STUDIES ON TUKRA AND POWDERY MILDEW DISEASES IN MULBERRY AND THEIR IMPACT ON THE GROWTH AND DEVELOPMENT OF SILKWORM, Bombyx mori L.

Author : M.GAYATHRI Title of the thesis : STUDIES ON TUKRA AND POWDERY MILDEW DISEASES IN MULBERRY AND THEIR IMPACT ON THE GROWTH AND DEVELOPMENT OF SILKWORM, Bombyx mori L. Submitted for the : Master of Science in Agriculture award of degree Faculty : Agriculture Major advisor : Dr.P.RAJENDRA PRASAD Department : Entomology University : Acharya N.G.Ranga Agricultural University Year of submission: 2007 The present investigation on Tukra and powdery mildew diseases in mulberry was undertaken in different villages of Chittoor and Ananthapur districts. The evaluation of different insecticides against mulberry mealybugs and different fungicides against Phyllactinia corylea causing powdery mildew and their toxicity on mulberry silkworm larvae were carried out in Insectary, Department of Entomology, S.V. Agricultural College, Tirupati. The results revealed that the tukra per cent disease index in tukra was minimum in V1 variety at Mamunduru village of Chittoor district and Ramunepalli village of Ananthapur district during summer season. The powdery mildew disease was observed only in winter season during the study period and it was very less in both Chittoor and Ananthapur districts. The maximum per cent disease index was observed in M5 variety of Mamunduru and Ramunepalli village whereas in V1 variety the disease was not recorded. The silkworm larvae fed with tukra affected leaves from hatching to spinning stage resulted decreased larval duration and increased larval weight. However, no significant difference was observed between all the treatments and control with regards to larval mortality per cent, cocoon weight, pupal weight, cocoon yield/10,000 larvae brushed and ERR. An increased shell weight and shell ratio was observed with feeding of high per cent disease index of leaves to the silkworm larvae. The maximum per cent population reduction of mealybugs was observed with methyldemeton 0.05 per cent and dimethoate 0.05 per cent treatments. DDVP and nimbicidine treatments were also found to be effective at all three concentrations. Studies on insecticidal toxicity to silkworm larvae recorded relatively shorter waiting period (3-5 days) for nimbicidine and DDVP at all three concentrations. Whereas longer waiting period (7-10 days) for dimethoate and methyldemeton at 0.05, 0.025 and 0.0125 per cent concentrations were noticed. Insecticidal treatments of mulberry leaves was also resulted in increased larval duration, decreased larval weight and cocoon characters. Methyldemeton (at all three concentrations) inspite of its high effectiveness against the mealybugs is not recommended to use on mulberry leaves due to its high residual toxicity and unfavourable effects on the larval and cocoon characters. DDVP and nimbicidine at all the three concentrations recorded shorter waiting period, satisfactory control of mulberry mealybugs and favourable effects on the economic character of silkworm. It is therefore recommended for extensive use in sericulture. Studies on effect of fungicides on conidial germination of phyllactinia under in vitro conditions revealed that maximum per cent of conidial inhibition was recorded with dinocap (0.20%) and minimum with thiovit 0.05 per cent. The influence of fungicide treated leaves on the mulberry silkworm resulted that the thiovit at all three concentrations were safe followed by mancozeb and dinocap at 0.05, 0.10 and 0.20 per cent concentrations. Fungicides immediately 1 DAS affected more in III instar larvae as compared to IV and V instar larvae. From 3 DAS onwards the toxic effect on the silkworm larvae was not observed, indicating that the fungicide treated leaves were safe for feeding the mulberry silkworm larvae in the later instars from three days after spraying. CHAPTER –