Plant-parasitic nematodes associated with weeds in developing agriculture with special reference to root-knot nematodes

Abstract

Plant-parasitic nematodes are ubiquitous, soil-borne pests that cause significant damage to a wide range of agricultural crops. A variety of weeds occurring in small-scale farming systems often serve as reservoirs for these parasites and compete with crop plants for light, water and nutrients. This study focussed on the association between plant-parasitic nematodes and weeds by identifying predominant nematode species or genera as well as weed species or genera that most frequently occur in small-scale farming systems in South Africa. A nematode survey was conducted at 44 sites located in the eastern (Eastern Cape, Limpopo, KwaZulu-Natal and Mpumalanga provinces) and western (Northern Cape province) resource-poor farming regions of South Africa. The sampled areas were divided into these regions based on the substantial difference in rainfall between east (wet) and west (dry). Thirty-seven weed species and 33 genera were identified as hosts of plant-parasitic nematodes during this survey which differed substantially with regard to their frequency of occurrence in the two regions. Cynodon dactylon had the highest frequency of occurrence for both the eastern and western regions. With regard to plant-parasitic nematodes, 20 species and 12 genera were reported for the first time in South Africa to parasitise weeds. Root-knot nematodes (Meloidogyne spp.), followed by Pratylenchus zeae, Helicotylenchus dihystera and Rotylenchus unisex were generally the predominant endo- and semi-endoparasites extracted both from root and soil samples in the two regions. Meloidogyne species identified by means of molecular techniques were M. javanica and M. hapla, with the latter species generally being predominant at some sites in the eastern region, but M. javanica was predominant at some sites located in both the eastern and western regions. In terms of the association between plant-parasitic nematodes and weeds, the four predominant nematode species or genera mentioned above had the highest frequency of occurrence in root as well as soil samples of C. dactylon. They, however, differed substantially in terms of their predominance on weeds for the two regions as well as when data were pooled over the two regions. Root-knot nematodes were predominant in the predominant in the roots of Chloris virgata and Flaveria bidentis in the eastern and western regions, respectively, while the genus was predominant in roots of Bidens bipinnata when pooled over the two regions. Pratylenchus zeae was predominant in roots of Cyperus esculentus in the eastern region and in roots of Sonchus oleraceus in the western areas. Although H. dihystera was predominant in roots of Nicandra physalodes and S. oleraceus in the eastern and western regions, respectively, it was predominant in roots of Hibiscus. trionum when data were pooled over the two regions. Rotylenchus unisex was predominant in roots of N. physalodes in the eastern region, while it was predominant in roots of Bryophyllum spp. occurring in the western region. Predominance of these plant-parasitic nematodes also differed with regard to numbers extracted from the soil samples for the weed species as well as for the two regions. Although this study focussed on the association of plant-parasitic nematodes with weeds, free-living and predatory members of the families Rhabditidae and Mononchidae were also extracted from these samples. Weeds identified during this study that maintain plant-parasitic nematodes, particularly root-knot nematodes, could have a negative impact on crop production when they are not eradicated timely and effectively. This problem is of particular significance in resource-poor, subsistence-farming systems where literacy and knowledge levels are low.