Vesicular-arbuscular mycorrhizal fungal infection in some tropical crops in relation to soil management practices
The dependency of maize (Zea mays L.) and bambara groundnut (Vigna subterranea) grown in acid tropical soils, on the mycorrhizal condition for improved growth and nutrient uptake, and the effects of various management practices on root infection by vesicular-arbuscular mycorrhizal fungi in maize plants were investigated. The effect of soil pH on mycorrhiza formation in perennial ryegrass and maize was also investigated using a temperate soil in which pH levels had been maintained in the field over a long period. Infection of roots of maize and bambara groundnut plants by vesicular-arbuscular mycorrhizal (VAM) fungi indigenous to the tropical soils, resulted in increased plant dry weights as well as increased uptake of nutrients. The population of VAM fungi in the soils consisted of several species in the genera Glomus and Gigaspora. The foliar application of two systemic fungicides (Triforine and calixin) to maize plants resulted in reduced infection with the application of calixin compared to control plants. Root infection by VAM fungi was not different from that of control plants when triforine was applied. Two phosphorus sources (triple superphosphate and Ghafsa phosphate rock) applied to soils at rates equivalent to 22kg ha-1 and 44kg ha-1 had varied effects on VAM fungal infection in maize roots. Phosphate rock applied at both rates and triple superphosphate applied at the lower rate (22kg ha -1) increased root infection compared to that for triple superphosphate at the higher rate. It was suggested that the increased availability of the relatively soluble triple superphosphate was responsible for the reduced infection. The effect of lime application on VAM fungal infection was dependent on the type of phosphorus fertilizer applied to the soils. Increased root infection occurred when lime was applied in addition to triple superphosphate in comparison to phosphate rock. Without lime application, increased infection occurred when phosphate rock was applied compared to triple superphosphate. In the temperate soil with pH maintained over a long period, no VAM fungal infection was found at pH values below 5.0 when ryegrass was the host, although infection occurred when maize was the host plant. There was an effect of host plant on the infectivity of the fungi present in the soils with low pH but infection was low in comparison with that in soil at pH values above 5.0 indicating that management practices which result in soil pH changes may influence mycorrhizal associations in different plant species to different extents. The application of phosphate rock was beneficial to mycorrhiza formation and it was suggested that fertilizer practices which involve the use of phosphate rock could confer additional benefits that can be derived from increased mycorrhiza formation.