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BOLETES OR SPONGE MUSHROOMS

Basidiomycota - Hymenomycetes

Boletes have well-developed caps and stalks and at first glance look like mushrooms. Underneath, however, boletes have tubes instead of gills and in this they resemble bracket fungi. Boletes differ from bracket fungi in that they are soft, fleshy and rot quickly. They are sometimes called sponge fungi. Bracket fungi, on the other hand, are leathery or woody and persist for long periods.

In boletes the spore mother cells (basidia) form a layer (hymenium) that lines the inside of the tubes. Tubes can be very shallow or more than a centimetre deep in some species. The mouths of the tubes appear as pores on the underside of the fruitbody. Pores vary in size and shape from species to species. In some species pores are circular, in others angular or maze-like. In some the pores are radially arranged and in others irregular. Pores can be more than a centimetre across or so tiny they are hard to see without a magnifying glass.

As a group, boletes are highly prized as edibles and many are listed as choice. Despite this, they are often overrated and have little flavour. Some, however, are truly delicious and amongst these is Boletus edulis, the king bolete. While most boletes are edible, some such as Boletus satanas are poisonous and others, such as Tylopilus felleus, are so bitter as to be inedible.

When collecting boletes you'll quickly notice that, more often than not, insects find the fruitbodies before you do and the mushrooms are often riddled with larvae and their tracks. Within moderation, however, this is not a problem for many collectors who happily consume them and consider a few larvae as an animal protein bonus.

Identification of Boletes

With a little experience a few of the common boletes can be recognized on sight. A number of characteristics are used to make identifications in this group. Shape, size and colour are always important. Also, chewing a tiny morsel establishes whether it is bitter or not. The flesh and/or pores of many boletes discolour on bruising and the shade and speed of staining is a useful diagnostic tool. Pore size, shape, and depth are important features and whether or not the pores are in rows or radially arranged can help. Is the cap smooth or rough, slimy or dry? Location is an additional aid. A number of boletes favour association with specific trees eg Suillus americanus is only found with white pine. It also helps to know the colour of the spores en masse. This information is obtained from a spore print. To make a spore print, cut the cap off a mature bolete and place it on a sheet of white paper. Cover with a cup or bowl and leave overnight. The following day the discharged spores will leave an imprint on the white paper! The colour will vary according to the thickness and freshness of the print. There are a number of insects called Mycetophilidae (fungus lovers), however, that specialize in stinging fruitbodies and laying their eggs in the flesh. So, when making a spore print from a bolete cap, you might end up next day with a writhing heap of white larvae with black heads. It is best to inspect the mushrooms for the presence of larvae before using for spore prints.

Comments on the Genera of Boletes

Boletes have had a checkered history. The various species have moved back and forward amongst different genera and each bolete has been known by several different names over the years. The species names themselves are undergoing considerable revisions. If you pick up a selection of three currently available books on mushrooms you might find the same fungus under three different scientific names. This suggests that there is a lot of uncertainty and difference of opinion about the classification system that should be applied to boletes. Hopefully, many of the questions will be answered by the new molecular techniques that are now being applied to the fungi. Until these matters are resolved, however, we will have to bear with the contradictions. Below, I have given some brief comments that might aid in distinguishing one bolete genus from another.

Strobilomyces: the single species treated here, S. strobilaceus, is recognized by its black, shaggy cap and stalk and grey pore surface.

Gyrodon: the single species, G. merulioides, recognized by its large, angular, radial pores, its off-centre stalk and its association with ash trees.  The backgrpound of this webpage is the pore surface of Gyrodon.

Gyroporus: Gyroporus cyanescens is the only species treated here. It has a yellow spore print, is off-white and stains brilliant blue on bruising.

Leccinum: species in this large genus are recognized by the dark-dotted stalks. The dots are mostly black or dark brown but are sometimes paler. Some species of Suillus e.g. S. granulatus have light-coloured dots on the stalks. Also, the tubes of Leccinum are never yellow as they can be in Sullius.

Tylopilus: the very common T. felleus has a pink spore print and the whitish pores turn pinkish in age. There is a network of ridges in the upper half of the stalk of this species and the flesh has a very bitter taste.

Chalciporus: the only species, C. piperatus, is recognized by its small size, peppery taste, cinnamon cap and bright yellow flesh.

Porphyrellus and Austroboletus are represented by a single species each. Both genera have red-brown spore prints. Porphyrellus bruises blue and Austroloboletus doesn’t stain. Some place both species in Porphyrellus.

Boletellus is reprented by two species with striate spores. Boletellus russelli is a very distinctive species with strongly fluted stalks that grows under oaks. Boletellus chrysenteroides is very similar to Boletus chrysenteron. It lacks the red pigment in the cracks in the cap.

Phylloporus is represented by a single species, P. rhodoxanthus, distinctive in having bright yellow gills rather than pores and therefore is more likely to be confused with a brown-spored gill fungus.

Boletus and Suillus: If the bolete you find is in none of the above genera then it may well be either a Boletus or a Suillus and the distinctions here are hazy for beginners. In general, if it has a ring and/or large, angular radial pores then it is a Suillus. If it is slimy then it is also likely to be a Suillus. Most Boletus are dry or, at best, slippery when wet and have small pores.