Association and impact of nematodes on weeds and leafy vegetables : with reference to Meloidogyne
Ntidi, Keikantsemang Nancy
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Although some African countries have attained some successes in addressing food insecurity during the past 20 years, acceptable levels of nutrition in humans still pose a challenge. African leafy vegetables (ALVs) are increasingly recognized as valuable sources of both micronutrients and bioactive compounds to contribute towards the diets of humans, especially in Africa. Since the 1980s, the value of Amaranthus spp. as a food crop has been recognised mainly due to its resistance to heat, drought, diseases and pests. However, a gap still exists regarding extensive studies being conducted on the occurrence of plant-parasitic nematodes and particularly Meloidogyne spp. in fields/gardens where ALVs are grown. Hence, knowledge on the effects of root-knot nematodes (Meloidogyne spp.) on the production of ALVs is scanty. Amaranthus spp. could be an easy and cost-effective way of addressing malnutrition, contributing towards rural people’s health, food security and household income through research and development. Production of ALVs is, however, hampered not only by diseases, but also by pests and in particular plant-parasitic nematodes of the genus Meloidogyne. Root-knot nematodes are reported as one of the biggest constraints of agri- and horticultural producers. The predominant Meloidogyne spp. that damage crops in South Africa are generally M. incognita and M. javanica. However, together with the latter species, M. arenaria and M. hapla, are the four economically most important species world-wide that cause damage to crops, while M. enterolobii is advocated as one of the emerging threat species. The long-term aim of this study was to determine the association and impact of nematode assemblages on indigenised Amaranthus spp. in developing agriculture in South Africa, with special reference to the Meloidogyne spp. complex. Furthermore, the ultimate goal was to identify a resistant or poor-host Amaranthus genotype to be used by smallholding farmers to alleviate root-knot nematode problems they experience. The technical part of this study focussed on assessing the host status of various weeds, including ALVs, to M. incognita and M. javanica in glasshouse and on-farm field experiments (see Chapter 2). Greenhouse studies showed Hibiscus trionum and Amaranthus tricolor were the most susceptible weed species while, Chenopodium carinatum and Datura ferox were the poorest hosts for M. incognita and M. javanica. With regard to the field experiments, Solanum retroflexum was the most susceptible weed to a M. javanica populations at Kuruman. Hibiscus trionum was the most susceptible weed species at both Mbombela and Potchefstroom, where a mixed (M. incognita and M. javanica) and single-species population of M. incognita occurred. The second objective was aimed at determining the association of plant-parasitic nematodes with weeds, including Amaranthus spp. This was done by conducting a survey at 10 localities situated in four provinces of South Africa where Amaranthus cruentus accession Arusha was grown as a food source (See Chapter 3). Part of this study entailed the identification (morphological, morphometric and molecular) of Meloidogyne spp. as well as that of the other plant-parasitic nematode species present in root and rhizosphere soil samples of accession Arusha. Eleven plant- parasitic nematode genera and 12 species were identified from rhizosphere soil and roots of accession Arusha. The predominant plant-parasitic nematode genus in root samples were Meloidogyne spp. (viz. M. incognita and M. javanica), followed by Helicotylenchus dihystera and Pratylenchus zeae. The third objective was to determine the host status of various Amaranthus genotypes to M. incognita and M. javanica in glasshouse experiments, followed by assessing the effect of varietal initial inoculation densities (Pi) the same root-knot nematode species on a good- (Arusha) and poor-host (Thohoyandou) accession (see Chapter 4). Four accessions (Oyaya, Red Stem, Local 33 and Thohoyandou) were identified as resistant to M. incognita with Rf <1, while for M. javanica only Red Stem and Thohoyandou were resistant. Reproduction parameters were significantly lower for all Pi levels for M. incognita compared to that of accession Arusha for both root-knot nematode species. Leaf- and root-mass data were inconsistent and warrants no further discussion. The fourth objective was screening of 13 Amaranthus genotypes at the same three on-farm sites where weeds were evaluated during a similar, former study on weeds (see Chapter 2). Eight plant-parasitic nematode genera were identified from rhizosphere soil and roots of Amaranthus genotypes from the three sites with Meloidogyne spp. being the predominant nematode species in both root and soil samples. Amaranthus acessions Thohoyandou, Local 33, ACAT Seedfair and Oyaya maintained relatively low population densities of Meloidogyne spp. at the different sites. The most susceptible genotypes at all sites were accessions Arusha, Joseph’s Coat, Bosbok Thepe, ex Botswana and Applebosch. Results from this study indicated that certain weed species are resistant and/or poor hosts (C. carinatum, D. ferox and Amaranthus accessions Thohoyandou, Local 33, ACAT Seedfair, Red Stem and Oyaya) to the two Meloidogyne spp. used in this study, while other were highly susceptible (H. trionum, S. retroflexum and Amaranthus accessions Arusha, Joseph’s Coat, Bosbok Thepe, ex Botswana and Applebosch). Therefore, production of susceptible ALVs may exacerbate the nematode problem that is experienced in local areas where smallholding farmers practise agriculture. Another outcome of this study is that several parameters should be used to identify resistance or the poor host status of plant species. This will optimise the use of root-knot nematode resistant and/or poor-host gentoypes by developing farmers, who generally have limited and marginal land available to grow food crops repetitively in fields where highly susceptible weeds (such as H. trionum, A. tricolor and others) grow simultaneously. Root-knot nematode problems in smallholding agricultural systems might thus be exacerbated when such susceptible weeds or ALVs are not removed timely and effectively. Also of importance is that the existing erroneous view that T. minuta are poor hosts to Meloidogyne spp. should be addressed by informing producers of the potential of this particular weed species to support relative high population densities of this nematode genus. Ultimately, results proved that the root-knot nematode resistant accession, Thohoyandou, was able to reduce population levels of both M. incognita and M. javanica substantially. However, accurate identification of the prevailing Meloidogyne spp. in fields of farmers should be known before this knowledge will benefit producers and can contribute to food security and sustainable crop production.