Garden fungi: harmful or beneficial?

With the arrival of drenching fall rainstorms, soils are replenished with moisture and mushrooms pop up with abandon, seemingly overnight. “Are they bad?” you ask. The answer: it depends.

There are more than 100,000 species of fungi worldwide. Approximately 10 percent of them cause plant diseases, while only one-tenth of a percent are harmful to animals. The vast majority of fungi are beneficial to the environment: they help decompose decaying plant matter into rich humus and nutrients, break down toxic chemicals into non-toxic ones, or act in ways that encourage plant growth.

The terms “fungi” and “mushroom” are often used interchangeably, but they’re not the same thing. Think of a fungus as a living iceberg with only the upper tip visible. The above ground protrusion we know as a mushroom is simply the visible reproductive “fruit” of the main, seldom-seen fungal structure called the mycelium, which is a tangled mass of microscopic, threadlike filaments called hyphae. The mycelium grows where the fungus’s food source is located: in the soil, in rotting wood or plant matter, or in a host plant. A mature mycelium produces fruiting bodies called sporangia, and a mushroom is simply a very large sporangium. Each sporangium contains vast numbers of miniscule spores (the fungal equivalent of seeds). Once the spores ripen they disperse, germinate, and sprout new hyphae. Most fungi require warm, moist conditions to grow.

Lawn fungus mushrooms (Photo by Jack Kelly Clark, UC IPM)











Most common garden mushrooms—“toadstool” types or puffballs—grow on or near the ground and are beneficial decomposing fungi. The same applies to the shelf-like mushrooms that grow on dead trees or wood. However, if fungal mats or mushrooms sprout from a living tree, this indicates a potentially severe problem, since rot is already present. The more insidious fungus Armillaria mellea actually attacks many trees and shrubs and produces clusters of golden brown mushrooms.

Puffball fungus with black spores (Photo by R. Michael Davis, UC IPM)











If you prefer to remove mushrooms from your lawn or garden because they’re unattractive or because they might be eaten by young children or pets, simply hand pick and dispose of them as they emerge. NEVER eat wild mushrooms unless you’re an expert in their identification. Some mushrooms are harmless, but many are poisonous and can cause symptoms ranging from temporary vomiting to permanent and fatal organ damage.

The beneficial mycorrhizal fungi deserve special attention. These fungi form symbiotic associations with the roots of many plant species; the word mycorrhiza literally means “root fungus” in Greek. Once these fungi colonize plant roots they spread their fine hyphae out into the soil, helping plants absorb water and nutrients far more efficiently than they could on their own. In exchange, plants “feed” the fungi with sugar compounds. Some plants are highly dependent on this partnership—especially oaks, conifers, and many native shrubs—and 90 to 95 percent of plant species are believed to benefit from fungal symbiosis.

Mycorrhizal fungi play an important role in helping plants survive drought conditions, but they can survive only in association with living plants, not in bare soil. To encourage a healthy population of these fungi, keep soil planted year-round, minimize soil-disturbing activities like rototilling, and avoid the use of pesticides and chemical fertilizers.

Some fungus species are pathogenic, and cause harmful plant and turf diseases such as anthracnose, powdery mildew, rust, root rot, leaf spot, and blights. Landscape conditions such as poor air circulation, excessive moisture, improper watering, and over-fertilizing encourage many of these fungal diseases. Proper cultural care practices are usually very effective in controlling the responsible fungi without the use of chemicals.

Most fungicides are preventive (not curative) and can only protect uninfected plant tissue; they must be used before infection occurs or when symptoms first appear. If it becomes necessary to use a fungicide treatment, research the affected plant and type of fungus carefully, choose the proper product, and apply it according to label instructions to avoid causing harm to yourself and the environment. Remember that pesticide labels are legally binding documents; it’s both dangerous and illegal to apply or dispose of pesticides in a manner inconsistent with the instructions.

For more information on fungi see the UC IPM Pest Note entitled, “Mushrooms and Other Nuisance Fungi in Lawns.”

For advice on gardening related questions, call the UC Master Gardener office at 209-953-6112, or use our website.




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