MSc

Different phenomena like mutation, recombination, natural selection, geographical
locations, etc would result in the accumulation of variation in plant pathogenic
microorganisms. These variants in the pathogen population often lead to loss of resistance
of the cultivar, leaving it highly vulnerable to the disease. Lasiodiplodia theobromae, the
causal organism of mango dieback is a highly variable pathogen known to occur in different
locations and affecting different mango varieties. In this context, variability studies of
Lasiodiplodia theobromae were undertaken to know the variation present among
Lasiodiplodia population in Kerala.
Purposive sampling surveys were carried out in five districts of Kerala viz., Thrissur
(AEU 5, AEU 6, AEU 10 and AEU15), Palakkad (AEU 10, AEU 22 and AEU 23),
Wayanad (AEU 20 and AEU 21), Kasaragod (AEU 11) and Kottayam (AEU 4 and AEU
9), thus covering 23 locations and 11 different agroecological units. A total of 37 isolates
of the pathogen have been collected from 17 different mango varieties. Per cent disease
incidence (PDI) of the different isolates ranged between 25-80 per cent whereas per cent
disease severity (PDS) ranged from 17.5-62.5 per cent.
General symptoms of dieback under natural conditions were necrotic leaves and
black discolouration on the stem and certain isolates showed vascular discolouration too.
Upon artificial inoculation of different isolates, a patch of black colour was observed and
the range of incubation days varied from 10-15 days while that of lesion size from 0.55-
1.1 cm.
Diversity in different cultural and morphological characters was also observed.
Cultural characters included colony colour (white, greyish white, grey, greyish black,
blackish grey, blackish white and black), colony colour on the reverse side of the Petri dish
(white and black colour), days taken to cover the Petri plate (2-5), days taken for pycnidia
formation (13- 40), days taken for sporulation (22-55) and growth rate (3.34-5.2 cm/day).
Certain cultural characters like texture, zonation and margin didn’t show any variation. All
the isolates had wooly texture, smooth margin and no zonation was observed in any of the
isolates.
Different morphological characters also showed wide variation. Morphological
characters observed were the colour, size and shape of conidia and the colour and size of
hyphae. The range of breadth of hyphae of collected isolates was 3.36-5.18 μm, the range
of length of conidia was was 20.83-23.85 μm, and the breadth of conidia was 10.68-13.32
μm. All 37 isolates invariably had cinnamon brown coloured ellipsoid conidia and brown
coloured hyphae.
The 10 most virulent isolates of Lasiodiplodia theobromae selected for vegetative
compatibility studies based on incubation days and lesion size, showed 32 compatible and
13 incompatible out of 45 combinations. The isolates PMuNe,, WMnMa and KoKBn were
compatible with maximum number of isolates (eight). Five isolates were selected for
pathogenic variability studies based on cluster analysis viz., TNaM (I1), PMuNe (I2),
WMnMa (I3), KPB (I4), and KoKBn (I5) and upon molecular characterization confirmed
as Lasiodiplodia theobromae.
Pathogenic variability of the selected five isolates of Lasiodiplodia theobromae was
evaluated against two traditional mango varieties viz., Muvandan and Priyur and four
commercial varieties of mango viz., Neelam, Bennet Alphonso, Banganapally, and Imam
Pasand. Variability was observed in pathogenicity parameters such as PDI (33.33-100%),
PDS (13.33-73.33%), lesion size (0.28-0.95 cm), and incubation period (9-17.33 days).
Based on Coefficient of Index (CI) value, Neelam, Bennet Alphonso and Banganapally
were categorized as susceptible while the other varieties, Priyur, Muvandan, Bennet
Alphonso and Imam Pasand were categorized as moderately susceptible to mango dieback
disease. All the isolates produced 100 per cent incidence on Banganapally, confirming its
susceptibility. Based on virulence index, three isolates viz., WMnMa (I3), KPB (I4) and
KoKBn (I5) with high value of virulence index were selected for their sensitivity to
different fungicides and biocontrol agents.
In vitro evaluation of the fungicides showed that these three isolates I3, I4 and I5
were completely inhibited by hexaconazole (0.2%), azoxystrobin (0.2%), Bordeaux
mixture (1%) and carbendazim + mancozeb (0.2%). In vitro evaluation of biocontrol agents
exhibited inhibition ranging from 14.82-51.33 per cent with maximum by Trichoderma
asperellum and Pseudomonas fluorescens being ineffective.
Based on the above parameters, it could be observed that the population of
Lasidiplodia theobromae exhibited significant variation in cultural, morphological, and
pathotypic traits, posing significant challenges to effective disease management