Plant diseases occur in most gardens; many on a yearly basis, others sporadically. Why disease occurs often is dependent on the weather. In fact, there are three conditions that must exist for a disease to be present: a host, a pathogen and a favorable environment.
Obviously, you must have a susceptible host plant for disease symptoms to occur. The word susceptible used here does not necessarily mean a damaged or less thrifty plant, although that may increase susceptibility. Even seemingly healthy plants may be susceptible to diseases.
Red Tip Photinia is one example of a plant that even when given perfect care will become infected with a disease (Entomosporium Leaf Spot). Research and plant breeding is used to develop new plant cultivars that have resistance to diseases, for example, the "Profusion" series of Zinnias which are resistant to Powdery Mildew.
Plant pathogens (i.e., bacteria, fungi, virus) may be present year-round but only cause disease when environmental conditions favor infection and disease development. For example, the fungus that causes rust on tall fescue overwinters as spores on infected grasses. The disease does not infect the plant until a period of warm days and cool nights occur coupled with heavy dew or rain showers that keep the turf wet for several hours giving the spores a chance to germinate and infect the plant. A few days later you see the rust symptoms, orange to reddish brown pustules on the leaf surface.
Dogwood Anthracnose, caused by the fungus Discula, is common on flowering dogwood (Cornus florida) and appears as large, brown irregular shaped blotches on the leaves. These blotches are often located along the leaf midvein.
Twigs are also infected and sunken brown spots, which develop on the tissue, will cause twig dieback. Infected leaves usually drop early; many times, defoliation will be severe. Fungicide sprays of Cleary's 3336 or Mancozeb on the leaves and twigs from bud break through summer can help prevent this disease.
Peony, a showy garden perennial, is another plant that suffers from diseases encouraged by excessive rain. Botrytis Blight is caused by the fungus Botrytis which attacks stems, buds and leaves. Young stalks discolor at the base, wilt, and fall over. Botrytis is most common in cloudy, rainy weather. Wilt and shoot death continue with wet conditions. Other symptoms include large, irregular shaped leaves or brown flower buds covered in a mass of gray, fuzzy, fungal spores.
To reduce disease pressure for next year, cut back diseased peonies to the ground in the fall and dispose of the infected material. Sprays of copper sulfate next season, when the shoots are 6 inches tall, will also help protect the plant.
Diseases encouraged by rainy weather are not limited to ornamental plants. Vegetables, especially tomatoes, may suffer from diseases when plants stay wet. Septoria Leaf Spot, Early Blight, Bacterial Speck and Bacterial Spot are all influenced by rainy, wet weather. Septoria Leaf Spot overwinters on tomato plant debris left in the garden. Spores are spread from the debris to the plant when raindrops splash them onto the lower leaves. The disease then spreads during warm, wet weather causing circular spots with gray centers on leaves, lesions on stems, and yellowing or dieback of infected leaves and petioles.
Early Blight also overwinters on debris and is spread by splashing rain. Irregular dark brown areas, with target-like concentric rings, are the major symptoms of this fungus. These rings may appear on leaves, stem or the fruit. Removing last year's tomatoes or rotating planting sites is the best way to avoid these diseases. A chlorothalonil-based fungicide will also provide control.
Bacterial Speck and Bacterial Spot are introduced into gardens on infected plants. They persist in the garden on infected tomato plant debris and the bacteria are spread to healthy plants by wind-driven rain during thunderstorms or when gardeners are working in the garden when the foliage is wet. Both bacterial diseases develop during periods of warm, humid, wet weather. Symptoms appear as small, raised, water-soaked circular spots on the fruit often with a green halo. Garden rotation and removing plant debris will reduce sources of infection; copper-based fungicides will control these bacterial diseases.
Knowing that rain encourages certain diseases may make you more aware of potential problems and more observant of your turfgrass, trees, perennials and vegetables. Researching your most prized plants and their associated disease problems will also help you know where to look for diseases and what, if anything, you can do to control them.
Kelly R. Jackson is the Christian County Extension Agent for horticulture. He can be reached at 270-886-6328 or visit Christian County Horticulture online at www.christiancountyextension.com.