Diseases of Perennials

Though it presents some unique challenges, disease control of perennials is very similar to that of herbaceous annuals. Number one is knowing what diseases are important and how to identify them. Before you plant anything, be sure that the material you are planting (seed, corm, rhizome, cutting, etc.) is free from diseases and insect pests. Purchasing propagation material from a reputable producer will help ensure high-quality starting material.

Corm, rhizome and bulb rots

Any time you place a fleshy structure such as a corm, bulb or rhizome in the soil, it can be subject to rot by a variety of pathogens. Common soil fungi, such as Fusarium sp., and bacteria, such as Erwinia sp., are very effective rot organisms if given the opportunity. Planting propagation material that has wounds (insect or mechanical) or freeze damage will allow these destructive organisms to enter and produce rot. Careful attention must be paid to the condition of the propagation material prior to planting. Also, always plant material in a well-drained medium.

Root diseases

Pythium or Phytophthora root rot are the most common root diseases of perennials. Caused by the fungi Pythium sp. or Phytophthora sp., they are most often seen in roots of plants growing in soil or media that is not well-drained, where the roots of the plants have been subjected to saturated conditions for an extended period of time. Symptoms include wilting and overall yellowing of the plant. Roots of affected plants will appear brown and mushy.

Disease-management strategies include planting in media that is well-drained and not allowing roots to sit in water too long. In conditions of high Pythium or Phytophthora, chemical treatments can be very effective.

Crown and stem diseases

Rhizoctonia crown and root rot. Though Rhizoctonia solanii can cause root rot, it is more active as a crown-rotting organism in perennials. The fungus is very common in most soils, attacking plants at the soil level and rotting them off at the crown. The fungus grows up the stem and forms a stem rot or canker (dark sunken area). Symptoms are plant wilt and overall lack of vigor. Once wilting is identified, the damage cannot be corrected.

Controls for Rhizoctonia consist of growing plants in media that is well-drained; growing plants in synthetic media that has not been supplemented with native soils; not allowing media temperatures to be too cool; and using chemical fungicides. Most perennials are susceptible to Rhizoctonia sp. if grown in a favorable environment for disease development.

Cottony Stem Rot. This disease is caused by the fungus Sclerotinia sclerotiorum, a soil-borne fungus that produces a resistant resting structure that can remain dormant in the soil for years. Under proper environmental conditions and in the presence of a susceptible host, the fungus can become active. The fungus attacks at the base or crown of the plant and moves rapidly up the stem. The fungal growth is very fluffy and white; in the latter stages of disease development, the fungus produces hard, black, resistant structures (sclerotia).

Plants affected with cottony stem rot Á should be destroyed. Susceptible plants should not be planted in soil infested with the sclerotia. Chemical controls can be used as a soil or media drench, but they will not inactivate the sclerotia. Some of the more common perennials susceptible to cottony stem rot disease are: ajuga, artemisia, aster, bleeding heart, columbine, shasta daisy, delphinium, dianthus, hyacinth, iris, liatris, lily, phlox, poppy, primrose, salvia, tulip and violet.

Southern blight. Like cottony stem rot, this disease has the potential to be very destructive. Many of the host plants affected by cottony stem rot are also affected by southern blight. Southern blight is caused by the fungus Sclerotium rolfsii and can be identified by the obvious growth of white fungal mycelium at the base of the stem, although the growth tends not to be fluffy. The southern blight organism also produces resistant structures (sclerotia), but unlike cottony stem rot, the sclerotia are spherical, brown and about the size of a mustard seed.

This disease is seen in periods of high temperature (80-90° F) and high soil moisture. The fungus attacks the host at the soil level and moves up the stem, rapidly decaying the tissue as it moves. Affected plants wilt, and in the latter stages, collapse. Dozens of sclerotia will be seen covering the affected plant stem.

Control strategies and the host list for this disease are very similar to those outlined for cottony stem rot.

Vascular wilts

Vascular wilt diseases occur when selected species of fungi infect the vascular tissue (xylem and phloem) of the plant. The infecting pathogen produces enzymes and toxins that break down the vascular tissue. Vascular wilts usually occur in more mature plants.

Verticillium wilt. This disease is one of the most common of the vascular wilts in perennials. It is caused by two species of the Verticillium fungus, V. albo-atrum and V. dahliae.

