Gardening: Wet weather woes

Eileen Ward

Wet weather can cause many plants to develop disease. Many of these rain storms happen during the evening and overnight hours, leaving plants wet for many hours. Fungus spreads rapidly when you have free water on leaf surfaces for too many hours.

Green sprouts in the rain

Brown patch disease of turf grass is caused by the fungus Rhizoctonia solani Kuhn. It affects both St. Augustine and bahia. In Southern Florida, it's mainly a fall and winter disease, but can be a problem in summer, as well. Grass blades may be killed in a few hours by this fungus. It infects the blade nearest the soil and disrupts transport of water and nutrients to the upper portions of the blade which then die rapidly. This leaves the lawn with brown areas which weeds invade before regrowth can occur.

When you pull on infected blades, they separate very easily at the base from the rest of the plant. The bottom of the blade is rotted and brown. The grass at the outer edge of the diseased area is often wilted and darker in color. In shady, moist locations, an all-over pattern of damage, rather than circular, may occur.

Take-all patch, Gaeumannomyces graminis var.graninis, looks very similar to brown patch, but does not respond to the same treatment. This stubborn fungus will require specific fungicides including Prostar, Heritage, Compass, Armada or Headway.

Two other leaf diseases of turf grass to watch out for are grey leaf spot and rust. Both like warm, wet weather and will cause grass blades to die, leaving brown blades throughout an otherwise green lawn. Unlike brown patch disease, nitrogen fertilizer can help the lawn recover from these leaf diseases.

Apply fungicide to the affected area plus a one- to two-foot border. Two treatments are sometimes necessary for complete control.

Ornamental diseases caused by fungi include the following:

  • Root rots, which are most often caused by excessive soil moisture. The plants will be wilting, yellowing and declining.
  • Stem rots affect the stem near the soil line. The stem will darken, soften and rot. There may be a smell.
  • Leaf spots can be light to dark, small or large, round or with irregular margins. Most leaf spots do not kill the plant, but can defoliate them. Rust spores of fungi are normally found on the undersides of the leaves.
  • Wilts affect plants in the water-conducting tissues so the plant cannot receive water and will die. This disease may progress slowly or quickly.
  • Powdery mildew grows on the upper and lower sides of leaves. It is white and then turns grey. It is not always serious and is common in low light areas.

For disease control, choose healthy plants and treated seeds for planting. Remove and destroy all dead or diseased plants and plant parts from the garden. After pruning, clean your equipment with a weak bleach solution to avoid spreading disease from plant to plant with your tools.

Allow space between plants. Thinning thick foliage allows more air movement and quicker drying. Spraying and dusting with fungicides should be done before disease begins or spreads too far. Fungicides protect healthy tissue from disease rather than curing infected tissues.

Soil borne diseases can be controlled by treatment of the soil with sunlight or fumigants before planting. Two good fungicides for ornamentals are copper or a systemic fungicide. Copper should be used sparingly as it can accumulate and become toxic if used too frequently.

The best horticultural method for preventing disease is to water in the early morning rather than in the evening. This allows the plants to dry with the sun before the water can begin activating the fungal organisms and spreading the disease. This process begins to happen after four or five hours of continuous moisture on leaf surfaces.

Because diseases turn plant leaves and grass blades brown, many people mistakenly believe they need to water more to turn the plants or lawn green again. This action will only exacerbate the disease problem as you add more water to fuel the fungus. If you suspect disease, allow the area to dry (good luck with that right now!), treat with a fungicide and when you resume watering, don't over do it.