Hendrika Fourie

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.

Indications of variation in host suitability to root knot nematode populations in commercial tomato varieties

The host suitability of 21 local, commercial tomato varieties were evaluated in concurrent greenhouse trials for resistance to Meloidogyne incognita race 2 and Meloidogyne javanica, respectively. M. incognita race-2-resistance identified in variety ‘Rhapsody’ during the latter study was subsequently verified in a follow-up microplot trial using differential initial population (Pi) densities and as well as in a field trial with four soil amendments. Substantial variation existed among the tomato varieties in the greenhouse screening with regard to resistance to the respective root-knot nematode species. Comparison of the different indicators of resistance used for the two species showed that labelling of specific varieties as resistant should not only be based on one criterium, since it could be insufficient. Strong non-linear relationships were shown in the microplot trial between Pi and Pf in the roots of both tomato varieties but nematode reproduction was poor on the resistant ‘Rhapsody’. Significantly lower Pf in roots and J2 in soil was obtained for ‘Rhapsody’ compared to the susceptible Moneymaker. In the soil-amendment field trial, ‘Rhapsody’ also maintained significantly lower M. incognita numbers compared to ‘Moneymaker’ in all treatments. These results confirm the superior resistance of ‘Rhapsody’ to local M. incognita race-2 populations used in this study. More frequent and extensive screenings of commercial tomato material are recommended in order to provide resource-poor producers with better options for improved and sustainable yields.

Identification and validation of genetic markers associated with Meloidogyne incognita race 2 resistance in soybean, Glycine max (L.) Merr

Host-plant resistance to Meloidogyne incognita race 2 is a useful and cost-effective tool for optimising soybean yield as well as profitability. Locally no nematicide is currently registered on soybean and most crops used in soybean-based rotations are also susceptible to M. incognita race 2. The identification of molecular markers and subsequent application of marker-assisted selection (MAS) is a quick and effective way to expedite a nematode resistance breeding programme. The soybean cultivars LS5995 (resistant) and Prima2000 (susceptible) were therefore used as parents in crosses to obtain a segregating F2 mapping population for the identification of genetic markers linked to the resistance trait in LS5995. Inoculation with ca 10 000 M. incognita race 2 eggs and second-stage juveniles (J2) was performed 16 days after plant emergence. Subsequently, the F2 population was screened with a number of SSR evenly distributed throughout the genome. A major QTL was identified on linkage group (LG) M between the markers Satt201 and Satt590 accounting for 62.4% of gall index values, while a QTL explaining 80% of the variation in eggs and J2 per root system in the segregating F2 population is situated between Satt567 and Satt201. A minor QTL accounting for 37.1% of the variation in gall index values was identified between the markers Satt500 and Satt358 on LG-O and corresponds to a QTL for M. incognita race 3 resistance described in other publications. Presence of the abovementioned markers was confirmed in the progeny of a successive F6 population, as well as in a number of exotic and local soybean genotypes. Meloidogyne incognita race 2 MAS could, therefore, be used in a breeding programme using markers Satt201, Satt358, Satt487 and Satt590, which were identified and validated in this study.

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.

Greenhouse and field evaluations of commonly occurring weed species for their host suitability to Meloidogyne species

Since weeds serve as hosts for nematode pests, the host status of 20 weed species (commonly occurring in fields of developing farmers) to Meloidogyne incognita and M. javanica, respectively, were investigated. Greenhouse studies showed that seven weed species had Rf values >1 for both nematode species indicating susceptibility, while 13 had Rf values ≤ 1 indicating resistance. Greenhouse results showed that Hibiscus trionum and Amaranthus tricolor were identified as the most susceptible and Chenopodium carinatum and Datura ferox the poorest hosts for M. incognita and M. javanica. For field experiments at Kuruman, Solanum retroflexum was the most susceptible weed to a M. javanica population, while the same was evident for H. trionum at Nelspruit, where a mixed population of M. incognita and M. javanica occurred and at Potchefstroom, where a population of M. incognita was present. Results from this study indicated that certain weed species are highly susceptible to root-knot nematodes and should be removed timely and effectively to prevent population level increases of root-knot nematode pests in the fields of farmers.

A microplot study on Ditylenchus africanus population densities and damage symptoms on groundnut in relation to commercial yields

Summary – Optimum kernel quality is important at all levels of groundnut production and utilisation because of its value as a direct or processed food. Kernel quality can be reduced by many factors but in South Africa the groundnut pod nematode, Ditylenchus africanus, is responsible for severe losses in groundnut crops. The objective of this study was to evaluate in microplots the effect of D. africanus on groundnut yield in terms of commercial grading principles. Differential initial population densities (Pi) of between zero to 7000 and zero to 20000 nematodes per plant were inoculated, respectively, in two separate trials over two consecutive seasons. Significant non-linear relationships were recorded between Pi and nematode densities at harvest (Pf), between Pi and downgrading of kernels (% unsound, blemished and soiled (UBS)) and between % UBS and income earned per tonne. No relationships were found between initial or final nematode population densities and kernel fresh weight. Sufficient evidence was provided that damage by D. africanus on groundnut could be expressed in terms of commercial yield grading stipulations. Therefore, it is possible to estimate the effect of this nematode in monetary terms and to consider control measures in terms of financial gains or losses.

Host suitability of selected South African maize genotypes to the root-knot nematode species Meloidogyne incognita race 2 and Meloidogyne javanica: A preliminary study

Abstract Thirty-one commercial maize (Zea mays L.) hybrids and open-pollinated varieties (OPV’s) were screened in separate greenhouse trials with a resistant inbred line MP712W as reference genotype for host suitability to Mel-oidogyne incognita race 2 and Meloidogyne javanica. Approximately 10 000 eggs and second-stage juveniles (J2) of the appropriate root-knot nematode species were inoculated on roots of each maize seedling 10 days after plant emergence. The numbers of eggs and J2 per root system were counted, while it was also calculated g- 1 root. In addition, percentage resistance in relation to the most susceptible genotype and nematode reproduction factors (Rf) were calculated for the maize genotypes screened. Substantial variation existed among the maize hybrids and OPV’s with regard to the nematode parameters evaluated. A number of genotypes could be regarded as highly resistant to M. incognita race 2 based on the fact that they supported less than 10% of the population of this root-knot nematode species, compared to that supported by the most susceptible genotype. Several hybrids and OPV’s were identified with Rf values less than one for M. incognita race 2 and M. javanica respectively, indicating antibiosis resistance to these parasites. Screenings of maize genotypes in this study have provided a clear indication of the genetic variability within the maize genome, also with regard to susceptibility of the crop to root-knot nematodes. This substantiates the fact that maize could not be regarded as a non-host to root-knot nematodes on a generic basis, particularly in terms of commercial hybrids. It is suggested that commercial maize hybrids are screened on a continuous basis against root-knot nematodes, which would facilitate selection of hybrids that are less susceptible to both nematode species but that would perform optimally in soils conducive to root-knot-nematode infestation.

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.

Achievements and challenges (Nematology in South Africa: a view from the 21st century)

This chapter highlights the core achievements within the science of Nematology in South Africa since 1982. In addition, the editors have attempted to highlight for the benefit of future decision makers and researchers, the key challenges for the future such that the progress achieved since 1982 can be built upon in the coming decades.