This document discusses variability in the fungus Rhizoctonia solani which infects many different host crops. It describes how R. solani exists as vegetative hyphae and sclerotia in soil and infects plants through soil or plant debris. Symptoms vary by crop but include damping off, root rot, stem canker and leaf blights. The document outlines 13 anastomosis groups of R. solani which vary in pathogenicity and host range. Studies examined cultural and physiological differences between isolates from soybeans as well as genetic variability of isolates from rice and potato crops. R. solani was found to have significant morphological, physiological and genetic variability depending on its host.

2. Variability in Rhizoctonia solani from
different host crops
Submitted By :
KAMAL SINGH PATEL
Roll No. 6375
Deptt. of Plant Pathology, College of Agriculture, Rewa
JAWAHARLAL NEHRU KRISHI VISHWA
VIDYALAYAJABALPUR (M.P.)
Credit Seminar
on

3. Introduction
 Rhizoctonia solani is the most widely known
and most studied species of genus
Rhizoctonia.
 R. solaniis soilborne Basidiomycete occurring
world-wide, with complex biology. Its highly
destructive lifestyle as a non-obligate
parasite causes necrosis and damping-off on
numerous host plant species. Because of the
lack of conidia and the scarcity of the sexual
spores, R. solani exists as vegetative hyphae
and sclerotia in nature.
 Sclerotia are an encapsulated, tight hyphal
clump that protects and preserves the fungus
over non-optimal times.

4.  The fungus is dispersed mainly via sclerotia,
contaminated plant material or soil spread by
wind, water or during agricultural practices
such as tillage and seed transportation. The
fungus can stay in the soil as a saprophyte for
long periods.
 The host range of R. solani is wide and it
causes various diseases on important crop
plants of the world (Table) including species
in the Solanaceae, Fabaceae, Poaceae etc.
 Disease symptoms include leaf blights, leaf
spots, damping-off, rots on roots, shoots
and fruits, canker lesions on sprouts and
stolons, and sclerotial diseases.

5. Rhizoctonia solani J.G. Kühn has a
significant economic impact in the
development and production of a wide
variety of crops. It is considered a
complex species because its
physiological and pathogenic variability.

6. Disease symptoms and
host range of different
anastomosis (AG) groups
of Rhizoctonia solani.

7. Table 1. Disease symptoms and host range of different anastomosis (AG) groups of Rhizoctonia
solani.Anastomosis
group
Symptomsa Host Reference
AG1
AG2
sheath blight
web blight
bud rot
root rot
stem canker
sheath blight
leaf blight
damping-off
Rice
common bean
soybean
sugarbeet
Potato
rice
sugarbeet
soybean
Sayler and
Yang 2007
Muyolo et al.
1993
Hwang et al.
1996
Herr 1996
Chand and
Logan 1983
Hashiba and
Kobayashi
1996
Herr 1996
Nelson et al.
1996

8. Anastomosis
group
Symptomsa Host Reference
AG4
AG5
stem canker
fruit rot
root rot
root rot
root rot
root rot
root rot
stem canker
root rot
root rot
potato
tomato
soybean
wheat
pea
common bean
cotton
potato
soybean
barley
Anguiz and
Martin 1989
Strahnov et al.
1985
Liu and Sinclair
1991
Rush et al. 1994
Hwang et al.
2007
Muyolo et al.
1993
Rothrock 1996
Bandy et al. 1984
Nelson et al.
1996
Rush et al. 1994

9. Anastomosis
group
Symptomsa Host Reference
AG6
AG7
AG8
AG9
AG10
AG11
AG12
AG13
Mycorrhizal
root canker
bare patch
minor
pathogen
minor
pathogen
not pathogenic
mycorrhizal
minor
pathogen
Cotton
cereals
potato
Lupin
cotton
Carling et al.
1999
Baird and
Carling 1997
Mazzola et al.
1996
Carling et al.
1994
MacNish et al.
1995
Eken and
Demirci 2004
Carling et al.
1999
Carling et al.
2002a

10. Culture and physiological variability in
Rhizoctonia solani
Rhizoctonia solani isolated from soybean growing in tarai
regions of Uttarakhand. Six isolates of R. solani has been
characterized on the basis of cultural and physiological
nature such as colony diameter, growth, colour and sclerotia
formation were recorded.
Potato Dextrose Agar (PDA) was found best for growth and
development. Two isolates covered the whole plates (90
mm) in 48 hrs. However, maximum number of sclerotia and
weight was recorded on Czapek Dox agar medium. Overall
radial growth supporting is recorded Corn Meal Agar
Medium.
Varied range of temperatures i.e. 10, 15, 25, 30, 35 and 400C
was tested and found better growth of different isolates of R.
solani at 10 - 400C, with an optimum growth temperature at
300C.

11. Isolates were grown on five broth media (Asthana &
Hawkers, Potato Dextrose Agar, Czapek’s Dox Agar,
Corn Meal Agar and Richards Agar) for fresh, dry
weight and oat meal broth culture filtrates of all
isolates was used in phytotoxic effects. It recorded
that maximum fresh and dry weight was observed on
corn meal agar medium.
The maximum reduction in radical and plumule
length of germinating seeds were recorded in
Haldichaur isolate.

