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.
Leaf spots, Alternaria, Botrytis, Colletotrichum, or Phyllosticta 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 is thiophanate-methyl. Consult the label for dosage rates and safety precautions.
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 topsoil around the plant. New soil may replace sclerotia-filled soil.
Diseases caused by Nematodes:
Foliar nematodes, Aphelenchoides spp.
These plant-parasitic worms attack virtually all plant parts and may cause leaf lesions, yellowing, necrosis and leaf drop, bud malformation, plant stunting, stem swelling or galls, and secondary fungal infections of necrotic tissues. The nematodes live and move in water films.
Reducing leaf moisture and removal of infected tissues, debris, or plants is important. Plants may be heat-treated to kill nematodes, but time and temperature combinations vary considerably between plant and nematode species.
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.
Garden slugs, Limax maximus.
Garden slugs sometimes infest greenhouses as well as moist, shaded outdoor gardens. These molluscs feed mostly at night, eating notches in the margins and holes in the interior of tender leaves, leaving a slimy, iridescent trail wherever they crawl. During the day they hide under rubbish.
Slugs can be controlled by lightly cultivating the ground in the spring to destroy dormant slugs and their eggs. A band of diatomaceous earth put around hostas will control slugs by rupturing their epidermis. Placing a small board in the flowerbed for the slugs to hide under during the day makes it easy to destroy many at one time. A bowl of beer, sunk into the ground, with a roof to protect from sun and keep large animals out, will act as a bait and the slugs will drown. If necessary, chemicals such as metaldehyde, which is among the compounds registered for use against this pest in Connecticut, will control slugs. Consult the label for dosage rates and safety precautions.