Plant-parasitic nematodes associated with weeds in subsistence agriculture in South Africa

Twenty-nine plant-parasitic nematode species belonging to 15 genera, as well as two fungus-feeding nematode genera and one species, were identified from soil and root samples of weeds collected from 67 localities situated in subsistence production regions of South Africa during a survey from 2005-2006. Of these, 16 nematode species from four genera were reported for the first time to infect or to be associated with weeds. The predominant endo- and semi-endoparasites identified both from soil and root samples were Meloidogyne species, Pratylenchus zeae, Helicotylenchus dihystera and Rotylenchus unisexus. Based on PCR technology, three Meloidogyne species, M. hapla, M. incognita and M. javanica, have been identified in roots of weeds sampled. Thirty weed species representing 26 genera were identified as hosts of Meloidogyne species, while 38 belonging to 35 genera were associated with various other plant-parasitic nematodes. Meloidogyne species were predominant in the roots of Chloris virgata, Cynodon dactylon and Amaranthus hybridus. The frequency of occurrence of Meloidogyne species, H. dihystera and R. unisexus was significantly and positively correlated with localities in terms of % clay and % silt. The same trend was evident for these three taxa in terms of rainfall, while the opposite was observed in terms of their frequency of occurrence and pH. Principal component analysis results substantiated the latter. It also suggested that all four predominant plant-parasitic nematode species tend to occur more frequently at localities where maize was mono-cropped as well as where maize and vegetables were planted in rotation. Cynodon dactylon, Cyperus rotundus, Datura stramonium, Tagetes minuta and Eleusine corocana had the highest frequency of occurrence of weeds sampled. Identification of weeds that maintain plant-parasitic nematodes, particularly Meloidogyne species, suggested that these pathogens can have a negative impact on crop production when not controlled timely and effectively.