12. Morphological characterization
 Isolates of R. solani were subcultured on
PDA poured Petri dishes in triplicate and
incubated at 25oC for 3-5 days. Mycelial and
sclerotial parameters were recorded for
each isolate. as virulent; 30-49%, as
moderately virulent and 10-29% as least
virulent.

13. Rhizoctonia Damping-Off and Stem Rot of Soybeans
 Rhizoctonia damage may occur at any time during the
growing season, but it is more severe on young
seedlings.
 Rhizoctonia solani can cause seed rot, root rot, and
lesions on hypocotyls. Damping-off occurs when
germinating seedlings are infected prior to or just after
emergence.
 On hypocotyls, lesions are reddish-brown and sunken.
Diseased seedlings collapse from the firm, dry canker
that girdles the hypocotyl.
 Diseased older plants become chlorotic, resembling
plants with nitrogen deficiency.

variability in Symptoms

14. Symptoms on older plants, or on those plants that
survive seedling infections, include the
characteristic sunken, reddish-brown cankers on
the lower stem near the soil surface.
Disease losses result from stand reduction in
newly planted fields and premature death of
diseased plants that produce undersized seed.
Rhizoctonia stem rot—yellowed,
stunted, infected plants in the field.
Premature yellowing caused by
Rhizoctonia solani, stem rot.

15.  Damage caused by Rhizoctonia is
frequently confused with diseases caused
by other seedling pathogens. It is very
difficult to identify the pathogen that
causes preemergence damping-off.
 The symptoms are very similar for
Pythium, Phytophthora, and Rhizoctonia.
 Rhizoctonia infected plants typically have
characteristic and distinct reddish-brown,
sunken cankers on the lower stem or
hypocotyl.
 Older plants with Phytophthora stem rot
have chocolate brown lesions that extend
up the stem several nodes on older plants.
Characteristic reddish-brown
sunken cankers on the hypocotyl
caused by Rhizoctonia solani.

16.  The most common symptom is wilting of the leaves, starting with the
oldest leaves, and is likely a consequence of damage to the vascular
system (Figures 1 and 3).
 Leaves may or may not become yellow. In some instances, the petioles
may blacken where they are attached to the crown (Figure 2).
 Crown rot infection probably is initiated when infected soil is thrown into
crowns during cultivation, by rain splash of infected soil into the crowns or
where the petiole attachment to the crown is covered with infected soil
(Figure 2).
Rhizoctonia Root and Crown Rot of
Sugarbeet
Figure 1. Wilting of oldest leaves fi rst and
yellowing of plants infected with R. solani.
Figure 2. Blackening of petioles and infection at
the soil level.

17.  Root rot infection typically starts at or just below the soil line
(Figure 3), and sometimes lower on the tap root (Figure 4).
 Root symptoms may range from scattered brown to
black lesions on the root surface to complete rotting of the root.
Initially, the disease may occur in a few patches in a fi eld. If signifi
cant inoculum is present, often as a result of continuous planting
of susceptible hosts, entire sugarbeet fi elds may be lost to the
disease when conditions are favorable for disease development.
Figure 3. Rhizoctonia root rot-infected plant
with yellow leaves showing infection
starting on the root just below the soil line.
Figure 4. Rhizoctonia solani-infected
plant with rot symptoms on the lower
part of the tap root.

18.  Genetic variability of R. solani on potato
Rhizoctonia disease symptoms on potato by
isolating field strains from important potato
growing areas. The majority of the samples
were taken from areas practicing intensive
potato cultivation and production of high
quality seed tubers.
In Rhizoctonia samples associated
with potato were isolated, of which 52.7%
originated from canker lesions on stems and
roots . Almost as many isolates were
assembled from sclerotia (42.7%) either from
tubers or from roots .

19. Genetic variability of Sheath blight of rice
Variability in 35 isolates of Rhizoctonia solani Kuhn causing
sheath blight of rice and weeds, was examined by
polymerase chain reaction-random amplified polymorphic
DNA (PCR - RAPD) analysis in conjunction with morphology
and pathogenicity studies.
 Studies on morphological characterization of R.
solani isolates showed that isolates were highly variable
both in mycelial and sclerotial parameters, with no
consistent characters related to virulence, origin, production
system or geographic origin. Pathogenicity of R. solani was
also evaluated and based on their virulence, isolates were
classified into different virulent groups.
 The similarity values of RAPD profiles ranged from
0.37 to 0.85 with an average of 0.61 among all the isolates.
The percentage polymorphism detected per primer varied
from 76.9 to 100% which suggested their utility in DNA
finger printing of the sheath blight isolates.

20. Conclusion
Rhizoctonia Solani is a multiphagous
pathogen and posses a great threat to
plants. It survives in soil through
formation of sclerotia which are hard
resting structures of the pathogen.
Sclerotia formation is not
formed in all diseases but present in
most of the crops. It can be effectively
manged using biological and chemical
control measures.