© Benaki Phytopathological Institute
Eslami
et al.
2
ease incidence is high (Kolte, 1984; Le, 2004;
Nguyen, 2004).
Sclerotium rolfsii
overwinters as myceli-
um or sclerotia in infected plant tissues and
soil. Under favorable conditions, hyphae or
germinating sclerotia infect the plant and
subsequently colonize and invade the root
and stem tissue with typical silky white my-
celium (Brewster, 2001). Infected plants be-
come yellow and then wilt, the collar root
turns brown and rots. In groundnut,
S. rolf-
sii
also infects the pegs and pods, leading to
yield losses.
Sclerotium rolfsii
is difficult to control by
physical and cultural practices due to its
wide host range of over 500 plant species
(Aycock, 1966; Punja, 1985) and persistent
sclerotia (Lakpale, 2007; Punja, 1985). To suc-
cessfully implement management practices
(e.g., chemical and biological) to control
S.
rolfsii
, knowledge of the distribution and di-
versity especially in pathogenicity and viru-
lence of the pathogen is essential.
Branch and Brenneman (1999) evaluat-
ed the resistance of mass-selected popula-
tions derived from combinations of cross-
es among two resistant and two susceptible
peanut cultivars. Fery and Dukes sr. (2002)
determined the cowpea resistance to
S. rolf-
sii
. There was significant variability in cow-
pea germplasm for resistance to southern
blight. In another study (Flores-Moctezuma
et al
., 2006), two onion isolates of
S. rolfsii
were inoculated to 51 plant species and dis-
ease severity levels were determined. Sub-
sequently, 12 out of 51 plant species were
selected for the determination of pathogen-
ic reaction to 20 isolates of
S. rolfsii
from dif-
ferent regions. Onion isolates produced vari-
able levels of disease severity for half of the
plants tested. Five plant species were sus-
ceptible or highly susceptible to all isolates.
Eleven sugar beet genotypes were eval-
uated at National Agricultural Research Cen-
ter, Islamabad, Pakistan, during the year 2009
for their resistance against root rot caused
by
S. rolfsii
(Farooq
et al
., 2011). Inoculation
of eleven genotypes with
S. rolfsii
exhibited
resistant response only in SD-PAK-09/07 and
moderate resistance in SD-PAK-07/071.
The results of a recent study showed that
S. rolfsii
isolates originating from ground-
nut, tomato and taro were all pathogen-
ic on groundnut, but displayed substantial
diversity of various genetic and phenotyp-
ic traits, including mycelial compatibility,
growth rate, and sclerotial characteristics
(Le
et al
., 2012).
The aim of this study was to identify the
virulence of different
S. rolfsii
isolates on a
susceptible local peanut germplasm and
determine the resistance of twenty peanut
genotypes to the most virulent isolate and
also the relationship between virulence and
mycelial compatibility groups (MCGs).
Materials and Methods
Isolates virulence determination
Seventy eight isolates of
S. rolfsii
from ten
different hosts in Guilan province with known
MCGs (Mehri
et al
., 2013) were applied for in-
oculation of a local susceptible peanut germ-
plasm in greenhouse conditions (Table 1).
Barley seeds were boiled in distilled wa-
ter for twenty minutes and then 12 gr of
seeds were added to each 100 ml Erlenmey-
er flask and autoclaved twice at 121
o
C and 1.5
atmospheres for thirty minutes. For each iso-
late a 5 mm disk of growing fungus on PDA
medium was transferred to the Erlenmeyer
flask containing sterilized barley seeds and
the cultures were maintained in the growth
chamber (27±1
o
C) (Sennoi
et al
., 2010).
The applied soil (1:1:2 clay, compost, sand,
pH=6.7) was autoclaved at 121
o
C and 1.5 at-
mospheres for thirty minutes and added to
the pots with 500 gr soil capacity. Seeds of
a local susceptible peanut germplasm were
sterilized with Sodium Hypochlorite 1% so-
lution for three minutes and rinsed with
sterilized distilled water three times, then
soaked in sterilized distilled water. The pea-
nut seeds were placed in a moist chamber at
25±5
o
C for 72 h to germinate.
When the mycelium covered all the bar-
ley seeds and enough sclerotia were formed,
each pot was inoculated with thirty infect-
ed barley seeds and the seeds were covered
