STUDIES ON STEM ROT DISEAS STEM ROT (Sclerotium oryzae Catt.) DISEASEE OF RICE
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Date
2024-05-16
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ACHARYA N G RANGA AGRICULTURAL UNIVERSITY
Abstract
Rice (Oryza sativa) is an important food crop and it is the staple diet of over
three billion people around the world, particularly in Asia. Rice diseases caused by
fungi are considered the main constraint in rice production and cause both qualitative
and quantitative losses. Stem rot disease caused by Sclerotium oryzae Catt. which was
considered as a minor disease earlier has now become one of the major limiting factors
in rice cultivation.
A roving survey on incidence of stem rot of rice in major growing regions of
Andhra Pradesh, Telangana and Tamil Nadu states of southern India was
conducted during Kharif- 2019. Disease incidence was varied among the locations.
The highest mean incidence (5.97%) was recorded in Andhra Pradesh with a range of
3.20 to 9.83, followed by Telangana (2.34) ranged from 1.02 to 3.97 and the lowest
mean per cent incidence (2.08%) was recorded in Tamil Nadu with a range of 0.00 to
3.51. Among all the areas surveyed the highest mean per cent incidence (12.92%)
was recorded in Karapa mandal with a range of 12.53 to 13.30 and the lowest
incidence (0.00%) was recorded in Devakottai block, Sivagangai district of Tamil
Nadu. Incidence was observed among all the prominent cultivars of particular region.
A total of 36 S. oryzae isolates were isolated from 60 diseased samples. Koch
postulated were proved on susceptible rice cultivar Prabhat (MTU3626). Among the
isolates, So15 has produced the highest mean per cent disease index (PDI) (39.51) with
maximum AUDPC (489.52 units2
) and an infection rate of 0.065 units day-1. There was
no correlation between the virulence nature of S. oryzae isolates with their geographical
regions from where they have been collected. Based on the above observation So15 was
considered as virulent isolate and used for the subsequent studies.
The variability in cultural characteristics among thirty-six isolates of S. oryzae was
carried out. There was difference in cultural characteristics such as radial growth of the
colony, margin, colour, zonations and sclerotial characters like time taken for initiation,
maturation, colour, position and pattern of sclerotia formation (whorls). The highest
significant negative correlation was observed between radial growth - sclerotia initiation
and radial growth- sclerotia maturation (r2
= -0.52, n=36, p<0.05). There was no
correlation between cultural characters and virulence nature of S. oryzae. Cultural
variability was evaluated in all the 36 isolates by cluster analysis and dendrogram
generated using R (Version 3.4.0) statistical tool. Three major groups (I, II and III) were
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formed. The group I included seven isolates, group II included a total of nine isolates
and group III consists the remaining twenty isolates.
ITS region of all the isolates were amplified with ITS1 and ITS4 universal
primers. All the isolates were appeared as identical at amplified fragment of 750bp. The
genetic diversity of S. oryzae isolates was studied using a total of 27 ISSR primers
which given repeated results out of 50 primers screened. A total of 424 alleles and
among them 4 monomorphic bands were identified. The percentage of polymorphic loci
ranged from 85.70 to 100 with an average 99.03 suggesting high diversity within S.
oryzae populations. Cluster analysis based on the UPGMA method using pooled ISSR
data (27 primers, 424 loci, 6206 bands) grouped thirty-six S. oryzae isolates into five
main clusters (I–V) based on their geographical origin at the Jaccard similarity
coefficient of 0.55.
The effect of mineral nutrition on the stem rot incidence was assessed under pot
culture. The PDI at the initial disease development (52DAS) was the highest in
treatments received recommended dose of N, P, Zn, Fe without K (T5) (14.40) and
recommended dose of N, K, Fe, Zn without P (T4) (12.35) denoting role of potassium
and phosphorus in disease incidence. At 66 days of crop age,the highest PDI (39.51)
was recorded in the treatment which received no potassium fertilization with 429.22
AUDPC units2
. There was no significant difference between treatments viz., double the
recommended dose of NPK (T2) (24.69; 288.06AUDPC units2
), recommended dose of
N, P, K, Fe without zinc (T6) (24.69; 259.26AUDPC units2
), recommended dose of N,
P, K, Zn without Fe (T7) (23.46; 257.82AUDPC units2
) and double the recommended
dose of N, P, K, Zn, Fe (T9) (23.46; 283.75AUDPC units2
) which shows that, there was
no significant role of Zn and Fe in disease development. The lowest PDI was recorded
in recommended dose of N, P, K, Zn, Fe (T8) (18.52; 237.66 AUDPC units2
) with
the highest per cent disease reduction (55.88%) over control. Hence there was a
need for balanced fertilization.
