Although plant nematodes are ubiquitous and pathogenic to a wide range of crops, research into these pests in the tropics has been concentrated on commodity crops. While modern intensive tropical agriculture has become unsustainable and often relies on the use of toxic nematicides to control damaging populations of plant nematodes, many traditional subsistence farmers have been able to suppress nematodes by promoting crop diversity and selecting for tolerance and resistance. This paper gives examples of sustainable systems which suppress nematodes. Other examples of once sustainable systems in which nematode problems now occur are discussed and reasons for this instability suggested. The need for farmers to retain responsibility for the development of sustainable systems is stressed. Nematologists and other plant protectionists are encouraged to take a wider view of the complex issues that affect the sustainability of agriculture in the tropics.

Los nemátodos de las plantas y la agricultura continua

This paper draws on the results of farming systems research (FSR) conducted over five years at eight sites distributed throughout Pakistan. The main research findings with respect to the irrigated plains, or favoured areas (four sites), and the marginal mountainous and rainfed areas (four sites) are summarized, with emphasis on the role of system interactions in technology development. In favoured areas, most system interactions occur between crops grown in multiple cropping patterns, whereas in marginal areas, crop-livestock interactions dominate. Appropriate research strategies for each kind of environment are discussed. It is concluded that although there is a need for quite different research strategies for favoured and marginal areas, the methods and perspective of FSR are equally appropriate and even critical for achieving future gains in agricultural productivity in each type of environment.

Estrategias de investigación agricola en zonas favorecidas y marginales: Experiencias recogidas durante la investigación de sistemas agrícolas en Pakistán

The performance of promising genotypes of faba bean developed for areas with limited rainfall was compared with the local landrace, ILB 1814, at Tel Hadya in Syria during 1989–1988 using different row spacings and sowing densities. In the last two years of the trials the performance of the improved genotypes and ILB 1814 was compared with and without supplemental irrigation. Yields of ILB 1814 were better than the highest yielding genotypes in some years and similar to them in others. In seasons with limited rainfall the best yields were obtained by using a narrow row spacing and dense sowing, and yields were significantly improved by irrigation. Grain yield was correlated with biomass and the number of pods per unit area. The percentage of radiation intercepted was highest in ILB 1814, particularly when sown with a narrow row spacing and at a high plant density. Soil water extraction, evapotranspiration and water use efficiency were improved by sowing at a narrowrow spacing and high plant density.

Los nemátodos de las plantas y la agricultura continua

Root systems of unpruned Cassia siamea and Leucaena leucocephala trees and regularly pruned hedges of C. siamea, L. leucocephala and Gliricidia sepium were examined in a semi-arid area in Kenya. Trenches were dug across rows of trees or hedges, soil profiles washed, and different sized roots counted in small 5 × 5 cm grids fully covering both faces of the trench. The roots were displayed using two computer software programmes written for this purpose: a spline technique that displays smoothed root densities in different shades; and a random dot method which displays roots as dots, using any specified scale. The fine roots of four-and-a-half-year-old C. siamea penetrated the soil to meet with rocks at depths between 0.60 and 2.0 m, and spread laterally to 9 m, traversing adjacent crop plots. Roots of eight-year-old L. leucocephala covered an extensive soil volume and included a greater density of fine roots than C. siamea at comparable distances. Above-ground pruning restricted the roots of hedges to a depth of 1.5 m, although roots of different hedges spaced 5 m apart intermingled freely in the alley space. L. leucocephala roots filled the alley very densely while those of G. sepium were sparse, particularly in the middle of the alley. Rooting density of C. siamea was intermediate. The dot method is more precise when rooting densities are low but when densities are high the splining method appears to be more visually helpful. These graphical methods greatly facilitate overall comparisons of sections of tree root systems, but quantification of root densities at different distances away from trees is still needed to understand more precisely the competitive effects of trees and associated crops.

Systemas de raices de los árboles

Bread wheat is grown under irrigation in Saudi Arabia. Since water resources are limited, any additional agricultural development must make use of sources of water that are less than ideal, for example, saline drainage water. A two year field study was conducted to compare the effects of irrigating bread wheat with normal water, with a 1: 1 mixture of normal water and drainage water, and with drainage water alone, at three levels of nitrogen fertilizer use. The mean electrical conductivity of the irrigation water was 2.6, 8.6 and 14.6 ds m−1, respectively, in the two seasons of the study. Irrigation with drainage water decreased yield and yield components but increased the protein content of the grain of the two wheat cultivars studied. Drainage water depressed straw yields more than it depressed the yield of grain. Nitrogen application increased yield and yield components in all the irrigation treatments, partially overcoming the adverse effects of the use of saline drainage water.

Efectos del agua salada y el nitrógeno en el trigo

The effects of levels of soil moisture and plant population on lodging and associated losses from mechanical harvesting of two lentil genotypes, one susceptible and one resistant to lodging, were investigated during 1986–87 and 1987–88 at Tel Hadya in northern Syria. High plant population increased lodging because the crowded plants had thin stems, while high levels of soil moisture caused lodging because biological yield was increased. When biological yields were high a two tonne increase in biological yield led to a one point increase in lodging score. Mechanical harvesting increased losses in biological yield due to lodging by 12% compared with yields obtained by hand harvesting under rainfed conditions and by 24% at the highest level ofsoil moisture. It is suggested that supplementary irrigation should be used in dry seasons in trials to select genotypes resistant to lodging.

Decaimiento de la lenteja

Coconut is harvested at bimonthly intervals (six picks per year) in all field experiments carried out by the Coconut Research Institute in Sri Lanka. Because recording of individual picks is costly and time consuming, it is of interest to know whether or not recording can be restricted to fewer picks without prejudicing the results of experiments. Yield data from five long-term field experiments conducted at five different locations showed that the sum of the first, third and fifth picks could provide a good estimate of the sum of six picks. Yield recording can therefore be confined to these three picks and the cost and time of recording in an experiment reduced by 50%. These three picks can conveniently be used to assess treatment effects on total annual yield.

Análisis estadístico del rendimiento de coco

A field technique to screen pearl millet genotypes for their emergence and survival at high soil surface temperatures is described. Genetic variation in seedling emergence and survival is shown and it is argued that this variation is largely due to tolerance of high temperatures rather than tolerance of soil moisture deficit, although some interaction occurred. An index for ‘thermotolerance’ is defined and genotypes are ranked accordingly for this trait, which is shown to be highly heritable. The technique is repeatable and allows a large number of genotypes to be screened at the same time.

A study conducted at Ibadan, Nigeria, showed that some community water supplies were slightly polluted by the pesticides applied on adjacent farms. There was also evidence of soil degradation as a result of agrochemical use, with differences in the physico-chemical properties of soils in fields where agrochemicals had been applied and in adjacent fields that had not been cultivated. Measures that can be taken to minimize some of the environmental dangers inherent in the application of agrochemicals are discussed.

The adaptation of extra short duration (ESD) pigeonpea (Cajanus cajan) genotypes to rainfed environments was studied on Alfisols and Vertisols at the ICRISAT Center between 1987 and 1989. Despite a slightly shorter crop duration, the grain yield of ESD genotypes was twice as large on Alfisols as on Vertisols. On both soil types, the rate of growth and grain yield were better in crops sown on time than in those where sowing was delayed. The population levels necessary to maximize yield varied among genotypes on Alfisols, where the grain yield of several ESD genotypes compared favourably with that of ICPL 87, a standard short duration genotype. However, none of the ESD genotypes yielded more than ICPL 87 on the Vertisols.