Perennnials are more lilkely to be disease-free than annuals or vegetables because perennial beds usually contain a mixture of plants, rather than a single species of cultivar (a monoculture). Monocultures draw diseases; and the pests that often spread them; like a magnet. Good cultural practices, like adding plenty of compost and other organic matter to the soil, mulching, and cleaning up plant debris in fall, also reduces the likelihood of diseases getting a foothold in your garden.
Diseases of perennials are caused by fungal organisms. The likelihood of occurrence is increased by poor or inappropriate condition or extremes of weather.
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Gray Mold (Botrytis)
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Photo credit: Oregon State University
This problem is caused by a number of soil-living fungi, the presence of which can be detected only by the damage they do. It occurs most often when seedlings are grown in wet, compacted potting soil and when seed is sown too thickly. Seedlings collapse as the result of fungus attack, usually at the soil surface. Perennials that have been raised in a propagator or a warm greenhouse are the most likely to suffer. Always use clean pots or flats together with fresh, bagged seed-starting mix; never use garden soil. Do not over firm the potting mix; sow seeds thinly; water with tap water, not water form a water barrel. Handle seedlings gently when pricking out. Water the potting mix with a liquid copper fungicide after sowing and when seedlings have emerged.
GRAY MOLD (BOTRYTIS)
This widespread problem is less troublesome in the perennial border than it is in a greenhouse. The fungus is especially virulent in cool, damp conditions, but some plants may suffer even at the height of summer if conditions are damp and humid. Especially dangerous to fully double flowers with soft or succulent petals. Dahlias and chrysanthemums are particularly susceptible. Sometimes the infection is confined to spotting, but it may attack old flowers as the petals collapse, then spread through the flower stem. Regular deadheading of susceptible plants is crucial. Remove flowers that have been damaged by either bad weather or frost. Remove diseased parts, and spray the plant with a systemic fungicide. This is caused by a number of different fungi and bacteria, each of which attacks only one type of plant. It is most troublesome in warm, wet weather. Dark blotches or spots disfigure the foliage of a wide range of perennials, including pinks, delphiniums, irises, and hellebores. In some cases there is little serious damage, but severe infections can be fatal. Picking off the infected foliage restricts the spread of the disease; spraying with a systemic fungicide is sometimes effective.
The dusty white coating of powdery mildew is especially troublesome in hot, humid weather and in locations with poor air circulation. Pulmonarias, aquilegias, asters, delphiniums, monardas, and phlox are among the plants affected.
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Damage: Fungus attacks the foliage, buds, flowers, and stems, covering them with a disfiguring white coating. The leaves may then turn yellow, and the whole plant may even die. Divide susceptible plants frequently to keep the clumps small, so that the air circulate freely. Spray with a systemic fungicide.
Many perennials are subject to root rot caused by a variety of fungi. This condition is often the reason for otherwise unexplained collapse. Soil-borne fungi attack roses, causing reduced growth and leaf yellowing at first, then poor flowering, and sometimes, eventually, sudden collapse. Keep plants growing well in conditions they enjoy. Some forms of root-rotting can lie dormant in the soil for years, so never replace an effected plant with another of the same type. No chemical treatment is yet available to control this disease.
These microscopic organisms cause severe damage to some plants and are becoming increasingly common. Viruses can cause a wide range of symptoms, including stunted growth; distorted flowers, foliage, and stems; pale or yellow streaking spotting; and mottling in many different forms. Sterilize the pruning shears or knife when taking cuttings of susceptible plants, as the virus is spread in sap. Control aphids, which are the main carriers of viral diseases. Dig up and then dispose of the infected plants.