Increased incidence of Pythium stem rot in cowpea treated with benomyl and related fungicides

In six replicated field trials during a 3-year period the incidence of cowpea wet stem rot caused by Pythium aphanidermatum was significantly greater in plots treated with benzimidazole (BZ) fungicides than in plots treated with non-BZ fungicides and nontreated plots. In laboratory studies, the growth of P. aphanidermatum in corn-meal agar was unaffected by the addition of up to 250 µg/ml active ingredient of the BZ fungicides. It seems likely that the broad-spectrum, yet selective, BZ fungicides favor the activity of P. aphanidermatum by suppressing antagonists and competitors

Decomposition of Carbon14 labelled ryegrass and maize under tropical conditions

This work is an extension of earlier studies on the rate of decay of plant material in the forest zone of Nigeria. Maize (Zea mays L.) leaves, uniformly labeled with 14C, were allowed to decompose for 2 yr under field conditions in an Alfisol (pH 7.0) or an Ultisol (pH 4.7). After two years, 15% of the original plant C still remained in the Alfisol. Decomposition was slower in the Ultisol, particularly during the first 26 wk. Maize leaves incorporated in soil at the beginning of the dry season initially decomposed more slowly than leaves incorporated during the wet season, but by the end of 2 yr the difference was small. Earlier experiments in Nigeria with labeled ryegrass (Lolium multiflorum Lam.) tops were continued: after 5 yr, 8% of the original ryegrass C remained in an Alfisol. After adjustment for differences in climate and soil texture, the current Rothamsted model for turnover of organic C in soil gave a close fit to the Nigerian data

Decomposition of Carbon-14 labeled plant material under tropical conditions

Ryegrass (Lolium multiflorum) and maize (Zea mays) tissue uniformly labeled with 14C were mixed with soil and allowed to decompose under field conditions in the open or under shade. The incubations were done in the forest zone of Nigeria, using a range of contrasting Nigerian soils. Of the ryegrass Coriginally added to the soil, 20% remained after 1 year, falling to 14% after 2 years. After 1 year the soil retained slightly less maize C than ryegrass C, but the difference was small and the overall pattern of decomposition similar. There was little difference between the rate of decomposition under shade or in the open, even though soil temperatures were considerably greater in the open. A soil containing 6% clay (Apomu series) retained slightly less maize C after 1 year than a soil with 16% clay (Egbeda series), but in general the decomposition rates in the different soils were similar. For ryegrass, the decomposition pattern was very similar under Nigerian conditions to that previously observed for the same plant material in England, except that the whole decomposition process was four times faster in Nigeria

The effects of clearing and cropping on organic reserves and biomass of tropical forest soils

Twenty Nigerian soil samples, mainly from field experiments at the International Institute of Tropical Agriculture (IITA), were used in a study of the effects of clearing and cropping on the C, N and S reserves of soils under secondary lowland rain forest. Soils that had been cropped for 2yr usually contained less total C and N than the corresponding forest soils. This decline was less rapid when crop residues (maize) were returned to the soil as a mulch than when they were removed. The soils were incubated in the laboratory either fresh, or after exposure to stress treatments (air-drying or fumigation). The amounts of N mineralised by the fresh untreated soils were small and not clearly related to the cropping history. Both air-drying and fumigation caused a marked increase in the mineralisation of N and the amount released was related to the cropping history. Cropping caused an even greater decrease in these ‘stress-labile’ N reserves than in total soil N, so that cropping causes a decline in both the quantity and “quality’ of soil organic matter. Again, the decline was less where crops were grown under mulches. A term “Per cent Stress-labile N” (PSN), defined as the percentage of the total N mineralised when fumigated soil is incubated under defined conditions, is proposed as a measure of the ‘quality’ of soil organic nitrogen. Under secondary forest the PSN ranged from 3.4–4.2%; on cultivation it declined, in one case to less than 1%. The amount of C in the soil biomass was calculated from the size of the flush of decomposition caused by fumigation. Biomass C usually decreased on cultivation, the decrease being less under mulches. There was a close linear relationship (r = 0.94) between the flush of N caused by fumigation and the amount of biomass C in a soil and a rough measure of biomass C can be obtained by multiplying the N flush by 8