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