| dc.description.abstract | Root-knot nematodes (Meloidogyne spp.) parasitize a wide range of agri- and horticultural crops worldwide, causing yield and quality losses. Meloidogyne arenaria, M. hapla, M. incognita and M. javanica generally are the four economically most important species that globally cause damage to crops, while M. enterolobii is advocated as one of the emerging threat species. The first aim of the study was to identify Meloidogyne spp. that occurred in 28 populations and were isolated from roots of crop plants received for diagnostic analyses and from research sites across six provinces of South Africa. This was done using morphological and molecular approaches. The second aim was to determine the pathogenicity of 11 selected Meloidogyne spp. populations identified during this study in a greenhouse trial. Deoxyribonucleic acid (DNA) was extracted from 20 mature, egg-laying females obtained from roots of crop plants that represented each of the 28 populations and subjected to the sequence-characterised amplified region (SCAR) - polymerase chain reaction (PCR) analyses. Phylogenetic analysis of the 28 populations was also done. The DNA bands of Meloidogyne spp. were compared to that of standard species (M. arenaria, M. chitwoodi, M. enterolobii, M. fallax, M. hapla, M. incognita and M. javanica respectively), that have been identified earlier and their identity hence confirmed, to ensure accurate results. In terms of the morphological identifications, various morphological characteristics (e.g. perineal patterns, shape of the lumen of the esophagi, shape of stylet knobs, presence of phasmids near tail terminus) as well as one morphometric feature (length of vulval slit) of 18 mature females were recorded. For the pathogenicity study, approximately 1 000 eggs and second-stage juveniles (J2) of the 11 selected Meloidogyne spp. populations were inoculated onto roots of a susceptible tomato cultivar (Rodade). Nematode parameters assessed 56 days later included egg-laying female (E.L.F.) indices, egg and J2 numbers and reproduction factors (Rf) / root system. Three (M. arenaria, M. incognita and M. javanica) of the four economically most important Meloidogyne spp. as well as the emerging M. enterolobii (= M. mayaguensis) have been identified as a result of both molecular and morphological identifications. None, of the Meloidogyne sp. that generally occur in colder areas (M. chitwoodi, M. fallax and M. hapla) and which have been reported earlier for South Africa, were identified during this study. An 82% similarity level was obtained when results from the molecular and morphological identification approaches were compared. Both identification interventions resulted in characterisation of the four Meloidogyne spp. contained within monoculture as well as mixed populations. Meloiodgyne incognita dominated and was present in roots of guava, maize, potato, soybean and sunflower. Meloidogyne javanica followed and was isolated from roots of guava, green pepper, maize, potato and sunflower. An important result that emanated from these research activities was the presence of the third ranked M. enterolobii, present in roots of guava, green pepper and potato. The fourth rank in terms of dominance was represented by M. arenaria which was contained in roots of maize only. Phylogenetic analysis of the 28 populations resulted in two major clusters that separated Meloidogyne spp. populations of M. enterolobii and M. javanica (as well as mixed populations of these two species and M. incognita) from those containing monoculture M. arenaria and M. incognita populations as well as complexes containing these two species. This result is interesting and warrants further investigation. Aggressiveness of the 11 selected Meloidogyne spp. populations differed substantially within and among species. The most aggressive population with the highest Rf of 203 was represented by a monoculture M. javanica population (obtained from potato roots), while a monoculture M. enterolobii population isolated from guava roots where the least aggressive (Rf = 18). Interestingly, the 2nd, 3rd and 4th most aggressive populations constituted mixed populations that contained combinations of M. enterolobii, M. incognita and M. javanica. Positive identification of M. enterolobii, which has been and still is easily confused with M. incognita in terms of its morphological identification, emanating from this study will contribute towards research aimed at studying the distribution, life cycle and pathogenicity of this emerging pest. The presence of M. arenaria in local maize production areas is also interesting and necessary to be considered when planning nematode management strategies. Knowledge generated on the aggressiveness of 11 Meloidogyne spp. populations also adds valuable and useful information that researchers and farmers can use to plan and construct management strategies to combat these pests in local crop production systems. Research related to this project’s aims is ongoing and will contribute towards baseline studies on the presence and incidence, pathogenicity and phylogeny of M. enterolobii as well as other economically important root-knot nematode pests. | en_US |