In the experimentation of effect of abiotic stress on stem rot incidence, the
highest PDI(24.69) was recorded under water stagnation conditions with a maximum of
273.66 AUDPC units2
besides, high initial disease development (10.29) and was on par
with control (21.60, 190.84 AUDPC units2
). The lowest PDI(9.26; 90.04 AUDPC
units2
) was recorded in drought and alkalinity conditions.
Twenty six rhizosphere bacteria were evaluated for their inhibitory effect on
mycelial growth of virulent S. oryzae isolate (So15) in vitro. Among them, RB17
showed the highest per cent inhibition (58.15%) and was considered as potential
bioagent and used for further studies. The preliminary cultural and biochemical studies
showed RB17 belongs to Bacillus sp. Sequencing of 16s rRNA (1560bp bp PCR
product) exhibited 93.15 per cent identity with reference B. subtilis strain ZW
(Accession no. CP092369.1) in NCBI database and was named as Bacillus subtilis
strain BsIIRR (Accession no. ON024393.1).
In vitro evaluation of efficacy of six organic amendments (OAMs) against the
virulent isolate of S. oryzae (So15) was carried out. Karanja cake have shown cent per
cent mycelial growth inhibition followed by vermicompost (76.67%) and poultry
manure (74.44%) at 5 per cent concentration. In evaluation of OAMs on sclerotial
germination, none of them were inhibited after 6hr of dipping period. Neem cake have
recorded the highest per cent inhibition (73.33%) which was on par with karanja cake
(70.00%) after 24hr of dipping period. Even though some organic amendments
inhibited mycelial growth failed to suppress sclerotial germination.
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The bio efficacy of twelve fungicides were evaluated against the virulent isolate
of S. oryzae (So15) in vitro. All fungicides at all five concentrations inhibited the
mycelial growth when compared to control except Validamycin 3% L at 250ppm.
Among the fungicides tested, Tebuconazole 250 EC (25.9% w/w) had completely
(100%) inhibited the mycelial growth of the pathogen at all five concentrations. In
evaluation of fungicides on sclerotial germination, none of them were inhibited after 6hr
of dipping period. There is a significant difference between treatments after 24hr
of dipping period. Hexaconazole 4% + Zineb 68% WP, Hexaconazole 5%
EC, Tebuconazole 250 EC (25.9% w/w) and Difenconazole 25% EC fungicides
inhibited one hundred per cent followed by Metiram 55% + Pyraclostrobin
(5%) WG, Propiconazole 25% EC and Azoxystrobin 18.2% + Difenconazole 11.4%
SC (86.67%).
The effective fungicide i.e., Tebuconazole 250 EC (25.9% w/w) and organic
amendment i.e., karanja oil cake were tested for their compatibility with the
bacterial biocontrol agent, Bacillus subtilis strain BsIIRR (Accession no. ON024393.1)
in vitro. The turbidity increased with increase in incubation time as in control which
showed compatibility of bacteria with fungicide and organic amendment. The in
vitro studies had revealed, Bacillus subtilis strain BsIIRR (Accession no.
ON024393.1), karanja cake and Tebuconazole 250 EC (25.9% w/w) as effective
against virulent S. oryzae isolate (So15)and were used for in vivo studies.
A biointensive management study for control of stem rot was conducted in farmer
field at Kaikaram village, Unguturu mandal of West Godavari district of
Andhra Pradesh during Kharif- 2020 and Kharif- 2021 on susceptible cultivar
MTU1121. Among the treatments, root dipping of seedlings with Bacilluss subtilis
strain BsIIRR (ON024393) at 10% + soil application of karanja cake @ 125 kg ha-1
+ spraying of Tebuconazole 250 EC (25.9% w/w) @ 1ml/L at 45 and 60 days of
crop age (T7) recorded the lowest PDI (15.74) and the highest yield (6.26 t/ha)
over all other treatments including control with B:C ratio of 1.76. Hence for
the cost-effective management of stem rot, root dipping of seedlings with Bacilluss
subtilis strain BsIIRR (ON024393) at 10% + soil application of karanja cake @ 125
kg ha-1 + spraying of Tebuconazole 250 EC (25.9% w/w) @ 1ml/L at 45 and 60
days of crop age proved to be the best