Wet weather during the hot summer or cooler winter months can cause disease problems for our lawns and shrubs. Fungus spreads rapidly when you have free water on leaf surfaces for too many hours. The evening and early night showers of summer wet plants and lawns going into the night, and the cooler winter weather lessens evaporation, both of which allow plants and soil to remain wet longer. That means you should water less during the cooler winter months than you do during the hot spring and summer months, once a week as opposed to two or three times a week. I often say winter is a great time of year to save on those water budgets.
Brown patch is a common disease of turf grass affecting both St. Augustine and Bahia. In Southern Florida it is mainly a fall and winter disease, but can also occur in the summer months. 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.
The first symptom of the disease is yellowing or chlorosis of the blades. The affected blades dry and turn various shades of reddish brown to straw brown. The fungus is most active at 73 to 90 degrees Fahrenheit. Excessive nitrogen fertilizer can fuel this fungus. A well-balanced fertilizer applied at moderate rates is important in reducing severity of this disease.
Take-all-patch is a serious disease fueled by the summer heat. It affects areas damaged by lack of water, insect or disease problems earlier in the year. The summer rains fuel this fungus and it begins to spread. Special fungicides are required to bring this under control and often the best treatment is to replace the area with new sod. More on this hard to control disease later...
Two other common leaf diseases of turf grass 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, and applying a fungicide will help protect the surrounding healthy grass from infection.
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 will also help, as it 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 out, treat with a fungicide and when you resume watering, don't over do it.