First, allow me to orient you with a little bit of Mycology 101. Specifically two basic sorts of mushrooms.
The mushroom is the result of sexual reproduction. The process? Not sexy. A couple of nuclei get together in a special cell and the process cascades: nuclear fusion and meiosis — where basically the genes get sorted — and finally a spore containing the recombined genes forms. What you see in a mushroom is the launching platform for the spore. The spores of all mushrooms are born externally on specialized cells. How and where those cells are placed distinguishes two major groups of mushrooms.
In one group the cells line thin sheets, or gills. So, if you look on the underside of the cap of one of these mushrooms you see the edges of a lot of gills radiating from the center, the stalk. The common supermarket mushroom is one of these, so is matsutake.
In another group of mushrooms the cells that bear the spores form inside tubes. If you look on the underside of one of these mushrooms you will see tiny pores. These are colloquially called boletes and the most famous bolete is the Cep.
This story is about one kind of bolete.
Many boletes and other gilled mushrooms form associations with the roots of trees and other plants to share nutrients — called mycorrhizae. It’s a mutualism wherein all partners get — and give — something. Some people think that the fungus body, filaments radiating through the substratum that you never see, connect trees and mushrooms in a kind of messaging system. These are good guys.
The ash-tree bolete — Boletinellus merulioides — is not a good guy.
At least not good for the ash trees.
Boletinellus merulioides is not mycorrhizal. It is in a symbiotic relationship with the ‘leafcurl ash aphid’ or ‘wooly ash aphid,’ Meliarhizophagus fraxinifolii. An ash pathogen.
This aphid has a complicated life, but it is inextricably tied up with several species of ash trees. Eggs, which overwinter in laid in cracks of the bark, hatch in the spring. After hatching, the aphids feed first on shoots, and then a new generation feeds on newly developing ash leaves. The leaves become distorted and ultimately form ‘pseudogalls,’ within which there is another generation of aphids. Eventually, as early as August, some of those aphids fly off and lay eggs. During summer some of the females change their behavior. They begin to feed on roots of ash trees. A new generation of males and females develops from those root-feeding aphids to complete the cycle.
But some of the root population remains in the soil all year, feeding on roots. They form a symbiotic relationship with the ash-tree bolete. The mushroom’s mycelium protects aphid by producing little knots of tissue around them. In return the honeydew produced by the aphid nourishes the bolete.
The ash ash aphid is native to North America but it has become a problem on European ash trees.