Diseases of Perennials

August 9, 2002 - 11:34

Learn to identify and prevent bulb, root and crown and stem diseases, vascular wilts and viruses before they get a stranglehold on your perennial crops and profits.

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. style='font-weight:normal'> 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.

About The Author

Steve Nameth is professor and associate chair in the Department of Plant Pathology at The Ohio State University, Columbus, Ohio. He may be reached via phone at (614) 292-8038 or E-mail at nameth.2@osu.edu.

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