STUDIES ON DOWNY MILDEW OF ONION (Allium cepa L.)
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Date
2018-07-05
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UHF,NAUNI
Abstract
ABSTRACT
Onion downy mildew disease is a serious threat to Onion production in India. Onion downy mildew is
caused by Peronospora destructor (Berk.) Casp. ex Berk. which causes heavy losses to onion worldwide as well
as in Himachal Pradesh. Therefore detailed investigations were undertaken with the objectives to record the
disease severity of downy mildew of onion in Solan district of Himachal Pradesh, and to study the
morphological, epidemiological parameters of the pathogen and device suitable management strategies with
SAR inducers and other chemicals. Survey revealed that disease was moderately prevalent in Solan district with
severity range varying between 4.78 to 58.65 per cent, maximum at Pandah (58.65 %) and minimum at
Dharampur (4.78 %). The pathogen was identified based on the morphological features as Peronospora
detructor (Berk.) Casp. ex Berk. Most distinguishing morphological features of pathogen were production of
hyaline, coenocytic mycelia having width of 3.7-6.8 μm, dichotomously branched sporangiophores and hyaline
pyriform/fusiform sporangia measuring in length (50.4-65.8 μm ) and width (24.6-25.2 μm). The pathogenicity
results were obtained within 8-12 days of inoculation. Among various temperature regimes, the maximum
germination (73.96 %) and germ tube length growth (202.98 μm) was observed at temperature 15º C whereas
RH levels of more than 95 per cent were suitable for germination as maximum germination (71.4 %) was seen at
100 per cent RH with maximum germ tube length (142.63 μm). Effect of different meteorological parameters on
progress of downy mildew of onion (cv. ‘Palam Lohit’) was studied under field conditions and it was observed
that the disease severity become apparent in last week of February with minimum disease severity of 3.55%
which gradually increase in respective months and assumed highest proportion (72.76%) in the second week of
April, 2018. The mean maximum temperature ranging from 21.5-26.6 oC, mean minimum temperature of the
range 6.7-17.23 oC, average relative humidity of 52-68%, cumulative rainfall of 1-4.5 mm favoured the disease
spread. In vitro evaluation of 4 non-systemic and 3 combi-products fungicides revealed that one systemic
fungicide Dithane M-45 and two combi-products Ridomil Gold and Cabrio Top were most efficacious. Highest
inhibition of sporangial germination (100%) was recorded in Ridomil Gold followed by Dithane M-45 and
Cabrio Top while Blitox 50 gave minimum inhibition even at higher concentrations. Best performing fungicides
under in vitro were tested under in vivo conditions along with other three SAR chemicals (Chitosan, Potassium
orthophosphate and BABA) and reported their effectiveness by reducing the disease severity to 12.5 per cent in
comparison of the control (69.5%) of onion downy mildew and increasing the average plant height, bulb yield to
43.9 cm, 26.7 t/ha, respectively.
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