A.H. McDonald

Identification of root-knot nematode species occurring in South Africa using the SCAR-PCR technique

Species of root-knot nematodes occurring in South Africa (Meloidogyne fallax, M. chitwoodi, M. javanica, M. incognita, M. arenaria and M. hapla) were identified and differentiated by the sequence characterised amplified region - polymerase chain reaction (SCAR-PCR) technique. Multiplex internal transcribed spacer (ITS)-PCR amplified a fragment in an unknown species for which no SCAR-PCR marker was available. M. fallax, a quarantine organism in Europe, was recorded for the first time in South Africa. The geographical distribution of M. chitwoodi is extended to four localities in South Africa. The SCAR-PCR technique reliably determined species composition, detecting mixed populations of M. hapla, M. fallax and M. chitwoodi on groundnut and of M. arenaria and M. incognita on Impatiens spp. In contrast, single species populations of M. javanica and M. incognita and the unknown species were detected in glasshouses.

Plant-parasitic nematodes in field crops in South Africa. 6. Soybean.

Twelve plant-parasitic nematode genera and 25 species were identified in soil and root samples collected from 17 localities in the soybean production areas of South Africa during the 1995/96 season. Predominant endoparasites found included Meloidogyne incognita , M. javanica , M. hapla , M. ethiopica , Pratylenchus zeae and P. brachyurus . Meloidogyne species occurred in 91% of all root samples, P. zeae in 87% and P. brachyurus in 33%. The occurrence of the three predominant endoparasites was not restricted to sandy soil, but they were also prevalent in soils with high clay contents. Other endoparasitic nematode species were Pratylenchus crenatus , P. teres , P. neglectus , P. thornei , Rotylenchulus parvus and Ditylenchus africanus . Nematodes found in soil included Helicotylenchus dihystera and Scutellonema brachyurus , which occurred in 78 and 71%, respectively, of all soil samples. Other nematodes found in soil samples were Helicotylenchus pseudorobustus , H. digonicus , H. microcephalus , Scutellonema truncatum , Rotylenchus unisexus , Mesocriconema sphaerocephalum , Paratrichodorus minor , Longidorus pisi , Xiphinema vanderlindei , X. elongatum and Tylenchorhynchus goffarti . Highest prominence values (PV) for Meloidogyne species were recorded on cultivars Prima, Bakgat, Tamboti, Hutcheson and Knap and the lowest on cultivars PAN812, SCS1, Nyala, Gazelle and A7119. Eight nematode genera and 19 species extracted during this study are reported for the first time in association with soybean in South Africa.

Host suitability of South African and foreign soybean cultivars to Meloidogyne Incognita race 2

Eighty-five local and foreign soybean (Glycine max (L.) Merr.) genotypes, including cultivars and breeding lines, were evaluated for host suitability to Meloidogyne incognita race 2 in two separate greenhouse experiments. In vivo reared M. incognita race 2 second-stage juveniles (J2) and eggs were inoculated on roots of soybean seedlings. Nematode egg and egg mass counts were made 56 days after inoculation (DAI). The number of egg masses as well as eggs per root system were counted, while reproduction factors (RF), egg-laying-female indices (ELF) and eggs per g of root were determined. Substantial variation existed among the soybean cultivars and breeding lines with regard to the parameters evaluated. Several local as well as foreign cultivars and breeding lines were identified with RF-values lower than 1 for M. incognita race 2. The commercial local soybean cultivar LS5995 had the lowest number of egg masses and eggs per root system, eggs per g of root, RF-values and ELF-indices in the present study. Based on RF-values this is the first report that LS5995 is resistant to M. incognita race 2. Results obtained in the present study could be used for the planning of crop rotation systems as well as the identification of resistance sources for breeding purposes.

Comparative cellular responses in susceptible and resistant soybean cultivars infected by Meloidogyne incognita

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 DAI. 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 DAI. 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 DAI. 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.

Host and yield responses of soybean genotypes resistant or susceptible to Meloidogyne incognita in vivo

Host and yield responses of soybean genotypes resistant to Meloidogyne incognita were verified in semi-field and field trials at four different localities in South Africa. The resistant cultivar LS5995 and susceptible cultivars Prima and Prima2000 were used and each trial consisted of two treatments: fumigated (control) and nematode-infected plots. The number of root-knot nematode eggs and J2 in roots of nematode-infected plants was significantly higher in Prima and Prima2000 compared with those of the resistant genotypes in all trials. Low root and soil nematode population densities were consistently recorded for LS5995 in all trials. Yield of the resistant genotypes did not differ significantly between the fumigated and the nematode-infected plants for all trials, while yields of the susceptible Prima and Prima2000 grown in fumigated plots were significantly higher than those in nematode-infected plots for one microplot as well as one field trial. LS5995 is no longer commercially available in South Africa but is used as a superior source of M. incognita-resistance in breeding programmes for introgression of resistance into high-yielding, locally adapted soybean cultivars.

Nematode Pests of Maize and Other Cereal Crops

Maize is the most important cereal crop grown in South Africa, followed by wheat and grain sorghum. Yields of grain crops usually vary considerably between years, mainly due to highly variable rainfall. The most widespread and frequently encountered nematode pests of maize are root-knot nematodes, with Meloidogyne incognita and M. javanica dominating. Lesion nematodes follow, with Pratylenchus zeae generally being the predominant of a range of species from this genus. However, other plant-parasitic nematodes of the families Criconematidae, Dolichodoridae and Hoplolaimidae and species of Longidorus, Nanidorus, Paratrichodorus and Xiphinema have also been recorded from maize-producing areas. The response to treatment with nematicides in field experiments generally has not given consistent results, due in part to fluctuating rainfall. As a result, nematodes are not considered by maize farmers as a primary yield constraint. In contrast with an extensive body of information being available for maize nematodes, limited nematology research has been done on other cereal crops. The most commonly occurring nematode pests associated with barley, grain sorghum, millet, wheat and rice are also referred to in this chapter.