INFLUENCE OF pH AND EC OF WATER ON THE EFFICACY OF BACILLUS THURINGIENSIS VAR . KURSTAKI AGAINST SPODOPTERA LITURA IN TOBACCO NURSERIES

Abstract

In agriculture water is mainly used for irrigation and the other major usage is for spraying plant protection chemicals. The quality of water used in agriculture generally depends on the total concentration of dissolved salts and pH. For evaluating the quality of water electrical conductivity (taken as measure of soluble salt content), chlorides and pH are measured. In tobacco cultivation the permissible limits of water for irrigation are EC <0.4 dS/m and pH 6.5-7.5 (Krishnamurthy and Nagarajan, 2001). In highly alkaline water (pH>8) many chemicals undergo a process called alkaline hydrolysis. This process causes the breakdown of the active ingredient into other compounds which can reduce the effectiveness of the pesticide over time. Very acidic water can also affect the stability and physical properties of some pesticide formulations. Similarly water with high salt concentration may denature the crystal protein in B.t formulations (Fast, 1981). However in many tobacco growing areas when the water is used for spraying in plant protection interventions, these limitations are going unnoticed and water suitability for spraying particularly for different biopesticides needed experimental investigation. Hence, experiments were conducted in tobacco nursery using water with different pH and EC on the efficacy of Bacillus thuringeinsis var.kurstaki (NBAII strain) against tobacco caterpillar to determine permissible limits of pH and EC.