Title: Rhizoctonia diseases in aboveground plant parts of agronomic and vegetable crops
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00066827/00001
 Material Information
Title: Rhizoctonia diseases in aboveground plant parts of agronomic and vegetable crops
Series Title: Plant Pathology Fact Sheet PP-41
Physical Description: Book
Language: English
Creator: Kucharek, Tom
Affiliation: University of Florida -- Florida Cooperative Extension Service -- Department of Plant Pathology -- Institute of Food and Agricultural Sciences
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: 1990
Spatial Coverage: North America -- United States of America -- Florida
 Record Information
Bibliographic ID: UF00066827
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 51341099

Table of Contents
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Full Text

Plant Pathology Fact Sheet

Rhizoctonia Diseases in Aboveground Plant

Parts of Agronomic and Vegetable Crops
Tom Kucharek, Professor and Extension Plant Pathologist in the Plant Pathol-
ogy Department, University of Florida, Gainesville, FL 32611. May 1990; Copied
February 2000.

Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean


Rhizoctonia solani is a soilborne fungus
that is common in Florida. It is capable of exist-
ing and growing in the soil in the absence or
presence of plants. The host range ofR. solani is
extensive. Most crops are susceptible to the fun-
gus as a seedling blight, root rot, fruit rot or
aboveground aerial blight. R. solani commonly
causes seedling blights and root rots in plants
and is therefore often regarded as a below
ground pathogen only. That perception is in-
correct. This fungus consists of different
"groups" within the species which provide for
a diversity of pathogenic potential. In warm and
humid climates, such as in Florida, R. solani
causes many plant diseases in aboveground
plant parts in addition to those caused in
belowground plant parts. This publication pre-
sents information about diagnosis and control
of plant diseases caused by Rhizoctonia in plant
parts above the soil surface. Plant Pathology
Fact Sheet 1 discusses this same fungus in rela-
tion to seedling blights.

The primary habitat of Rhizoctonia is the
soil. When conditions are suitable, hyphae of
Rhizoctonia may penetrate plant tissues below
the soil surface. The fungus grows as small
threads hyphaee) in soil. It is abundant in and
on dead and living plant litter, which is on or
near the soil surface. Also, sclerotia (small pin

cushion-like masses of hyphae) may be formed
by the fungus and allow for long-term survival
in soil or water (Fig. 6). As decomposition of
plant litter occurs, beneficial fungi and bacte-
ria colonize such litter, and Rhizoctonia is re-

In some situations, the fungus may cause
diseases in plant parts above the ground.
Rhizoctonia is often characterized as a non-
spore-forming fungus, but under certain con-
ditions with some isolates of the fungus, ba-
sidiospores are ejected above the soil surface
or hyphae grow upward on the plant. When
either of these situations occur during warm
temperatures with high humidities, infection
structures are formed, foliar plant tissues are
penetrated, and disease occurs.


Rhizoctonia causes different types of
symptoms in plant parts above the soil surface.
These symptoms may be classified into four cat-
egories: leaf spots, blights, lower stem rots, and
fruit rots. These diseases are commonly
misidentified in the field because of the mis-
conception that Rhizoctonia is limited to the
infection of plant parts in the soil. Also, Rhizoc-
tonia-induced diseases on aboveground plant
parts sometimes appear similar to other dis-


a disease called soil rot (Figs. 15 and 16). Where
Rhizoctonia causes an aerial blight, the fruit
becomes greasy (Fig. 8) and seed is reduced in
size, rotted, or not produced.

Sometimes the fungus, Pythium sp.
causes fruit rots that initially appear similar to
rots caused by Rhizoctonia. As the disease
progresses, however, Pythium-induced dis-
eases often form large masses of pure white
hyphae (mycelia) (Fig. 17). Cucumber fruit may
be infected with both fungi.


Control of Rhizoctonia-induced diseases
in aboveground plant parts begins prior to the
planting of the crop. Avoid planting in low and
wet, or poorly drained land. The soil should
be plowed with a mold board plow to bury old
plant debris which reduces the amount of
Rhizoctonia near the soil surface where it sur-
vives best. Soil preparation should be com-
pleted 30 days prior to planting so that the ben-
eficial soil microflora have ample time to rot
old debris and become established as natural
biological controls against Rhizoctonia. Never
plant a crop into freshly tilled soil where green
plant matter has not decomposed. Diseases
caused by Rhizoctonia are commonly more se-
vere in a crop which follows legume crops such
as beans, southern peas, or soybeans. Therefore
crop rotation that excludes legumes should be

Other cultural controls include the
avoidance of deep seeding, deep transplanting,
dense planting schemes, excessive overhead ir-
rigation, and excessive fertilization (especially
nitrogen) that promotes rank foliar growth. Re-
member, aerial blights caused by Rhizoctonia
are more likely to be a problem where exces-
sive humidities exist during warm weather.
Crops should be planted when soil tempera-
tures are ideal for rapid seed germination or
plant growth. This reduces the amount of in-

oculum in young plants that can serve as a
source of disease later in the season. Purchase
healthy transplants or propagation stock from
a reputable transplant producer.

Fruit rots of vegetables caused by
Rhizoctonia and certain other fungi that origi-
nate in the soil can be reduced significantly by
trellising the crop or by using a full-bed mulch
(e.g., plastic) on which the fruit can form. Any
method that reduces direct contact of soil with
fruit would serve the same purpose.

