Autumn is a time when day length shortens and daytime temperatures begin to decline. These environmental cues induce macrofungi (mushrooms and similar large species) to transition from repeating cycles of asexual reproduction, (through vegetative hyphae and asexual spores), to sexual reproduction. This produces spores capable of overwintering and surviving extreme cold temperatures and harsh conditions.
Similarly, many tree species in southern forests begin losing their leaves and producing fruits (mast) in the form of nuts that serve as reproductive seeds for the generation of new trees in the spring and provide food for wildlife. Some macrofungi use the energy and nutrients derived from fallen leaves and dead limbs to produce fruiting bodies that proliferate after fall rains. Like mast, these fall fruiting bodies provide food and energy sources for wildlife during autumn and sometimes well into winter. There are three major groups of macrofungi: decomposers, parasites and pathogens, and mycorrhizae.
The mycorrhizal fungi of fall
Two similar boletes, the golden-pored bolete (Boletus auriporus) and the red and yellow bolete (Boletus bicolor) form mycorrhizal (symbiotic) associations with shallow roots of oak trees (Quercus spp.). They commonly begin appearing in Mississippi Delta forests during late summer and into autumn. Both species have a bright yellow pore-like surface under their caps, much like polypore fungi.
- The edible golden-pored bolete has a broadly convex, pinkish cinnamon to dark reddish-brown cap, often smooth to wrinkled and with a slightly pungent odor. The brilliant yellow pore surface turns brick-red when bruised. The stalk is pale yellow at the apex but streaked with pinkish to reddish-brown coloration along the length and with abundant white mycelial growth at the base.
- The red and yellow bolete is distinguished by its dark red to purplish-red cap, developing deep cracks on the margins and fading to tan with age. The bright yellow pore surface turns dark blue when bruised. This edible species can be confused with the similarly-colored sensitive bolete (Boletus sensibilis), which is reportedly poisonous.
The smooth chanterelle (Cantharellus lateritius) is frequently seen on the ground under oak trees along woodland paths. It is considered a choice edible species of equal culinary value to the well-known golden chanterelle (Cantharellus cibarius). The cap is bright orange to yellow, smooth and initially funnel-shaped, becoming flattened with a central depression and inwardly rolled margin. The stalk is similarly pale yellow, long and tapering towards the base. The fruiting bodies tend to have a fragrant odor and darken to a near blackish color with age.
Several Russula species may be observed during autumn months. The beautiful flavid (yellow) Russula is aptly named for its bright yellow cap with bright orange, depressed center. The gills are white and thick on the reverse and often forked near the stalk. The stalk is pale yellow and tends to be thickest in the middle, tapering near the apex and base and often soft, spongy, or hollow inside. These edible mushrooms emerge from the soil in association with roots of mixed broadleaf tree species. The modest Russula (Russula modesta) occurs in similar habitats but is shorter and starts out with a white bloom, although the caps quickly turn a gray-green to olive-green.
The scarlet-red capped Russula (Russula pulchra) is also abundant in broadleaf forests containing oaks. This edible, mild-tasting species is distinguished by its beautiful intense red to pink cap, white thin and forked gills and thick tapered white stalk with a slight blush of red.
Parasites and pathogens
There are several major pathogenic macrofungi capable of causing devastating diseases and significant mortality to hardwood (broadleaf) tree species of Delta woodland forests and throughout the South. Some of these fungi may cause enough damage to dead heartwood and living sapwood to predispose trees to wind throw or trunk breakage during heavy windstorms.
The varnish conk fungus (Ganoderma lucidum) is among the most destructive species due to its ability to kill trees by causing white root-rot and butt-rot of infected trees. This fungus effectively attacks healthy trees of many Delta hardwood species, including wild and domestic pecans, oaks, maples, beech and others. The fruiting bodies (conks) form from decayed wood at the base of trees or from shallow, rotted roots close to the tree and are recognized by their dark red, smooth, and shiny lacquer-like caps with dark amber to white margins.
These distinctive semicircular or fan-shaped conks may persist well into the winter and often continue to form on the stumps and roots of dead trees long after the trunk has died or been removed. Despite its destructive nature, chemical extracts from contextual tissues of this species have been found to have medicinal properties useful in treating human ailments, including certain respiratory and cardiovascular diseases, as well as specific types of cancer. Other secondary metabolites from the fungus are capable of lowering blood pressure and increasing blood flow.
