MSc

Yard long bean (Vigna unguiculata subsp. sesquipedalis (L.) Verdc.) is
believed to be selected and developed from cowpea (Vigna unguiculata (L.) Walp.)
for its long, succulent pods which are used as a vegetable. In Kerala, it is one of the
most preferred vegetables having very high amount of protein, iron, calcium,
vitamin A, Vitamin C and dietary fibre. It is considered as a remunerative crop
under protected condition owing to its high market demand. However, incidence of
diseases is a major setback hampering the production of yard long bean under
protected conditions among which, powdery mildew is the most devastating one.
In this background, the present study was undertaken to characterize the pathogen
causing powdery mildew of yard long bean and to formulate a management strategy
for the disease under protected cultivation.
Purposive sampling surveys were conducted in seven locations of Thrissur
district and the disease severity varied from 1.67 to 67.33 per cent. The results of
the survey indicated that the severity of disease was more during pod bearing and
harvesting stage. Since powdery mildews are obligate parasites, characterization
was done based on the microscopic observation of pathogen present on the leaves.
The fungi produced hyaline, branched and septate hyphae. The conidiophores were
erect and cylindrical on which conidia were born in chains. Variability was
observed regarding conidia and conidiophore characters of powdery mildew
collected from different locations, based on which the isolates were grouped into
two viz., PM 1 and PM 2. PM1 type was observed in all locations except
Vellanikkara. Based on the morphological characteristics of conidia and
conidiophores, it was identified as Erysiphe polygoni. PM 2 type obtained only
from Vellanikkara in which conidia and conidiophore characters were similar to
Podosphera sp. which is very rarely reported on legumes. Hence, its identity was
further confirmed as Podosphaera xanthii by molecular characterization. The
rRNA-ITS sequence was deposited in NCBI Genbank database with accession
number MH645799. This is the first report of powdery mildew of yard log bean
incited by Podosphaera xanthii.
In-vitro evaluation of 17 treatments including fungicides, biocontrol agents
and botanicals by spore germination technique revealed that all the treatments
caused cent per cent inhibition of conidial germination. For taking forward eight
promising treatments to the field experiments, they were tested in-vitro on detached
leaves by artificial inoculation of conidia from infected leaves. Based on the per
cent leaf area infected, two systemic fungicides, one contact fungicide, two
biocontrol agents and one botanical were selected for field evaluation.
Field experiments were conducted simultaneously inside polyhouse and rain
shelter to evaluate the performance of selected fungicides, biocontrol agents and
botanicals against powdery mildew. Among the treatments, low disease severity of
4.33 per cent and 7.67 per cent was recorded in T1- difenoconazole and T2 –
tebuconazole respectively in polyhouse and these treatments were statistically on
par. In rain shelter also, T1- difenoconazole and T2- tebuconazole recorded low
disease severity of 7.67 per cent and 10.67 per cent respectively. The performance
of wettable sulphur at lower and higher concentration did not differed significantly.
All the four non-chemical treatments were equally effective in managing the disease
both in polyhouse and rain shelter. Correlation analysis between the meteorological
parameters and disease severity revealed that per cent disease severity was
negatively correlated with temperature and relative humidity both in polyhouse and
rain shelter.
Analysis of population of phylloplane microflora showed that, there was a
drastic reduction in the population of phylloplane fungi and bacteria after spraying
chemical fungicides which is an indication of the toxicity and non-selectivity of
these chemicals. Survival ability of biocontrol agents sprayed on the leaves were
studied and found out that both Trichodema viride and Pseudomonas fluorescens
survived on the leaves for seven days. Residue analysis of difenoconazole, the most
effective chemical fungicide revealed that the compound with initial deposition of
0.21 mg kg-1 dissipated to 0.09 mg kg-1 after seven days in polyhouse whereas, the
residue after seven days in rain shelter was 0.19 mg kg-1. The faster degradation of
the chemical inside polyhouse may be attributed to the higher temperature prevailed
during the experiment.
Evaluating the results various experiments in the present investigation, it
was found that, even though chemical fungicides provided best disease control,
considering their toxic effect on beneficial non target microflora on the phylloplane
and the residue left on edible pods, biocontrl agents such as Trichoderma viride and
Pseudomonas fluorescens which exhibited consistent performance with moderate
disease control and sufficient survival on the leaf surface would be ideal to control
powdery mildew of yard long bean if applied at right time. Moreover, frequent
application of systemic fungicides with single site action can result in the
development of resistant strains of pathogens. So such chemicals should be adopted
only if the disease severity is very high and cannot be managed with biocontrol
agents.