August 2002

Diseases of Perennials By Steve Nameth

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

Corm, rhizome and bulb rots

Any time you place a fleshy structure such as a corm, bulbor 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., arevery effective rot organisms if given the opportunity. Planting propagationmaterial that has wounds (insect or mechanical) or freeze damage will allowthese destructive organisms to enter and produce rot. Careful attention must bepaid 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 rootdiseases 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 isnot well-drained, where the roots of the plants have been subjected tosaturated conditions for an extended period of time. Symptoms include wiltingand overall yellowing of the plant. Roots of affected plants will appear brownand mushy.

Disease-management strategies include planting in media thatis well-drained and not allowing roots to sit in water too long. In conditionsof 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, itis more active as a crown-rotting organism in perennials. The fungus is verycommon in most soils, attacking plants at the soil level and rotting them offat the crown. The fungus grows up the stem and forms a stem rot or canker (darksunken area). Symptoms are plant wilt and overall lack of vigor. Once wiltingis identified, the damage cannot be corrected.

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

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

Plants affected with cottony stem rot ç should bedestroyed. Susceptible plants should not be planted in soil infested with thesclerotia. Chemical controls can be used as a soil or media drench, but theywill not inactivate the sclerotia. Some of the more common perennialssusceptible to cottony stem rot disease are: ajuga, artemisia, aster, bleedingheart, 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 bysouthern blight. Southern blight is caused by the fungus Sclerotium rolfsii andcan be identified by the obvious growth of white fungal mycelium at the base ofthe stem, although the growth tends not to be fluffy. The southern blightorganism also produces resistant structures (sclerotia), but unlike cottonystem rot, the sclerotia are spherical, brown and about the size of a mustardseed.

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

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

Vascular wilts

Vascular wilt diseases occur when selected species of fungiinfect the vascular tissue (xylem and phloem) of the plant. The infectingpathogen 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. Itis caused by two species of the Verticillium fungus, V. albo-atrum and V.dahliae.

The most common and most obvious symptom associated withplants infected with Verticillium sp. is a wilt. Wilted plants may"recover" from the wilt in the evening or in times of abundantmoisture. In some cases, only one side of the affected plant will wilt. Plantsinfected with Verticillium sp. will show dark or discolored vascular tissuewhen cut open. This is a key diagnostic symptom associated with most allvascular wilt diseases.

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

Fusarium wilt. Fusariumwilt is caused by the fungus Fusarium oxysporium. Like Verticillium, thisfungus is very common in most soils. However, the perennial hosts that itinfects are less than those of Verticillium. Symptoms and methods of çcontrol associated with Fusarium wilt are similar to those of Verticilliumwilt. Common hosts for Fusarium wilt are China aster, chrysanthemum anddianthus.

Foliar diseases

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

Powdery mildew. Thisdisease rarely kills the host outright. However, severe infection can cause thehost to defoliate prematurely. Also, the cosmetic damage caused makes someplants unsalable. The most prominent symptom associated with powdery mildewinfection is the presence of white, fluffy spots on the leaves, flowers orstems of the affected plant. Under conditions of extreme infection, the wholeplant appears to be dusted with snow.

The most effective way to control or manage powdery mildewis to grow plants that are resistant to the disease and modify the environmentto inhibit disease development. Do not allow leaves to remain wet for extendedperiods of time. Chemicals can be used to control powdery mildew if used earlyand 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 stemrust of perennials are caused by a group of fungi that produce masses ofrust-colored spores as part of their life cycle. In most cases the spore massesare more prevalent on the underside of the leaf surface, but not always. Likepowdery mildew, rust very rarely kills the plant outright. It is relativelyeasy to diagnose in that the spore masses are abundant and obvious.

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

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

The best way to control Botrytis blight is to grow plants inan environment that is not conducive to disease development. Low relativehumidity and plenty of air movement around and across the plants goes a longway toward keeping disease to a minimum. Just about every type of herbaceousperennial is susceptible to Botrytis blight.

Viruses

Diseases of herbaceous perennials caused by plant virusesare not as common as diseases caused by fungi. However, they can be just assignificant depending on the virus and the host. There are many differentviruses that infect perennials, and most of them induce symptoms that aresimilar. Cucumber Mosaic Virus is probably the most common virus found inperennials but not necessarily the most destructive. Viruses such as ImpatiensNecrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV) can causesevere damage to the plants they infect. Symptoms associated with virusinfection include, but are not limited to: plant stunting, yellowing of entireplant, 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 willremain infected for life. Even if plants die back to the ground, the roots andcrown are still infected, and when the new growth appears in the spring, itwill also be infected. Most plant viruses are moved about by insects, primarilyaphids, thrips and whiteflies. Insect control is essential for keeping virusesto a minimum. Plants infected with a virus should be removed and destroyed.Never propagate from virus-infected plants.