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 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, Phytophthora spp.
Downy mildews typically cause 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, 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, fungicides may be applied to the plants. Among the compounds registered for use in Connecticut is thiophanate-methyl. Consult the label for dosage rates and safety information.
Leaf spots and blights, Alternaria, Kabatiella, Colletotrichum or Cercospora 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.
Among the compounds registered for use in Connecticut are sulfur and thiophanate-methyl. Consult the label for dosage rates and safety information.
Stem rot, Sclerotium rolfsii or Sclerotinia sclerotiorum.
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 Nematodes:
Root-knot nematodes, Meloidogyne hapla.
The northern root-knot nematode, M. hapla, is a sedentary endoparasite, meaning that it infects host roots after hatching from eggs, stimulating the formation of a small gall containing specialized feeding cells, and feeds in the same location through several molts to produce several hundred offspring. Because most of its life cycle is inside roots, it may be spread to new locations with vegetative propagation material. This nematode is parthenogenetic, a single female can reproduce without males, resulting in a new generation every 28 days under ideal conditions. The galls produced on roots interrupt translocation and act as a nutrient sink. As a result, plants may be stunted, wilt easily, and show signs of nutrient deficiency. The nematode has a wide host range, but a number of ornamentals, including Rudbeckia, Aster, and others, have been shown to be resistant.
Growing resistant plants or rotating to small grains can greatly reduce or eliminate nematode populations in infested soil.
Flower thrips, Frankliniella tritici, F. occidentalis.
Hemerocallis multiflora, H. citrina, and H. thunbergi may be injured by these thrips, which feed on flower buds and branch tips. Entire clusters may die before blooming. The mouthparts of this insect shred plant tissue and then suck up the resulting plant sap. Corky scars several inches long result. If control is needed imidacloprid, applied as a soil drench in spring may provide some systemic suppression. Sprays of insecticidal soap or spinosad, which are among the compounds registered for use against these pests in Connecticut, will also suppress thrips. Consult the labels for dosage rates and safety precautions. Control is very difficult because the thrips feed in areas inaccessible to sprays, and flower thrips have developed resistance to nearly all of the insecticides available to homeowners.