Asterisk Lichen

Arthonia radiata is a crustose lichen with an immersed thallus, often separated from its surroundings by a thin brown line. The thallus is typically pale, ranging from white to pale grey, sometimes with a brown or olive tinge, and often forms a mosaic-like pattern on its substrate. Its reproductive structures, called apothecia, are black and can appear in various shapes—rounded, star-like (stellate), or elongated. These apothecia can measure between 0.15 and 2.2 mm across, and are usually flat or slightly convex. They lack a frosted appearance, often referred to as being non-pruinose.

Under a microscope, the apothecia have a thin epithecium (the upper layer) that is brown or olive-brown, reacting to certain chemical tests by turning pale green. The internal structure includes a clear hymenium (spore-producing layer) and a hypothecium (the layer beneath) that is also colourless to pale olive-brown. The paraphysoids (sterile filaments within the hymenium) are narrow, 1–2 μm wide, but can expand to 3 μm, and they are often capped with darker brown tips. Arthonia radiata produces spores that are cylindric-obovoid to cylindrical, with 3 internal walls (septa) dividing the spore into 4 cells. These ascospores measure between 15 and 20 μm long and 4.5 to 6 μm wide, and the uppermost cell is not enlarged. …

Arthonia radiata is a widespread and common species. It grows on the smooth bark of many trees and shrubs.

Lichenicolous fungi that have been recorded growing on Arthonia radiata include Muellerella polyspora, Stigmidium arthoniae, and an undescribed species of Opegrapha.