This time of year, as the weather starts to cool and we get a little more moisture in the air, we start to see some fungi in the garden. These might be mushrooms growing in the lawn or at the base of a tree, or conks growing out from the trunk. Other smaller fungi can produce white, black or orange spots, fuzzy material or residue on leaves. Still others make strange forms we might not recognize as fungi, such as puff balls, earth stars, galls or masses on branches or soil.

Some of these peculiar protuberances are caused by the closely related slime molds, such as the so-called “dog vomit” slime mold seen in the spring. This mold produces a bright yellow, frothy-looking growth on soil, often near the base of trees or other plants.

Fungi play an important role in the environment. They help decompose plant material that cannot easily break down otherwise.

But they can also cause plant diseases and problems in the garden. Fungi generally need moisture for reproducing, which they do by means of spores. Generally spread by wind or water, spores are a little like tiny seeds. The powdery substance you feel on the underside of a rose leaf infected with rust is the rust spores. You can sometimes see spores on the underside of a mushroom, or in the smoky substance from a puffball.

Althought some fungi are edible, others are deadly poisonous and some are major allergens.

In the fall, powdery mildew often develops on the leaves of vegetables and ornamentals. It may look like white spots or a dusty coating on the leaves. Leaves infected with powdery mildew will eventually turn yellow, die and fall off.

Powdery mildew prefers cool, moist places. It does not do well in full sun and heat, so it appears first on plants in shady areas.

To avoid powdery mildew, plant in sunny locations and provide good air circulation. Keep plants pruned properly to encourage air circulation and don’t over fertilize. Too much fertilizer promotes fast, weak, leafy growth that will need more water. Excess nitrogen in the soil — what the plants can’t use — washed into our waterways and promotes algae growth.

For many vegetable crops and deciduous plants, there’s no good reason to treat for powdery mildew since the plants will soon be removed or go dormant. Most treatments can prevent the fungus, but they don’t cure it. This means you need to treat before you see symptoms.

Treatments need to thoroughly cover all susceptible plant parts. Mild to moderate symptoms can be treated with horticultural oils or plant-based oils. Follow package directions carefully. These oils can damage plants if used within two weeks of a sulfur application, or when temperatures are high or if plants are drought stressed.

University of  California Cooperative Extension has three free “Pest Note” publications related to controlling powdery mildew. You can read these publications online or pick them up in the Napa County Master Gardener office (address below).

Mushroom are generally beneficial in the garden because they break down organic matter. They grow on buried wood, dead roots and other plant debris. Removing the mushrooms will not remove the fungi from your yard because most of the fungus is actually underground or in the plant tissue. The mushroom is only the fruiting body.

It’s like harvesting an apple from a tree. You remove the seeds but the tree lives on. For fungi, spores are the equivalent of seeds. They can travel great distances on the wind.

Because some mushrooms are toxic, you may want to remove them to keep them from being eaten by small children or pets. If you suspect mushroom poisoning, contact California Poison Control at 800-222-1222. If possible, keep a piece of the suspect mushroom or take a photo of it so you can accurately describe it to poison-control or medical personnel. If you suspect your pet has eaten a toxic mushroom, contact your veterinarian.

University of California Master Gardeners do not identify mushrooms, nor can we determine if they are edible or not. But we do have Pest Notes available on mushroom control. 

To learn more about the biology of mushrooms and other fungi,  consult the Web sites of the Mycological Society of San Francisco (http://www.mssf.org), the Bay Area Mycological Society (  http://www.bayareamushrooms.org) or the Sonoma County Mycological Association (http://www.somamushrooms.org).

Napa County Master Gardeners (http://cenapa.ucdavis.edu) answer questions Monday, Wednesday and Friday, 9 a.m. to noon, at the UC Cooperative Extension office, 1710 Soscol Ave., Suite 4, Napa, 253-4221 or 877-279-3065.