Reports on Plant Diseases
RPD No. 902
- Rhizoctonia Disease of Cabbage and Related Crops
] [ Disease Cycle ] [ Control
The fungus Rhizoctonia solani
(Thanetophorus cucumeris synonym Pellicularia
filamentosa) causes damping-off and wirestem of cabbage,
cauliflower, and other crucifer seedlings in the seedbed; bottom rot and
head rot of older plants in the field and storage; and root rot of turnip,
radish, rutabaga, and horseradish. Losses occur as reduced stands and
lowered yield and quality.
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Damping-off. Seeds may decay,
especially in cold, wet soils. Stems of young seedlings become light
brown and water-soaked near the soil line. Affected seedlings quickly
wilt, topple over, and die. Damped-off plants commonly occur in circular
patches in plant beds or along sections of rows. A number of other
common soil-borne fungi produce the same symptoms. Damping-off develops
most rapidly at temperatures of 75°F (23°C) and above in wet
soils following heavy rains or over-irrigation.
Wirestem. Wirestem is the most
common and destructive phase of Rhizoctonia disease. The stem above
and below the soil line shrivels and darkens, becoming tough and woody
or wiry. Generally, infected plants do not fall over but remain erect.
In wet soil, the decayed outer tissues of the stem may slough off
when the plant is pulled (Figure 1). Growth of diseased seedlings
aftr transplanting to the field or garden is usually slow. Severely
diseased plants soon die. Even if the plants are able to produce new
roots above the affected stem after transplanting, they are generally
stunted, unthrifty, and they produce small heads.
Bottom rot. This midseason disease
is often a carryover from wirestem or from new infections that occur
when the outer leaves touch damp, infested soil. The lower leaves
droop, decay, and turn black, but remain attached. Some plants may
recover and produce heads. In cabbage, bottom rot usually develops
into head rot.
Head rot. A firm to slimy dark
decay of the bases of the outer leaves and heads of cabbage develops
between early head formation and maturity. The outer leaves of the
head wilt, become pale, and turn brown to black near the main stem
(Figure 2). As leaves are killed, they dry at the base and remain
held in place by the margin of the blade that folds over the top of
the head. Foliage leaves die and drop off, exposing the stem beneath
the head. A cobwebby brown mold (mycelium) of Rhizoctonia solani may
be conspicuous on decayed tissues and between the head leaves in damp
weather. Small brown sclerotia may develop and be visible on parts
or the whole surface of the head. Dark brown sunken spots are produced
inside the head. A firm, persistent, dark decay continues to develop
in transit and storage. Secondary rot-producing bacteria commonly
invade the diseased tissue and rapidly turn an infected head into
a slimy, foul-smelling mass.
Root rot. Turnip, radish, rutabaga
and horseradish roots may decay in the field and in storage. The rot
is usually dark brown, slightly sunken, and semiwatery to spongy.
Horseradish root tissue is light yellow to grayish tan and usually
rather dry. Infected tissue separates easily from the advancing edge
of the rot (Figure 3). A cobwebby, creamy-white to brown surface mold
and irregular, chocolate-brown sclerotia help to distinguish Rhizoctonia
from other root rots. The fungus may enter through leaf scars, injuries,
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Wirestem of cabbage.
Rhizoctonia head rot of cabbage.
3. Horseradish roots infected by Rhizoctonia.
The Rhizoctonia fungus
is widely distributed throughout the world in practically all soils suitable
for plant growth. No soils in Illinois are known to be free of the fungus.
It attacks hundreds of different kinds of plants and produces a wide range
of disease symptoms on the same host plant depending on the time of infection.
The most important vegetable hosts, besides cabbage and related plants
(crucifers), include beet, beans, carrot, celery, cress, cucumber, eggplant,
lettuce, onion, pea, pepper, rhubarb, spinach, tomato, and sweet potato.
The fungus can be subdivided into strains based on the
isolates host or tissue preference and optimum temperature to cause disease.
For example, strains of Rhizoctonia that attack potato do not normally
attack crucifers, and vice versa. The strains that infect cereals differ
from those that cause disease in sugar beets, legumes, and most vegetables.