The most common and most obvious symptom associated with plants infected with Verticillium sp. is a wilt. Wilted plants may “recover” from the wilt in the evening or in times of abundant moisture. In some cases, only one side of the affected plant will wilt. Plants infected with Verticillium sp. will show dark or discolored vascular tissue when cut open. This is a key diagnostic symptom associated with most all vascular wilt diseases.

The best way to control Verticillium wilt is to not introduce it into your operation and be sure that the cuttings you purchase are wilt-free. Also, try not to blend native soil into your mix. Chemical treatments are not very effective once the disease becomes established. Common hosts for Verticillium wilt are aster, chrysanthemum, coreopsis, delphinium, peony, poppy and phlox.

Fusarium wilt. Fusarium wilt is caused by the fungus Fusarium oxysporium. Like Verticillium, this fungus is very common in most soils. However, the perennial hosts that it infects are less than those of Verticillium. Symptoms and methods of Á control associated with Fusarium wilt are similar to those of Verticillium wilt. Common hosts for Fusarium wilt are China aster, chrysanthemum and dianthus.

Foliar diseases

Foliar diseases are the most common diseases associated with herbaceous perennials. In most cases, these diseases are caused by fungi; however, bacteria can incite some significant foliar diseases.

Powdery mildew. This disease rarely kills the host outright. However, severe infection can cause the host to defoliate prematurely. Also, the cosmetic damage caused makes some plants unsalable. The most prominent symptom associated with powdery mildew infection is the presence of white, fluffy spots on the leaves, flowers or stems of the affected plant. Under conditions of extreme infection, the whole plant appears to be dusted with snow.

The most effective way to control or manage powdery mildew is to grow plants that are resistant to the disease and modify the environment to inhibit disease development. Do not allow leaves to remain wet for extended periods of time. Chemicals can be used to control powdery mildew if used early and with the proper application. Common hosts for powdery mildew include: phlox, aster, dahlia, delphinium, rudbeckia, lupine, dianthus, yarrow, columbine, chrysanthemum and coreopsis.

Rust. Leaf and stem rust of perennials are caused by a group of fungi that produce masses of rust-colored spores as part of their life cycle. In most cases the spore masses are more prevalent on the underside of the leaf surface, but not always. Like powdery mildew, rust very rarely kills the plant outright. It is relatively easy to diagnose in that the spore masses are abundant and obvious.

Keeping the relative humidity down and not allowing free moisture to stand on the leaf surface for extended periods will help discourage disease development. There are a few good fungicides labeled for the control of rust. Common hosts for rust include: aster, daylily, dianthus, chrysanthemum, hollyhock, pansy, phlox and iris.

Botrytis blight. This is one of the most common diseases of perennials, both in the production facility and in the landscape. Caused by the fungus Botrytis sp., Botrytis is sometimes referred to as gray mold. This fungus has a very wide host range. Disease severity can range from minimal to plant death. Initial stages show up as small, irregular brown spots on the leaves, flower petals, flower buds or stems. Under humid conditions, the spots may spread rapidly, and the infected tissue may look soft and be covered by the gray fluffy growth of the fungus. The disease is most prevalent during the cool, wet weather of early spring to early summer. If left unchecked, small plants can rot, and more established plants can succumb to leaf blight or stem canker.

The best way to control Botrytis blight is to grow plants in an environment that is not conducive to disease development. Low relative humidity and plenty of air movement around and across the plants goes a long way toward keeping disease to a minimum. Just about every type of herbaceous perennial is susceptible to Botrytis blight.

Viruses

Diseases of herbaceous perennials caused by plant viruses are not as common as diseases caused by fungi. However, they can be just as significant depending on the virus and the host. There are many different viruses that infect perennials, and most of them induce symptoms that are similar. Cucumber Mosaic Virus is probably the most common virus found in perennials but not necessarily the most destructive. Viruses such as Impatiens Necrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV) can cause severe damage to the plants they infect. Symptoms associated with virus infection include, but are not limited to: plant stunting, yellowing of entire plant, ring-like spots on the leaves, deformed leaves, blackening of stems, plant distortion and leaf mosaic.

There are no chemicals that can be applied to plants to “cure” them of virus infection. Once a plant is infected, it will remain infected for life. Even if plants die back to the ground, the roots and crown are still infected, and when the new growth appears in the spring, it will also be infected. Most plant viruses are moved about by insects, primarily aphids, thrips and whiteflies. Insect control is essential for keeping viruses to a minimum. Plants infected with a virus should be removed and destroyed. Never propagate from virus-infected plants.