STUDIES ON EPIDEMIOLOGY AND MANAGEMENT OF CERCOSPORA LEAF SPOT OF CASTOR (Ricinus communis L.) IN AGRO-CLIMATIC CONDITION OF ASSAM
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Date
2021-11
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Abstract
Castor (Ricinus communis L.) is the primary host plant of the eri silkworm
Samia ricini Donovan, a domesticated polyphagous multivoltine lepidopteran insect
which is responsible for producing the eri silk. The healthy and robust growth of the eri
silkworms primarily depends upon the quality and quantity of castor leaves, which
ultimately reflects in the qualitative and quantitative parameters of the cocoons
produced. Cercospora leaf spot of castor, caused by Cercospora ricinella Sacc. & Berl.
is one of the major foliar diseases encountered in the cultivation of castor, causing
substantial injury to leaves intended as feed for eri silkworms. Study on occurrence and
epidemiology was done during three seasons i.e. pre-monsoon (Mar-May), monsoon
(Jun- Aug) and post-monsoon (Sept-Nov) by correlating weather parameters such as,
temperature, rainfall, number of rainy days, relative humidity and bright sunshine hours
with disease incidence. It was inferred that, disease incidence was prevalent at high
levels during the pre-monsoon season (Mar-May) followed by monsoon as compared to
post-monsoon season. Disease incidence is positively correlated to maximum
temperature (significant), minimum temperature, RH (at P.M.), rainfall and number of
rainy days (non-significant) and negatively correlated to RH (at A.M.) and BSSH (nonsignificant).
A total of 8 treatments were taken for the experiment of management of the
disease in pot condition. The treatments included a phytoextract of Tulsi (Ocimum
sanctum) @ 5%, a bio agent (Trichoderma viride) @ 5% and a chemical fungicide
(Copper oxychloride) @ 0.2 % and their combinations, applied as foliar spray at 15
days interval. None of the eco-friendly treatments were found superior than chemical
fungicide inclusive treatment, Tulsi extract + T. viride + Copper oxychloride, where
lowest disease incidence was recorded (7.13%). Among eco-friendly treatments, Tulsi
extract + T. viride (12.48 %) was found to be the best. Treatments containing T. viride
promoted the growth of plants in terms of leaf area, shoot length, root length, dry and
fresh weight of root, thus showing dual roles as bio control agent and plant growth
promoter. Biochemical analysis revealed the highest total chlorophyll and flavonoid
content in plants treated with Tulsi extract + T. viride + Copper oxychloride whereas,
MDA (Malondialdehyde content) in leaf tissues was found to be highest in control and
lowest in pots treated with Tulsi extract + T. viride + Copper oxychloride. Another
investigation was conducted to find out the effect of treated castor leaves on eri
silkworm growth. Silkworms fed with castor leaves treated with T.viride + Tulsi extract
showed the best results followed by worms fed with leaves treated with Tulsi extract +
T. viride + COC.