Plant Health ProblemsSee Perennials for a detailed discussion of problems that may occur and are common to most herbaceous ornamentals.
Diseases caused by Fungi:
Crown rot, Sclerotium delphinii.
Plants wilt and die. The characteristic cobweb-like mycelium may be found at the base of the plant containing seed-like sclerotia.
Removal and destruction of infected plants and removal of infested soil is advisable.
Downy mildew, Peronospora myosotidis.
Pale spots appear on the upper sides of the leaves, with a downy corresponding growth on the lower sides.
Among the compounds registered for use in Connecticut is mancozeb. Consult the label for dosage rates and safety precautions.
Leaf spots, Cercospora or Phoma spp.
Leaf spots are very common, typically sharply delimited necrotic areas on plant leaves caused by a wide variety of pathogenic species. Leaf spots usually are favored by wet conditions and may become important if a large number of lesions are present or if they start to coalesce.
Under those conditions, control may also be achieved with the use of fungicides applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are thiophanate-methyl and sulfur. Consult the label for dosage rates and safety precautions.
Powdery mildew, Erysiphe or Oidium spp.
These fungi are obligate plant parasites which grow vegetatively on the plant leaf surface, sending haustoria, structures which absorb food from the host, into epidermal cells. The white mildew seen on the leaf is a combination of vegetative mycelium and spores borne in chains on upright conidiophores. Wind-dispersed mildew spores can germinate without free water under high humidity conditions, and disease is often severe when conditions are humid but dry. Small black over-wintering structures called perithecia are often found in powdery mildew affected areas.
Control may also be achieved with the use of fungicides applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are potassium bicarbonate, ultra fine oil, sulfur, triadimefon, or thiophanate-methyl fungicides. Consult the label for dosage rates, safety precautions, and directions for use.
Rust, Puccinia spp.
The term rust refers to both the disease and pathogen causing the disease. Symptoms of rust infection include rust-colored spores or gelatinous horns in powdery pustules on leaves or stems. Surrounding tissue is discolored and yellowed, and plants are often stunted.
Control of heteroecious rusts may be aided by removal of the alternate host, but for most perennials, control may also be achieved with the use of fungicides applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are sulfur and mancozeb. Consult the label for dosage rates, safety precautions, and directions for use.
Smut, Entyloma spp.
Smuts are diseases caused by fungi. While most smut diseases attack the reproductive parts of plants such as flowers, replacing them with a sooty black mass of fungal spores, smut diseases of perennials often result in light spots on leaves and are called white smuts.
This disease is not typically serious, and usually can be controlled by removal of infected plant debris.
Diseases caused by Phytoplasmas:
Aster yellows, phytoplasma.
The pathogen is a prokaryotic organism without cell walls. It infects the phloem of susceptible plants and causes a general yellowing and dwarfing symptom. The phytoplasma is spread by a leafhopper vector.
Infected plants should be removed and destroyed. Early season control of the leafhopper vector and removal of weed hosts may help prevent re-infection.
Diseases caused by Physiological/Environmental Factors:
Hot weather and drought can wilt and kill forget-me-nots. Consideration of environmental factors as well as laboratory analysis will help determine causal agent. Rhizoctonia solani can also cause rotting of the roots with consequent wilting due to lack of water in the plant.
The green peach aphid, Myzus persicae, feeds on foliage of forget-me-not and can be controlled with applications of malathion, imidacloprid or insecticidal soap, which are among the compounds registered for use against this pest in Connecticut. Consult the labels for dosage rates and safety precautions. Natural enemies, such as lacewings, ladybugs, and syrphid flies, usually provide adequate control.
Potato flea beetle, Epitrix cucumeris.
This is a small, black, jumping flea beetle that feeds upon the leaves of potato, tomato, tobacco, and many other plants, eating out small round holes from the underside, but leaving the upper epidermis. The remaining tissue soon dies, however, and falls away, leaving holes through the leaves that turn yellow and later turn brown and die. This beetle is not much more than 1/16" long. It lives through the winter under rubbish and in other sheltered places. The overwintering beetles lay eggs in the soil in June. The larvae feed upon tubers and roots of the host plants. An abnormal growth sometimes takes place around each injury. The larvae transform and beetles emerge early in July.
These beetles feed as long as the plants are green and temperatures are favorable. These insects then hibernate until the following spring. If the flea beetles are controlled in the spring when they first begin feeding, fewer beetles will be present in the fall and the following spring. See Flea Beetle fact sheet.