Plant Health Problems
See Perennials for a detailed discussion of problems that may occur and are common to most herbaceous ornamentals.
Diseases caused by Fungi:
Basal or root rot, Rhizoctonia solani, Fusarium spp. or Pythium spp.
Roots and basal stems are rotted, black or brown and collapsed. Sunken lesions may occur at the soil line. These fungi must be distinguished microscopically. However, Pythium tends to predominate in wet soils and Rhizoctonia in well-drained soils.
Control with fungicides is not reliably effective and may be prohibitively expensive, so removal of infected plants is important.
Downy mildew, Bremia.
This pathogen typically causes leaf spots with downy white or gray patches under the leaves. The downy growth results from the production of spores called sporangia which are wind-dispersed between plants. Disease is usually favored by cool wet weather.
Control may include cultural means of reducing humidity and leaf wetness. 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 is mancozeb. Consult the label for dosage rates, safety precautions, and directions for use.
Gray mold, flower and leaf blight, Botrytis cinerea.
This fungus occurs everywhere and commonly infects senescing or damaged plant parts such as old flowers, causing a fuzzy gray mold which is easily blown around. From these tissues it moves into healthy stems and leaves, causing a damaging blight. Disease is favored by cool wet conditions and the presence of overripe fruit or old flower petals.
Sanitation is the most important means of control. Remove dead flowers before gray mold develops. If disease has moved into leaf or stem parts, 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 is thiophanate-methyl. Consult the label for dosage rates, safety precautions, and directions for use.
Wilt, Verticillium albo-atrum.
This trouble usually appears when plants are about to flower. Wilting and yellowing of lower leaves progresses up the plant. Black or brown discoloration of the vascular system is evident in the stem and roots. Early infection results in stunting.
Soil sterilization, including pots or flats, prevents this trouble.
Stem rot, Sclerotium rolfsii.
Symptoms include yellowing of lower leaves followed by wilting and death of the rest of the plant. A white cottony mass of mycelium growing around the crown or on the soil near the crown distinguishes this crown rot from others. In this fungus web may be found the whitish to cream color or buff or reddish-brown seed-like sclerotia the size of a pinhead.
Control may include removing and destroying all infected plant parts and removal of top soil around the plant. New soil may replace sclerotia-filled soil.
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.
Aster leafhopper, Macrosteles quadrilineatus.
This insect feeds by sucking the juices from the plants and is responsible for the transmission of aster yellows from diseased to healthy plants. In the spring, the insects feed on diseased wild plants and then carry the virus to cultivated asters, marigolds, calendula, chrysanthemums, cosmos, dahlia and gaillardia. The adults are about one eighth of an inch long and greenish gray in color. Control of aster yellows is difficult because ornamental plants are continually reinfested by leafhoppers who have fed on diseased wild plants. Apply carbaryl, which is among the compounds registered for control of this pest in Connecticut, to manage leafhoppers. Imidacloprid applied as a systemic to be taken up by the roots will also provide season-long control. Consult the label for dosage rates and safety precautions. Whenever possible, keep young plants in a fine-screened cage. Discarding diseased plants throughout the season will also help.