Damping off is caused by fungal pathogens that infect seedlings at soil level, girdling the stems and causing death. Infected seedling will develop tan-colored, soft tissue at the base of the stem. The plants fall over and usually die. Once established and actively growing, plants are seldom affected by damping off. Control measures include maintaining optimum soil moisture and planting into well-drained soils that are not overly wet. Maintaining air circulation is important. In extreme cases, a soil drench of a registered fungicide can be applied to the soil surface. However, by the time damage is observed it may be too late for effective control.
Leaf spots are caused by numerous fungal (occasionally bacterial) pathogens that penetrate and kill leaf tissue. Symptoms usually start and are worse on older leaves. These diseases are usually worse following periods of wet weather and high humidity.
Removal of all dead plant material at the end of the growing season helps prevent many leaf spot diseases the following year. In-season control usually requires use of a registered fungicide.
Powdery mildew is caused by fungal pathogens. The classic symptom is a whitish, powdery growth present on leaf surfaces. Heavy infections cause distortions on new growth. Infections are often worse during summers preceded by damp spring weather. Plants grown in shade are more prone to infection with powdery mildew
Prevention involves growing plants in a sunny location and making sure there is plenty of space and air movement around plants. Control of severe infections may require the use of a registered fungicide.
Root and Stem Rots
Root and stem rots are caused by fungi (occasionally bacteria) that live in the soil. Infected plants initially develop mild wilting symptoms that become progressively worse and may eventually cause death.
Soil pathogens are difficult to control. They can best be prevented by planting resistant varieties, avoiding overly wet soil conditions, and destroying infected plants.
White mold is caused by a fungus that overwinters in the soil. It infects plant stems that touch moist soil surfaces. Symptoms include a slimy, white mold that girdles and collapses the infected tissue. Leaves above the girdled stem wilt and die. In advanced stages, small gray structures that look like mouse droppings form inside a hollowed stem.
Prevention is the best strategy and involves staking stems off the ground, spacing plants to allow air movement around foliage, and irrigating infrequently to allow intermittent drying of the soil surface.
Not technically alive, viruses are small pieces of genetic material that disrupt plant function. Symptoms vary widely and usually include some combination of stunting, yellowing, mottling, or leaf and stem distortion.
Consistently effective control measures for viruses are rare. The best methods include using resistant varieties or preventing the infestation of organisms (usually insects) that transfer the viruses from one plant to another. Prevention also involves removing and destroying any infected plants.
For a more complete discussion of care for annual flowers, including selection, planting, general management, and pest control, see chapter 19 of the Idaho Master Gardener Handbook.
Additional information on insect and disease management in annuals is provided in this list of publications from the University of Georgia.
Diagnosis information and specific control measures for diseases in the landscape is available from the University of Kentucky.