. guiyangense holotype (HGUP 230079; culture GUCC 23-0130) came from Iris sp.; setose ascomata, asci and ascospores plus ITS+LSU phylogeny delimited the taxa. The authors' uncertainty matters: T. xishuiense fell outside the core Trichomerium clade, close to Formicomyces, and was retained mainly because its sexual morphology matches the genus. “Sooty mould” describes appearance and epiphytic position. The study did not trace carbon sources, demonstrate feeding on honeydew, identify an insect partner or test effects on the plant. Two sites cannot establish full distributions, endemism, abundance or threat status.