Unless you want to make a Russian faint, do not tell him or her that you have never been mushroom-picking. It is akin to telling an American that you have never seen a Disney movie, or have never eaten pizza. In a Russian's eyes, you simply aren't human until you have gone traipsing through the woods in search for various forms of fungi.
Russians are taught the types of edible mushrooms and where to find them from childhood, until it becomes second nature. You could bring a Russian into the woods and they would probably find the nearest chanterelle just by smell. Mushrooms are dried, salted, pickled, fried, and used for soup. They have been picked for centuries, and despite the fact that nowadays you could probably go to a market and buy some, most Russians with an access to a good forest and a woven basket or two will go mushroom picking at least once each year.
In the late summer or early fall, soon after a rain, a Russian family will set out to the nearest forest with baskets and shears or scissors. The most prized mushroom is the White Mushroom, known as the king of all mushrooms, followed by various other mushrooms from the boletus family, many of then named after what trees they grow under, their caps, and whether they will poison you or not when eaten raw (Ex: "Syroyezhka," or "Raw-edible").
If you look up the kinds of mushrooms Russians are likely to pick, you may notice that some mushroom guides will list a few of them as poisonous. So how is it, you may ask, that Russians continue to eat them without dying? Are they insane? Or perhaps they possess magical powers?
Well, you are right on both accounts! But mostly it is that their immune systems have strengthened over centuries of consuming poison, and where a German or an Englishman might die an untimely and painful death, a Russian might just have a bit of a head ache.
Of course, there are some mushrooms even most Russians can't eat. Some examples would include Poganka (Death cap) and Mukhomor (Fly Agaric). Death caps are easy to identify, because they look like their name -- all pale and sickly looking. Fly Agaric, on the other hand, while well known to cultures that engage in mushroom picking, is known to many others as "that cool-looking mushroom from Mario":
To be fair, Fly Agaric has been used in the past by shamanistic tribes to induce visions -- but this experience tended to include intense stomach cramps and drinking someone else's pee.
So in conclusion, no need to make anyone faint! If a Russian asks you whether you have been mushroom-picking, nod vigorously and invite them to go with you. If you do not know how to identify mushrooms, do not bring a guidebook! -- you will be scorned. Instead, just grab any mushrooms you see and shove them in your mouth. Any Russians around you will be touched by your enthusiasm.