Residents can help prevent the spread of plant disease such as Phytophthora, according to Igor Lacan, an urban horticulture adviser for U.C. Cooperative Extension San Mateo-San Francisco Counties.

"Basically, look in the nursery. Go to a reputable nursery and ask if they follow best-management-practices protocol. If you ask and the nursery person looks at you blankly, maybe you want to go somewhere else," he said.

Customers can also ask nurseries if the potting soil is pasteurized, which kills the harmful organisms. And look to see if the plant is in good shape, not wilting and not next to a plant that is wilting or drooping, he said.

Unfortunately, a Phytophthora infection won't cause symptoms in the early stages. When people bring home new plants, Lacan recommended putting them in quarantine from other plants for four to six weeks. Runoff from watering be contained so as not to contaminate other soil or plants, he said.

If plants show signs of wilting, develop oozing cankers or have rotten roots, the plants should be pulled and destroyed. They should not be composted, he said. If you think you have a Phytophthora-infected plant, the entire plant can be double bagged and taken to a local Cooperative Extension office for examination, he said.

Soil and pots should not be used again and can be bagged and disposed of in the trash. Phytophthora expert Ted Swiecki said that chemical treatment may help to suppress the activity of the microorganisms, but it will not make the pathogen go away.

"Infestations in urban sites pose a risk for spread if shoes or equipment get contaminated with inoculum in soil, especially when soils are wet, and subsequently transported elsewhere," he said. Unless transported along creeks, in urban areas the risk of transport downstream is not that high, he added.

Best practices for gardeners also include scrupulous cleaning of garden tools and equipment and even shoes. Spraying surfaces with 70 percent isopropyl alcohol can help disinfect tools and other surfaces, Lacan added.