Chemical controls include seed treat-
ment with an effective fungicide, soil fumiga-
tion with a broad spectrum liquid or gaseous
fumigant, and sometimes a fungicide spray.
Seed treatments reduce the amount of early sea-
son inoculum. Likewise, soil fumigation re-
duces the initial fungal population. Where soil
fumigation is used, it is important to avoid re-
contamination of fumigated soil by not mov-
ing soil from unfurnigated areas into the fumi-
gated area. Contamination of fumigated soil is
commonly done with tillage equipment, soil on
footware, and flowing water from excessive rain
or irrigation, Aerial blights and sometimes fruit
rots caused by Rhizoctonia can be reduced by
fungicide sprays. Contact your County Exten-
sion Office for current information.

Figure 1. Tobacco leaf with shotholes.

Leaf spots may be 2 inches or more in
diameter or less than 1/4 inch in diameter. All
spots begin from a single point of infection and
therefore the earliest sign of a leaf spot may be
no more than a pinpoint-sized dark spot (Fig.
1). After infection, the fungus grows on and in
the leaf and causes small spots (Fig. 1) or larger
spots that have concentric rings with alternate
light or dark areas (Figs. 2, 3, and 4). Coloration
of spots varies from a light brown to dark brown
(Figs. 1, 2, 3, and 4). In some situations, the cen-
ter of a spot will dry, become brittle, and sepa-
rate from the leaf causing shot holes (Fig. 1) or
large torn areas (Fig. 3). The target-shaped spots
in tobacco leaves (Figs. 2 and 3) are similar in
appearance to spots caused by Alternaria tenuis
(brown spot). Microscopic diagnoses are rec-
ommended for such situations to determine
which organism is causing the disease.

Blights caused by Rhizoctonia are clas-
sified differently from spots because they tend
to be larger in size and are not necessarily
round. Blights caused by Rhizoctonia are com-
monly called aerial blights. Diseased tissue is
irregular in shape. Blighted leaves have light
brown spots that are lobed (Figs. 5, 7, and 8).
Tissue that has been recently infected may be
greasy in appearance due to the breakdown of
cells in the tissue (Figs. 4, 5, and 6). Some of the
peanut leaflets in Fig. 4 have leaf spots and
blights. Early in the morning or shortly after a
wetting event, hyphae (strands) of the fungus
can be seen on or between tissue (Fig. 6). These
strands are sometimes confused with the web-
bing produced by spider mites. Sometimes
aerial blights caused by Rhizoctonia are re-
ferred to as web blights.

Rots of plant parts just above the soil
surface may be an extension of disease just be-
low the soil surface (Figs. 9, 10 and 11). Rotted
areas begin as small reddish-brown orange dis-
colorations on the tissue and later the fungus
causes sunken or cratered rots (Figs. 9 and 10).
When such infections occur, plants may have
discolored leaves and upper plant parts due to

the reduction of upward translocation of nutri-
ents and water from the soil. Later plants may
fall over (lodging) or die. With crops such as
celery (Fig. 9) and onions (Fig. 10), the edible
portion of the plant becomes unusable. With
salad crops such as lettuce, endive, and spin-
ach, which naturally have leaves that touch the
soil, a rot occurs on the stem and lower leaves
(often called bottom rot). Such rots may not be
noticed until the plants wilt, die, or are har-
vested. Also, secondary soft rot bacteria often
follow such infections; this results in mushy tis-

Brace roots of field corn which support
the plants may be rotted which results in lodg-
ing and lower yields (Fig. 11). A disease called
limb rot occurs in peanut vines that are in con-
tact with the soil. Lesions of limb rot may be
two inches or more in length, light brown in
color, and may contain concentric rings. Pegs
of peanuts may also have lesions caused by
Rhizoctonia. These lesions vary in color, and
may be infected with other fungi also. Labora-
tory tests are essential for such diagnoses. Rots
of plant parts at or just above the soil surface
sometimes originate during the seedling stage
or on transplants.

Fruit rots caused by Rhizoctonia occur
either because the fruit is in direct contact with
the soil or is part of an aerial blight infection.
Symptoms in fruits are variable when infected
with Rhizoctonia, but they usually occur on the
side of the fruit that contacts the soil. In cucum-
bers, lesions begin as slightly raised blister-like
(edema) spots that are dry or greasy. Later, these
spots enlarge, become reddish-brown-orange
and sunken (Fig. 12). This disease is called belly
rot and has caused culling or more than 50% of
the fruit. A similar disease occurs in winter
squash (Fig. 13) and pumpkin (Fig. 14) fruit
where the fruit is in contact with the soil. With
winter squash, the infected area often has raised
bumps (edema) or netting (Figs. 13 and 14).
When fleshy fruits, such as tomatoes or beans,
are in contact with the soil, Rhizoctonia causes


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Figure 2. Tobacco leaves with round brown

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Figure 4. Peanut leaflet with spots and

Figure 3. Tobacco leaves with large spots and
blight-type lesions.

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Figure 5. Peanut leaves with aerial blight.





Figure 6. Aerial blight in peanuts.

Figure 7. Southern pea leaflets with aerial

Figure 8. Aerial blight in soybeans.

Figure 9. Stalk rot in celery.

Figure 10. Bulb and leaf rot in onions.


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Figure 11. Brace root rot in field corn.

Figure 13. Belly rot in spaghetti squash.

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Figure 12. Belly rot in cucumber.

Figure 14. Belly rot in pumpkin.





Figure 16. Soil rot in beans.

Figure 17. Cottony leak in cucumber caused
by Pythium spp.

Figure 15. Soil rot in tomato.

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