In high humidity, plants cannot make water evaporate (part of the transpiration process) or draw nutrients from the soil. This leaves plants susceptible to disease. It’s important to treat each disease with the correct fungicide at the correct dose that will control future problems.
"Red thread" disease is among the first lawn diseases of the growing season; however, it has become a year round problem. It is more prevalent during cool/warm, humid conditions but cool, wet weather and prolonged overcast conditions create the ideal environment for it to thrive. In addition to the pinkish-red color, lawns affected by red thread have grass blades that look ragged and seem to be dying back from the tip of the leaf. The affected grass will appear in irregularly shaped, straw-like patches that range in size from 2 inches to 3 feet in diameter.
"Summer patch" is a fungal disease that usually appears between June and September, during periods of high humidity when daytime temperatures are over 85 degrees and nighttime humidity is high. This root-destroying fungus is recognizable by the circular patches of dead grass it causes to appear on the lawns it infects.
"Dollar Spot" begins as clusters of small, round straw-colored spots that are about the size of silver dollars but grow into larger, irregular patches. This disease favors high humidity and temperatures ranging from 60 to 85 degrees F. If untreated, this disease can persist in the fall when temperatures begin to cool.
Roses are susceptible to "Blackspot" and "Rust" and appear as dark or rust spots on the leaves. These diseases are a product of warm, humid, rainy conditions and overhead watering. The roses should be treated with a fungicide and be sure your sprinklers are adjusted properly to avoid watering of the roses. Drip irrigation for roses is highly recommended to avoid problems.
"Powdery Mildew" is a white or gray powdery material that appears on the new buds and leaves of plants. Many commonly grown annual and perennial flowering plants, as well as ornamental grasses, can be infected. This disease attacks the plant from the top down. The mildew appears when warm days and high humidity are combined with cool nights. It is best to prune and remove infected plant material followed by the application of the proper fungicide. Good air circulation and plentiful sunlight is important to help prevent powdery mildew.