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Marin IJ Articles

Fungi: it’s important to know which are nuisances and which require action

  • Nanette Londeree
  • What do blue cheese, conks, penicillin, athlete’s foot and soy sauce have in common? Fungus! These represent both the good and the bad that come from this amazing group of living things.

    Neither plant nor animal, fungi inhabit a world mostly hidden from view, the mushroom-like things popping out of the ground often the only sign of their presence. Overall, they are immensely important for the well-being of our planet, playing the vital role of decomposer, breaking down dead plant and animal material. They are nature’s incredible recyclers.

    This time of year, mushrooms, in myriad colors, shapes and sizes, can decorate a landscape. Interesting forms pop up out of wood chips that line walkways, dot vegetable beds topped with rich compost, and speckle lawns in places where birds or other wildlife may have left their calling cards. You might observe a clump of honey-colored mushrooms at the base of a tree, rusty-colored spots on rose leaves, talcum-powder-like coatings on shrubs or reddish-brown lesions on blades of grass. All are indicators of the presence of a fungus; the challenge is to differentiate those that are simply a nuisance versus those that might warrant action.

    The majority of fungi are saprophytes — they absorb their food from dead organic matter. There’s a small percentage that are parasitic, deriving their food from other living organisms, and are the ones that generally cause disease in plants and animals. Yeasts are the single-celled form of fungi, while mushrooms and other fruiting bodies (such as conks, puffballs and stinkhorns) are the recognizable multicellular forms that house, then disseminate fungal spores (something like the flower of a plant dispersing its seed). Fruiting bodies range in size from microscopic to a foot across or more.

    A few examples of bad fungi include the common soil-borne Armillaria mellea. It can infect a wide range of trees and shrubs along with some herbaceous perennials such as begonias, dahlias and geraniums. Also known as oak root fungus or honey fungus, its clusters of golden-hued mushrooms are very obvious, usually growing on the wood of roots or the lower trunk of diseased trees.

    Plants infected with rust have orange, reddish or yellowish colored fruiting bodies and spore masses; this disease attacks ornamental and fruit trees, roses and other shrubs, flowers, vegetables and lawn grasses. Numerous species cause this disease and most weaken the plant over time.

    A huge number of fungi are responsible for the disease called powdery mildew; each species attacks a specific type of plant. Hosts range from towering trees like valley oaks, to landscape shrubs, perennial and annual flowers, many vegetable crops and lawns. Unlike other types of fungi that prefer moist environments, this disease can flourish in warm conditions and is prevalent during our dry summer conditions.

    So what’s a gardener to do when mushrooms appear? First and foremost — nothing. No matter how interesting or appetizing a mushroom may look, the Mycological Society of San Francisco website emphasizes that you “always seek out expert assistance until you are experienced and absolutely sure of an identification. Proper identification of mushrooms is critical for your safety and health!”

    Next, consider where the mushrooms are growing — if it’s on dead material (tree stumps, wood chips, compost), then likely the fungi are simply doing their job of decomposing organic material. If you observe symptoms on living plants, try to identify the culprit that’s causing them. The University California’s integrated pest management website, ipm.ucanr.edu, is loaded with photos and information to aid you in identifying potential problems, whether on your roses, in your lawn, on your trees or in your vegetable bed. Marin Master Gardeners can help, too, online, by phone, at the Novato help desk or at our booths at the Civic Center, Novato, Mill Valley and Point Reyes Station’s farmers markets.