The buff-tan oak Inonotus or weeping polypore (Inonotus dryadeus) causes a similar white destructive root-rot and butt-rot on oak species and rarely on maples and elms. Usually the large, buff-colored fruiting bodies are thickly attached to woody substrates at the ground line or on roots some distance from the trunk. The caps are initially white, turning tan and eventually dark brown with age, frequently exuding amber yellow to dark brown liquid droplets. The fruiting bodies may overwinter and persist for several years, eventually turning nearly black and with a cracked surface. The fungus enters the host through wounds at the base of the tree. In oak trees of urban residential yards, mechanical wounding caused by lawn-care tools provide opportunities for entry into the lower trunk or surface roots.
The cauliflower mushroom (Sparassis spathulata) also occurs on the ground at the base of primarily oak trees, causing a brown root-rot and butt-rot, but occasionally it is found on well-decayed wood in older forests. The fruiting bodies consist of rounded clusters of short, flattened branches, resembling the shape of thin leaf lettuce, but with a fibrous texture and having multiple points of attachment to the ground. The flesh of young specimens is edible when cooked slowly. This species is sometimes confused with Sparassis crispa which is common on conifers and has smaller branches, more tender flesh, and a long tapering root-like stalk. The fungus is known to produce several antifungal metabolites that may protect trees from attack by other fungal parasites.
Decomposer fungi break down coarse woody debris and organic matter derived from dead and dying trees. Thus, they serve the important function of effectively recycling carbon and nutrients back into the soil. This facilitates the growth and regeneration of young forest trees in the understory. Decomposing woody materials also provide sustenance to many other organisms.
After white-rot decay, bear Lentinus (Lentinus ursina) forms white to buff-colored, convex and fan-shaped mushrooms on dead sapwood of broadleaf trees, especially oaks, maples, and beech. The fruiting bodies develop into a shelf-like, overlapping growth form similar to the palatable oyster mushroom (Pleurotus ostreatus), but the bear Lentinus differs by having a hairy cap with irregular margin, ragged light pink gills, and inedible bitter-tasting flesh. The hairy light brownish upper surface of the mushrooms resembles bear fur from which it gets its name. Secondary metabolites, consisting of sesquiterpenoid derivatives, have been isolated from this fungus which have potential biomedicinal applications as antibacterial agents with antibiotic properties.
The violet-toothed polypore (Trichaptum biforme) commonly forms on fallen oak woody debris and appear as thin gray to tan, hairy, sessile, fruiting bodies with hairy to smooth caps, having concentric growth zones, superficially resembling the turkey tail fungus (Trametes versicolor) but lighter and with less contrasting-colored zones. The underside has a pink to purple fertile pore surface with angular pores. The most intense color occurs in new growth at the margins. The fruiting bodies may form singly or in shelving clusters which usually coalesce laterally into larger structures.
A separate, smaller group of macrofungi, known as sac fungi, consist of several important fungal groups that form their sexual spores inside of a sack-like structure (ascus). Some common fungi in this group include the morels, truffles, and cup fungi. Some smaller groups of sac fungi are capable of decaying wood and are commonly found as decomposers of woody debris found on the forest floor in Delta woodlands. (Brewer’s or baker’s yeast fungi, used in the proofing and secondary rising (fermentation) stages for making leavened bread, also belong to this group, but these are microscopic forms that are invisible; and thus, not included among the macrofungi.)
The dead man’s finger fungus (Xylaria polymorpha) forms single or small groups of dark, club-like fruiting structures (stromata) on a cylindrical stalk. In clusters, the dark fruiting bodies resemble the appearance of human fingers that have been burnt or rotted to form charcoal-like masses from which the fungus is named. The internal tissues are usually white, just below the dark outermost superficial crust that contains the fertile, sexual spore-forming tissues. The fruiting bodies typically start forming in summer and have a brownish surface color, due to the production of asexual spores (conidia), until environmental cues and lower air temperatures induce the formation of the sexual spore-forming stages. This decomposer fungus can cause a white-rot type decay of dead woody debris, derived primarily from broadleaf trees such as oak, beech, and occasionally other hardwoods.
Editor’s note: This article is one of four in a seasonally themed series by Dan Wilson. Although written originally for mushrooms in the Mississippi Delta, most of these mushroom species are also found across the Southeast and beyond.
For more information, email Dan Wilson at firstname.lastname@example.org.