With their seemingly infinite variety of color and form, bedding plants are a staple of any garden. Bedding plants can add a splash of color to dull landscapes, bland patios, or uninviting entrances. With the unpredictable weather we've had so far this year, it would be no surprise if we experience more 'weird' weather throughout this season. Along with this 'weird' weather we often see several common problems on bedding plant in the home garden including root rots, grey mold, and powdery mildew.
This group of diseases causes plants to have stunted growth, yellowing, weak flower color, dieback of stems, sudden wilting, or a combination of these. The most prevalent culprits, Pythium, Phytophthora, Fusarium, and Rhizoctonia, are encouraged by cool and wet soils. Root rots can be separated into two types. The first type, caused by Pythium and Phytophthora, produce a soft, black to brown rot of outer layer of the roots. The second, resulting from Fusarium and Rhizoctonia, produce a dry rot usually with a pink to red shade. However, proper diagnosis should be obtained since drought and low nitrogen can produce similar symptoms.
Management of these diseases is easier if we prevent rather than attempt to cure, especially since we won't be able to cure a plant infected with root rot. Healthy plants should be planted in sites with good soil drainage. Water management is the most important factor. Improve soil drainage by using raised beds, incorporating sand or organic matter to improve soil structure, and redirecting down spouts away from plantings. By providing proper nutrition and light requirements, we can minimize plant stress and lower the likelihood of root diseases. Excessive fertilization can predispose plants for root diseases.
Grey Mold (Botrytis cinerea) can affect all above ground parts. This pathogen has a wide host range and can survive in soil and plant debris for long periods. This disease is encouraged by overcast, cool, and humid conditions. The disease usually begins on older, fading leaves and flowers. Symptoms begin as soft, tan to brown dead areas that rapidly enlarge under moist conditions. If the humid conditions persist, dense masses of fuzzy spores are produced on infected tissue.
Grey mold can ordinarily be control by using cultural practices. For this pathogen, humidity is crucial for a successful infection. Proper plant spacing allows more air movement around the plants and lowering the humidity. Irrigation, when needed, should be done in the morning so that the foliage will dry more quickly. Sanitation of fading and dead leaves and flowers removes the foothold required to invade healthy tissue. If this disease a persistent problem, all infected and weakened tissue and plants should be removed before a chemical application is applied for control. Fungicides labeled for gray mold control include captan, mancozeb, chlorothalonil, Thiophanate-methyl and copper products. These fungicides are often sold in local garden centers by many different companies such as Ortho, Scott's, Ferti-lome, Bonide, Hi-yield and many others.
Powdery mildew is characterized by patches of white to grayish, dry, dusty growth usually on the upper sides of the leaves. Severe cases can cause leaf distortion and tissue death. Powdery mildew does not usually kill the plant but it can cause the plant to become unattractive. This disease, unlike many others, doesn't require leaves to be wet to infect, only high humidity and warm temperatures (68–86 °F). Although there are several types of powdery mildew pathogens, they all produce similar symptoms on plant parts. These pathogens are very host specific. For example, the powdery mildew on lilac cannot infect geraniums.
Disease-free, healthy plants, should be selected and planted where plant vigor can be maintained (well-drained soil, proper light, adequate spacing). Remove and destroy infected plant parts and all dead leaves that might harbor the fungus. This decreases the likelihood that the fungus can survive the winter in your garden. Fungicide applications may become necessary if the disease becomes severe. Products containing myclobutanil, propiconazole, sulfur, neem oil, potassium bicarbonate These chemicals can be found in products sold locally by companies such as Ortho, Safer, Scott's, Ferti-lome, Bonide, Hi-yield, and many others.
If you are in doubt as to what problem is plaguing your garden plants, it is always best to seek advice from your local county extension office or from a plant diagnostic laboratory. Your local county office location can be found at the University of Missouri extension website (http://extension.missouri.edu). Information about the MU Plant Diagnostic Clinic can be found at their website (http://plantclinic.missouri.edu).
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REVISED: December 5, 2011