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The effect of forty different pesticides on two Rhizobium isolates from green gram Vigna radiata (L.) Wilczek were studied. Among the different pesticides tested, 10 have shown inhibitory activity. The pesticides which did not show any effect on Rhizobium isolates in zonal inhibition technique were tested for their stimulatory effect on Rhizobium isolates in broth. Dimethoate and monocrotophos were found to have stimulatory effect on both the isolates. Five pesticides which showed either inhibitory or stimulatory effect were tested for their effect on nodulation. Among these, captan severely affected the nodule number, size, leghaemoglobin and nitrogen content. Simultaneous application of Rhizobium and pesticides in the field was more toxic, and maximum reduction in nodule number was also observed. But when time gap between pesticide and Rhizobium application increases, the toxic effect reduces.
The legume- Rhizobium symbiosis has been exploited agriculturally and suitable Rhizobium
strains are used to inoculate seeds of legume before sowing for inducing proper nodulation
and nitrogen fixation. In modern agriculture, application of pesticides is an accepted practice
towards controlling pests and diseases of plants. Pesticides may directly affect the free living
population of nodular bacteria in soil or indirectly influence the extent of infection and thus
the number of nodules (Niewiadomska, 2004). Realizing the pesticide toxicity on nodulation
process, much work has been conducted on this aspect, but the work was confined mainly
to a few major legume crops like soybean, groundnut etc. Hence, the present work has been
taken up to study the effect of pesticides on Rhizobium and nodulation of green gram.designated as NU isolate. Other isolate was procured from Regional Agricultural Research
Station, Lam, Guntur and designated as LGG isolate.
To study the effect of pesticides on Rhizobium isolates, sterile filter paper discs were dipped
in different concentrations (1, 5, 10, 50, 100, 500, 1000 ppm) of pesticide solution and
air dried. After drying, discs were again dipped in pesticide solution. Thus prepared discs
were placed on YEMA plates containing lawn culture of Rhizobium isolates and incubated
for 24-48 h at 28±2 oC. After incubation, the plates were observed for zone formation
and the diameter of the inhibition zone was recorded.
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