DeceasedThe area planted to soybean in South Africa has increased by 54% since the 2009 growing season, mainly as a result of the increasing demand for protein-rich food and fodder sources. Moreover, the introduction of advanced technology, namely the availability of genetically modified herbicide tolerant soybean cultivars also contributed towards increased soybean production. The omnipresence of plant-parasitic nematodes in local agricultural soils, however, poses a threat to the sustainable expansion and production of soybean and other rotation crops. Meloidogyne incognita and M. javanica are the predominant nematode pests in local soybean production areas and those where other grain-, legume-and/or vegetable crops are grown. The lack of registered nematicides for soybean locally, crop production systems that are conducive to nematode pest build-ups as well as the limited availability of genetic host plant resistance to root-knot nematode pests, complicate their management. Research aimed at various aspects related to soybean-nematode research, namely, audits of nematode assemblages associated with the crop, identification of genetic host plant resistance in soybean germplasm to M. incognita and M. javanica, the use of molecular markers that are linked to such genetic resistance traits as well as agronomic performance of pre-released cultivars that can be valuable to producers and the industry are accentuated in this review. Evaluation of syntheticallyderived as well as biological-control agents are also discussed as complementary management tactics. It is important that lessons learned through extensive research on soybean-nematode interactions in South Africa be shared with researchers and industries in other countries as they might experience or expect similar problems and/or challenges.
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 oí 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 dactyton 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.
Meloidogyne incognita, a predominant nematode parasite of soybean in South Africa, increasingly threatens production of the crop as it is expanding to maize-producing areas infected by this nematode. The parasitic relationship between M. incognita and soybean were compared on a susceptible and a resistant cultivar in terms of nematode penetration, development, reproduction and fecundity as well as histopathology studies. Second-stage juveniles (J2) of M. incognita were inoculated on roots of a resistant (LS5995) and a susceptible (Prima2000) cultivar in three concurrent but separate glasshouse trials. For pre-infectional studies, root systems of plants were harvested 2, 4, 10, 16 and 20 DAL Sampling times for the post-infectional experiment were 4, 10, 20 and 30 DAI, whilst those for the histopathology experiment were 2, 4, 10, 20 and 30 DAL J2 penetrated roots of both cultivars in comparable numbers 2 DAI but vermiform J2 numbers were significantly lower in roots of LS5995 at 4, 16 and 20 DAL Final (Pf) J2 population density (vermiform plus swollen individuals) in roots of Prima2000 was significantly higher at all sampling times than those in roots of LS5995. Development of M. incognita J2 to third-(J3) and fourth-stage juveniles (J4) was significantly affected by cultivar susceptibility and time, being slower in LS5995. Development of J2 to J3 and J4 or into mature females was also consistently slower in LS5995 for the duration of this experiment. Adult females in roots of Prima2000 produced significantly more (98%) eggs per egg mass and also maintained significantly more egg and J2 numbers (98.5%) per root system 30 DAI than those in roots of LS5995. Histopathological observations showed that J2 penetrated roots of both soybean cultivars and migrated intercellularly to undifferentiated provascular tissue 2 and 4 DAI, with pronounced cellular changes taking place. A hypersensitive reaction was observed 2 DAI in roots of the resistant cultivar. From 10 to 30 DAI giant cell formation in the differentiated vascular tissue in the roots of LS5995 differed substantially from those in roots of Prima2000. Giant cells that developed in roots of the LS5995 were smaller and fewer compared to those in Prima2000. Giant cells in roots of LS5995 also contained empty as well as sub-optimal giant cells with thicker cell walls than those reported for resistant soybean cultivars in earlier studies. The induction, development and maintenance of giant cells in LS5995 proved to be typically retarded.
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