Studies on variability and management of Alternaria spp. causing Leaf blight of oilseed brassica
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Date
2020
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DRPCAU, Pusa
Abstract
In the world, Oilseed brassica, called as Rapeseed-Mustard is the third most important oilseed crop after soyabean and palm. Bihar produces 0.11 mt from an acreage of 0.09 mha with a productivity of 1241kg/ha.
As far as 35- 45 % losses had been reported in Indian mustard, even more losses in rapeseed up to 70%, due to Alternaria leaf blight disease caused by Alternaria brassicae and Alternaria brassicicola.
Survey had conducted for the natural occurrence of the disease prevailed in different districts of Bihar and in a range of 25.67 to 69.67 per cent disease incidence was recorded.
The black points were the first seen symptoms on the lower leaves which coalesce to form spots of brown to black coloured lesions. Disease spreads rapidly forming circular spots on middle and upper leaves. Upon severity target board like concentric rings were formed which leads to defoliation of the leaves. Symptoms were seen on the stem and siliquae deteriorating the crop and causing damage to seeds.
Disease samples were collected from the surveyed fields and isolation of the pathogen from the isolates had done on the PDA medium. Out of ten isolates collected, seven were identified as Alternaria brassicae and three were identified as A. brassicicola. Pure cultures were maintained for further studies of morphological and cultural variability among the isolates. Mycelial growth of the pathogen and colony characters were considered to study the morphological variations. Radial growth of mycelium varied from 53.0 to 77.2 mm in diameter and five different colony colors were observed among the pure cultures.
Pure cultures of ten isolates were inoculated on the five different solid media to check the cultural variability. Radial growth of the mycelium varied on different media from 39.3 to 76.3 mm in diameter. Conidial dimensions were also measured to check the variability. The average conidial length, breadth and beak length varied from 36.5 to 183.0 μm, 6.55 to 22.0 μm and 2.8 to 63.35 μm respectively. Hence it was concluded from the experiment that morphological and cultural variability exists in the pathogen causing Alternaria blight in oilseed brassica.
Some new fungicides had evaluated both in vitro and in vivo experiments for next line as management practices because some popular and effective fungicides may likely to banned in future. Among the six different fungicides tested in in vitro, Trifloxystrobin 50WG was found more efficient in controlling the pathogen. The experiment conducted with three different concentrations and found that the radial growth of mycelium inhibited by Trifloxystrobin 50WG was minimum at all concentration tested viz. 39.3 mm @ 250ppm, 24.7 mm @ 500 ppm and 0.7 mm @1000ppm.
The field experiment had conducted with three replications under eight treatments with six fungicides. Plots treated with seed treatment of Carbendazim 50WP@2g/kg of seeds followed by foliar sprayed with Trifloxystrobin 50WG@1ml/l of water were found superior to the other treatments. Least PDI (20%.5) maximum 1000 seed weight (4.1g), highest yield (1430 kg/ha), highest yield increase over control (591.7kg/ha) and highest CB ratio (1:71) was recorded from this treatment.