Effects of improved fallows on the diversity of plant-parasitic nematodes in the agroforestry systems of Western Kenya
Kandji, Serigne Tacko
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In recent years, fast growing nitrogen-fixing shrubs of the family Fabaceae, Sesbania sesban (L.) Merr., Crotalaria grahamiana Wight & Arn. and Tephrosia vogelii Hook. f., have been intensively used in western Kenya to improve soil fertility, control soil erosion and increase crop yield. Local farmers have recognised the benefits of rotating these plant species with crops, and this has facilitated the rapid spread of this improved fallow technology in the area. Research has been conducted to understand the implications of this new land use system vis-a-vis the physical, chemical and biological characteristics of the soil. The scope of this study was to investigate the impact of this practice on the abundance, diversity and damage potential of plant-parasitic nematodes. There was a dramatic increase in nematode populations when the improved fallow shrubs were planted in agricultural lands. Spiral nematodes (Scutellonema spp. and Helicotylenchus spp.), root knot (Meloidogyne spp.) and root lesion (Pratylenchus spp.) nematodes were the most abundant species in the area. Spiral nematodes had high populations in all the improved fallows, while root knot nematodes were most abundant in the Sesbania and Tephrosia fallows. The highest populations of root lesion nematodes were found in the Crotalaria fallows and in the cultivated plots. The distribution of plant-parasitic nematodes was correlated to that of some soil physico-chemical properties of the soil, which validates the potential of nematodes as biological indicators of soil status. Nematode abundance fluctuated strongly during the year with peaks observed at the onset and in the middle of the rainy seasons and the lowest populations in dry seasons. Although all nematodes were sensitive to seasonal changes, this study showed clearly that the species had different survival strategies. For example, root lesion nematodes, Pratylenchus spp., which were dominant in the study site, showed very strong fluctuations in the year. This behaviour, however, contrasted with that of Scutellonema spp., which had fairly constant populations over the year. Planting crops and fallow trees in rows had a significant impact on the horizontal distribution of nematodes. Most species were aggregated along the rows, with higher populations in the rows located in the lower part of the slopes. Other distribution patterns such as clumping were also observed in this study, especially with root knot and spiral nematodes. The nematode communities that emerged when shrubs were planted in the fields negatively affected subsequent crops. Beans were very sensitive to root knot nematodes and suffered heavy loses in the Tephrosia soil. Maize was resistant to root knot nematodes, but was damaged by the spiral nematodes, Scutellonema spp., especially in low soil fertility. Beans were also sensitive to the spiral nematodes. This is a reminder that the interactions between plants and pest organisms such as nematodes must be considered among other criteria when choosing trees or shrubs to be planted in croplands. Since these interactions can be deleterious to crop growth (and consequently yield), a genuine understanding of the host range and the pathogenic function of the plant-parasitic nematodes occurring in the area may be a prerequisite. Neglecting the role of nematodes in the improved fallow systems in western Kenya may well jeopardize the future of agroforestry in the area.
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