MSc

Dolichos bean (Lablab purpureus L.) is one of the oldest legume crops cultivated
throughout the tropical regions in Asia and Africa. It is a multipurpose crop grown for pulse,
vegetable and forage purposes. Several diseases affect this crop at different stages which
hamper its production and productivity. Systematic documentation on diseases of dolichos
bean has not been undertaken so far in Kerala. In light of this, the current study aims to
identify and document diseases of the dolichos bean, symptomatology, etiology and
characterization and in vitro management of the pathogens associated.
In order to catalogue and characterize various diseases associated with dolichos bean,
purposive sampling surveys were carried out in the main dolichos bean growing areas of
Thrissur district during 2021-2023. From the 14 surveyed locations, the symptoms like leaf
spot, leaf blight, stem blight, collar rot, pod blight, inflorescence blight, wilt and ring spots
were observed. Leaf spots, leaf blight and stem blight were the major symptoms observed
from these locations. The collected samples included nine leaf spots, ten leaf blights, a ring
spot, nine stem blights, two collar rots, one wilt, one pod blight and one inflorescence blight.
Depending upon the type of symptoms observed, the incidence and severity of associated
diseases from each specific location were assessed. The maximum severity in terms of per
cent disease index (PDI) of 63.3 per cent was recorded for Rhizoctonia leaf blight (VLKLB1)
and the lowest was observed in Alternaria leaf blight (TOTLB) with 10 per cent severity.
The higher disease incidence was recorded for the ring spot (VLKRS; 40 %) samples and the
least disease incidence was recorded from the wilt-affected areas of Vellanikkara (2 %).
Cercospora and Alternaria were major leaf spot and leaf blight pathogens documented
during the survey. Stem blight caused by Colletotrichum was the most common disease
observed during the survey with the highest severity observed in case of CHRSB (43.3-PDI).
The major collar rot pathogens in dolichos were Rhizoctonia (VLKCR1) and Sclerotium
(VLKCR2), with a documented disease incidence of 16.66 and 3.33 per cent respectively.
Isolation of associated fungal pathogens yielded a total of 29 isolates. The
characteristic symptom produced by each isolate under artificial inoculation was studied and
compared with the symptom development under natural conditions. The pathogenicity was
established by MBIM (Mycelial Bit Inoculation Method), progression and time taken for the
symptom development was recorded. By comparing the cultural and morphological features,
the pathogens were tentatively identified up to the genus level. The pathogen associated with
VLKLS1 leaf spot was identified as Myrothecium sp. and that with VLKLS3, MANLS,
POTF1LS1, THIF2LS and THIF3LS were confirmed to be caused by Cercospora sp. Leaf
spots VLKLS2, PALLS and leaf bights like ELAF1LB, THIF1LB, THIF2LB, THIF3LB,
KIPLB, TOTLB, TEKLB were identified as Alternaria sp. Leaf blights VLKLB1, CHRLB
and collar rot VLKCR1 were found to be caused by Rhizoctonia sp. The pathogen associated
with leaf and inflorescence blight (VLKLB2 and VLKIB) was found as Choanephora sp. In
addition, stem blight, a leaf spot and pod blight (POTF1LS1, VLKSB, CHRSB, KOZSB,
MIQSB, ELAF2SB, KIZSB, POTF2SB, THIF2SB, TEKSB, VARPB) associated pathogen
was identified as Colletotrichum sp. The typical viral ring spot samples collected were
serologically characterized by DAC-ELISA. An absorbance of 1.1 against Watermelon silver
mottle virus antiserum was obtained, confirming the presence of the Tospovirus.
Molecular characterization of selected fungal pathogens was confirmed by analysing
the amplified specific LSU or ITS regions of pathogens in the BLASTn program of NCBI.
Based on this, the leaf spot pathogens VLKLS1, VLKLS3 and PALLS were identified as
Myrothecium inundatum, Cercospora glycinicola and Alternaria alternata. Similarly, the
collar rot pathogens VLKCR1, VLKCR2 was identified as Rhizoctonia solani and Athelia
rolfsii. The stem blight associated pathogens (CHRSB and POTF2SB) were identified as
Colletotrichum dracenophilum and wilt-associated pathogen was confirmed as Fusarium
oxysporum.
The complete inhibition against Rhizoctonia solani (VLKCR1) was observed with
the fungicides copper hydroxide, Propineb, mancozeb, Bordeaux mixture, tebuconazole,
carbendazim, trifloxystrobin (25 %) + tebuconazole (50 %), carbendazim (12 %) + mancozeb
(63 %), azoxystrobin (18.2 %) + difenoconazole (11.4 %) at all the three concentrations. In
the case of Myrothecium inundatum (VLKLS1), copper hydroxide, Propineb, mancozeb,
Bordeaux mixture and tebuconazole showed complete inhibition at all the three dosages.
Copper hydroxide, Propineb, mancozeb, tebuconazole, carbendazim, carbendazim (12 %) +
mancozeb (63 %), azoxystrobin (18.2 %) + difenoconazole (11.4 %) were highly effective
against Colletotrichum dracenophilum (CHRSB) at all the three dosages. In vitro evaluation
of Alternaria alternata (PALLS) showed cent per cent inhibition in Propineb, mancozeb,
Bordeaux mixture and tebuconazole at all three concentrations evaluated.
The spore germination assay was carried out for two selected pathogens,
Myrothecium inundatum and Colletotrichum dracenophilum. The complete inhibition of
Myrothecium inundatum spores was observed at all three doses of the fungicides tested, i.e.,
chlorothalonil, copper hydroxide, Propineb, mancozeb, Bordeaux mixture, tebuconazole,
carbendazim, trifloxystrobin (25 %) + tebuconazole (50 %) and azoxystrobin (18.2 %) +
difenoconazole (11.4 %). Chlorothalonil, copper hydroxide, Propineb, mancozeb,
tebuconazole, carbendazim, trifloxystrobin (25 %) + tebuconazole (50 %) and azoxystrobin
(18.2 %) + difenoconazole (11.4 %) fungicides at all three concentrations inhibited the
germination of Colletotrichum dracenophilum spores.
Among the biocontrol agents, Trichoderma talc formulation showed higher inhibition
(100 %) against all fungal pathogens tested. In PGPM consortia, 80 – 87, 45 – 72 and 81 –
89 per cent inhibition was observed against Myrothecium inundatum, Colletotrichum
dracenophilum, Alternaria alternata. An inhibition of 70 – 100, 33 – 68 and 53 – 66 per cent
was recorded against Myrothecium inundatum, Colletotrichum dracenophilum, Alternaria
alternata. But, the PGPM and PGPR-II formulations were less effective against R. solani.
Pseudomonas fluorescens showed no inhibition against Rhizoctonia solani and Alternaria
alternata in both dual culture assay and poisoned food technique. P. fluorescens against
Colletotrichum dracenophilum and Myrothecium inundatum showed 32 to 82 per cent
inhibition.
In continuation with the present findings, studies need to be conducted on the
molecular characterization of more viral and fungal pathogens of dolichos bean. Further,
field evaluation of the in vitro effective fungicides and bioagents for the integrated
management of diseases of dolichos bean need to be conducted

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