Suillellus mendax, a species described from Italy in 2014 and subsequently confirmed in Cyprus and France, is very similar to S. luridus and found under the same host-trees. It produces more robust fruit bodies with a markedly tomentose cap, has a reticulum that is less pronounced and often restricted to the upper part of the stem, and is mostly found on acidic rather than calcareous soil. Microscopically, S. mendax has more elongated, narrowly fusiform (spinde-shaped) spores than S. luridus, measuring (12.4–)13.3–14.7(–15.5) × (4.5–)4.9– 5.5(–5.7) μm, and with a higher spore quotient of 2.7. Collections from southern Europe previously classified as Boletus caucasicus on the basis of a yellow subhymenial layer (the flesh in the cap tissue immediately above the tube layer known as Bataille’s line), have been shown to phylogenetically correspond to either S. luridus or S. mendax. As shown by Vizzini and colleagues, the name Boletus caucasicus has been invalidly published (nomen nudum) and the Bataille's line is not reliable for discriminating between species in the Luridi complex, as it can be randomly present or absent in both S. luridus and S. mendax.
