George M. Kariuki

Mitochondrial haplotype-based identification of ethanol-preserved root-knot nematodes from Africa.

The asexual root-knot nematodes (RKNs) (Meloidogyne spp.) exemplified by Meloidogyne incognita are widespread and damaging pests in tropical and subtropical regions worldwide. Comparison of amplification products of two adjacent polymorphic regions of the mitochondrial genome using DNA extracts of characterized RKN strains, including 15 different species, indicate that several species are derived from the same or closely related female lineages. Nevertheless, M. javanica, M. enterolobii, M. incognita, and other key species could each be assigned unique mitochondrial haplotypes based on polymerase chain reaction fragment size and restriction cleavage patterns. M. arenaria isolates did not group as a single haplotype, consistent with other reports of diversity within this species. To test the utility of this assay, we characterized ethanol-preserved samples from 103 single-species isolates from four countries in sub-Saharan Africa (Benin, Nigeria, Kenya, and Tanzania). Mitochondrial haplotypes corresponding to M. javanica and M. incognita were the most prevalent. Samples from western Africa included several instances of M. enterolobii but this species was not detected in samples from East Africa. This protocol provides progress toward a standardized strategy for identification of RKN species from small, preserved samples and a rational starting point for classifying species present in regions where previous knowledge has been limited.

Efficacy of Selected Agroindustrial Wastes in Managing Root-Knot Nematodes on Black Nightshade in Kenya

Black nightshade is commercially cultivated in Kenya as a source of nutrition and income to the rural populations. Besides insect pests, root-knot nematodes (RKN) are important production constraints of this vegetable. Little information is available on the efficacy of Tithonia diversifolia Hemsl (TD) and agro-industrial wastes of pyrethrum marc (PM) and tea residue (Tres) on RKN. Greenhouse experiments were conducted to determine the optimum levels of amending soils with TD, PM, vegetable waxy resins (VWR), Tres and cattle manure (CM) at their respective rates for management of RKN. The amendments were incorporated into the soil 14 days before sowing the seeds. Thereafter 21-day-old seedlings were inoculated with ten egg-masses, with four replications arranged in randomized complete block design. Plant growth and disease parameters were assessed and subjected to ANOVA. Disease severity and population reduced significantly at levels 2 and 3 for most amendments with the highest top biomass recorded in CM, TD and PM. Higher levels of Tres and VWR caused stunting and reduced biomass. Lower severity occurred in VWR, Tres, PM, and CM at 20%, 28.0, 9.0, and 9.0 g/kg soil, respectively, with reproduction ranging from 0.3 to 3.7 recorded on amended soils with Tres at 32.0 g/kg, TD at 8 g/kg soil and CM at 9.0 g/kg. Higher yields, lower severity and reproduction were found on soil with PM, Tres, TD, CM and VWR at 9.0, 28.0, 8.0, 9.0 g/kg soil, and 20.0%, respectively. These amendments are alternatives in ecofriendly management of RKN and other plant parasitic nematodes.

The Impact of Suppressive Soils on Plant Pathogens and Agricultural Productivity

Soil is a primal element of agricultural production. It consists of complex mix of organic and inorganic matter. Suppressive agricultural soils exist globally and the biological basis of suppressiveness has been depicted for majority of the soils. However, varied land uses and management have a great impact on them, as well as their production system sustainability. Abiotic factors have a great impact on soil suppressiveness. For instance, the soil pH, level of calcium, nitrogen form, and the availability of other nutrients in the soil play key functions in the management of diseases. Sufficient crop nutrition renders plants more resistant to or tolerant of disease. A number of commercial products consisting of beneficial, disease-suppressive organisms such as Flavobacterium spp., Trichoderma spp., Gliocladium spp., Streptomycetes spp., Pseudomonas spp., and Bacillus spp. have been reported. These products are applied through various ways including seed treatments, soil inoculation, compost inoculation, and soil drenches. They have plant growth-promoting rhizobacteria (PGPR) that colonize plant roots and trigger plant growth and/or decrease occurrence of plant disease. These PGPR serve as aggressive colonizers, plant growth stimulators, and biocontrol. This chapter describes how suppressive soils affect or influence plant pathogen suppression in the soil and how they contribute to agricultural productivity. Different types of suppressive soils which include fungi-suppressive soils, bacteria-suppressive soils, and nematode-suppressive soils have also been discussed highlighting the contribution of these types of soils to agricultural productivity.