Here’s the down-low on battling downy mildew

Downy mildew causes yellowing of older leaves so that it first looks like a nutritional problem.

The leaves then turn brown, new growth may be twisted, the undersides of leaves show white downy/cottony growth or purplish spots, then complete defoliation occurs. Cool, moist weather as well as overhead irrigation that keep leaves wet favors development of downy mildew.

These symptoms differ slightly from powdery mildew, which forms white spots on leaf surfaces; powdery mildew can occur in both humid and dry weather. Downy mildew occurs in wet conditions.

Downy mildew spores overwinter in fallen leaves, or can be windblown for miles. It mainly attacks grapes, cucurbits and impatiens in home gardens. Buy resistant varieties and monitor for symptoms, remove and destroy infected plants, cut back grapevines and remove infected clusters, and irrigate early in the day to keep leaves dry.

Spring-blooming snapdragons are especially susceptible to rust fungus problems. Yellow spots appear on the leaf surface, the plants seem stunted, and then ugly orange/rust-colored pustules form on the undersides of the leaves. Rust spores are spread by wind and splashing water; warm springtime temperatures (50 to 55 degrees at night, 70-75 degrees during the day) and humid rainy spring conditions that allow leaves to remain wet for six to eight hours daily are ideal for rust fungus proliferation.

When planting spring-blooming annuals, avoid overcrowding to provide for good air circulation. Monitor for signs of rust infection when weather conditions are ideal for its spread, and promptly pull out all infected plants. Clean up all debris and dispose of the debris in the green waste bin. Change out mulches that can harbor dormant fungal spores.

Fungicides containing chlorothanil can be sprayed to treat for both downy mildew and rust fungi. Fungicides must be sprayed on schedule, usually every five to 18 days in dry weather. In many cases, it’s easier and cheaper to change cultural practices and to pull out young, infected plants and replace them with resistant varieties or other species.

Anthracnose fungus infects sycamores, evergreen elms, oaks and ash trees here in the Central Valley. It too is spread by splashing rain or water in spring when symptoms develop on the first flush of new twig and leaf growth. Anthracnose does not spread in dry conditions. This season’s rains will bring more problems with anthracnose and other fungal problems than we’ve seen in many years.

Symptoms of anthracnose vary with the tree species. We most commonly see sycamore anthracnose, which causes leaf tissue to turn brown along the veins. Infected trees may drop their early crop(s) of leaves, but a new crop should re-sprout.

Anthracnose spores overwinter on fallen leaves and twigs. Prevention or reduction of fungal infection begins with thorough, regular cleanup underneath any trees that previously showed signs of anthracnose. Overhead watering should be eliminated.

Because anthracnose disease occurs only in wet conditions and because new leaves usually regrow, it seldom seriously threatens the health of infected trees here. If branch die-back is severe or if cankers appear on the bark, have a certified arborist diagnose the extent of the infection.

Spraying fungicides to control anthracnose in mature trees does not always provide complete coverage and it is also expensive since several applications may be needed. The University of California does not recommend root injections of fungicides. Cleanup of all debris and ground level irrigation are better controls.

This story was originally published March 2, 2017 at 12:46 AM with the headline "Here’s the down-low on battling downy mildew."