Using Neem to Control Charcoal Rot of Chickpea

Abstract

Charcoal rot is an important soilborne disease that infects a wide range of plant species including chickpea, caused by Macrophomina phaseolina resulting in serious yield losses. This study was undertaken to control this disease by a natural fungicide agent, the neem tree which has a well-known fungicidal effect in most of its parts. In vitro experiments examined neem seed oil and neem leaf extract to determine their inhibitory effect on the growth of two strains of M. phaseolina. Neem oil extract promoted the mycelial growth of both strains, while neem leaf extract reduced the growth of both strains. Glasshouse experiments examined the effect of neem leaf powder on the development of charcoal rot of two cultivars of chickpeas: desi and kabuli. Neem powder reduced the symptoms of infected plants and promoted their growth so that it was the same as in uninfected controls. However, there was a slight phytotoxic effect when neem was applied in the absence of the pathogen. A glasshouse experiment showed that neem powder could be safely used on the beneficial microbes Rhizobium and mycorrhiza. In order to optimise rate and application time, neem leaves were prepared in different formulations, pellets, capsules, aqueous extract and powder. These were used in rates 0.5%, 1%, and 1.5% w/w in pot experiments, applied either pre- or post-emergence of chickpea seeds. Neem pellets at 0.5% applied pre-emergence had the optimum effect on charcoal rot symptoms with less phytotoxicity to the plant. The effect of neem pellets and neem powder on control of charcoal rot of chickpea was examined under field conditions. Neem powder reduced disease symptoms and increased shoot dry weight of the plant. Neem pellets controlled the disease and also had a large growth promotion effect. Neem pellets increased the number and the dry weight of rhizobium nodules. Neem reduced the population of soil fungi but not of bacteria. Tests of growth inhibition against a suite of typical soil fungi showed a range of responses from high to no inhibition. Neem extracts did not inhibit growth of the important chickpea pathogen Phytophthora medicaginis and neem pellets did not control Phytophthora root rot of chickpea in glasshouse experiments. Neem shows promise as a control method for charcoal rot and some other soilborne diseases, which does not have a negative effect on beneficial microbes.