The strain of Rhizoctonia solani causing bottom rot and head rot of cabbage
grows at temperatures ranging between 48° and 91°F (8° to
32°C). Cabbage may become infected at temperatures ranging between
53° and 90°F (11° to 32°C) with an optimum between 77°
to 80°F (25° to 26°C). A strain exhibiting a host preference
for turnip causes disease when temperatures range between 53° and
90°F (1° to 32°C) with an optimum of 66° to 77°F (18°
to 25°C). Another subdivision of Rhizoctonia solani is based on the
ability of mycelia from different isolates to fuse or anastomose. Isolates
that demonstrate this compatibility form an anastomosis group. Isolates
within the same anastomosis group are more similar than isolates from
The fungus survives indefinitely in the soil, passing
through unfavorable conditions primarily as small, hard, chocolate-brown,
kernel-like bodies called sclerotia. The sclerotia are extremely resistant
to cold, heat, drought, and most chemicals. They germinate in damp weather
by forming delicate threads, or mycelium, that spread through the soil
for several inches and penetrate roots and leaves of susceptible plants
with which they come in contact. Sufficient moisture on the surface of
the host is required for penetration through natural openings, wounds,
or intact tissues to be successful. Once Rhizoctonia has penetrated the
plant, it proceeds to invade other tissues regardless of external moisture.
However, high moisture and humidity are conducive to accelerated disease
development in the field and in storage. After the fungus has depleted
nutrients in the host’s tissue, or as other environmental conditions
become unfavorable, the mycelium produces sclerotia to complete the life
The fungus is also capable of infecting the seed without
exhibiting symptoms. It remains in the seed in a quiescent state until
the seed begins to germinate; the fungus then attacks the seed. Crucifer
seed infected with Rhizoctonia is also a primary source of inoculum; mycelium
radiating from a colonized seed infects adjacent seeds and seedlings causing
seed decay and damping-off.
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When plants are grown in a hotbed, coldframe, or
flat, disinfest the soil before seeding. Use steam or a soil fumigant.
Carefully follow the manufacturer’s directions regarding timing
and method of application. A number of soil fumigants are restricted-use
chemicals and must be applied by a person licensed to purchase and
handle the chemicals. Avoid recontamination of steamed or fumigated
soil with nonsterile soil.
Grow plants in well-drained, fertile soil. Fertilize
based on a soil test and suggestions given in Illinois Extension Circular
1354, Illinois Homeowner’s Guide to Pest Management. Avoid overwatering,
deep planting, overcrowding, and overfertilization with nitrogen.
Alteration of the sowing date will reduce the risk of infection from
soilborne inoculum for crops where the seed requires high temperatures
Before seeding, treat all crucifer seed with hot
water, or plant seed already treated. In addition, treat the seed
with a registered seed-protectant fungicide.
In plant beds that have not been disinfested,
control can be achieved by thoroughly incorporating a fungicide dust
into the upper three inches of soil before planting or applying a
fungicide soil drench after planting. For information on the current
fungicide recommendations, refer to Circular 1373, Midwest Vegetable
Production Guide for Commercial Growers.
When transplanting, discard all seedlings with
discolored stems and roots.
Do not plant crucifers for at least three years
in fields where bottom rot, head rot, and root rot have occurred.
Cultivate the soil as soon as possible after heavy
rains to aerate it and thus make conditions less favorable for infection.
Avoid covering parts of leaves with soil when cultivating. Banking
soil around plants creates conditions favorable for development of
bottom rot and head rot.
Harvest crucifer root crops when the soil is comparatively
dry so that a minimum of soil will adhere to the branch roots.
Discard cabbage, turnips, rutabagas, radishes,
and horseradish that show severe Rhizoctonia infection. Healthy or
slightly affected produce may be stored for a month or slightly longer
without excessive loss if the temperature is held at 32° to 34°F
(0° to 1°C) and the humidity is kept as low as practical without
causing wilting or shriveling.
A reduction in the inoculum level of Rhizoctonia
has been reported following cropping with cereals and incorporating
the crop residue into the soil.
The development of resistant varieties of crucifers
to Rhizoctonia is slow.
For the latest information on recommended cultivars for
growing in the Midwest read Circular 1373, Midwest Vegetable Production
Guide for Commercial Growers, available from Information Technology and
Communication Services, University of Illinois, P345, 1917 S. Wright St.,
Champaign, IL 61820. Also consult current seed catalogs and trade publications.
Information concerning insecticides, weed control, varieties, and other recommendations
can be found in the Illinois Homeowners' Guide to Pest Management, available
at your nearest Extension office.
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