Kgabo Pofu

Managing the Phytotoxicity and Inconsistent Nematode Suppression in Soil Amended with Phytonematicides

High incidence of phytotoxicity and inconsistent results in nematode suppression of soil amended with phytonematicides had been limiting the adoption of phytonematicides in various countries. The efficacy of phytonematicides depended on allelochemicals as active ingredients, which are naturally phytotoxic. Plant species respond to increasing concentrations of allelochemicals through density-dependent growth patterns, which have three phases, namely, stimulation, neutral and inhibition, with each phase having a range of concentrations. The curve-fitting allelochemical response dosage model was used for two triterpenoid phytonematicides, nemarioc-AL and nemafric-BL phytonematicides, to develop the non-phytotoxic concentrations of the products within the stimulation phase of tomato (Solanum lycopersicum) plants. The concept called for the development of the application interval, culminating in the formulation of the application frequency and the dosage, which ameliorated the incidence of phytotoxicity. The application intervals of the derived non-phytotoxic concentrations for the products were such that the life cycle of the root-knot (Meloidogyne species) nematodes was continuously disrupted, thereby ensuring consistent results in nematode suppression in soils amended with phytonematicides.

Host-status and host-sensitivity of wild Cucumis species to Meloidogyne incognita race 4

Abstract Wild watermelon (Cucumis africanus) and wild cucumber (Cucumis myriocarpus), which are highly resistant to the southern root-knot nematode (Meloidogyne incognita) race 2, have the potential for serving as seedling rootstocks for the highly nematode-susceptible watermelon (Citrullus lanatus) cultivars. In South Africa, due to high labour costs, most cotton farmers are turning to watermelon production. In these regions, there is high incidence of M. incognita race 4. A pot experiment in the greenhouse was, therefore, initiated to investigate the host-status and host-sensitivity of Cucumis africanus and Cucumis myriocarpus to M. incognita race 4, when inoculated with 0, 200, 600, 100, 1400, 1800 and 2200 eggs and second-stage juveniles (J2s). At harvest, 56 days after inoculation, the reproductive factor values at all levels of inoculation were less than one. Penetration indices on both plant species were greater than one, suggesting that the resistance was post-infectional and therefore, introgressible. Sex (male: female) ratios of M. incognita race 4 on the two Cucumis species were greater than one, suggesting that more J2s were converted into males. Infection of Cucumis species by the test nematode had no effect on yield components of the two plant species due to poor feeding sites. The two Cucumis species were, therefore, resistant to M. incognita race 4, with post-infectional resistance, where the sex ratio was skewed towards maleness. Thus, the two Cucumis species have the potential to be used as nematode-resistant rootstocks in watermelon production.

Improving survival of inter-generic grafts of nematode-susceptible watermelon cultivars and nematode-resistant Cucumis species

Abstract Inter-generic grafting of highly nematode susceptible watermelon (Citrullus lanatus) cultivars onto nematode-resistant wild watermelon (Cucumis africanus) and wild cucumber (Cucumis myricoarpus) had on average 36% graft survival ratios, which was attributed to unequal stem diameters at the graft union. Cucumis species had smaller stem diameters at the graft union, whereas Citrullus had bigger ones. The objective of the study was to improve inter-generic graft compatibility of Citrullus and Cucumis through optimizing the sizes of stem diameters during grafting. Cucumis species were raised in a 160-hole seedling tray and primed seeds of Citrullus were raised in a 200-hole seedling tray seven days after emergence of Cucumis in order to reduce the stem diameters. At grafting, stem diameter ratios in various treatments were equal to one, whereas at 66 days after grafting the ratios were grater than one, with the exception of those of intact plants. Survival of grafts from grafting to 66 days after grafting was 100%, translating into relative improvement of 186%. Improved survival of grafts would invariably promote the potential uses of inter-generic grafting among wild and cultivated watermelon in management of soil-borne pathogens.

Density-dependent Growth Patterns of Meloidogyne javanica on Hemp Cultivars: Establishing Nematode -sampling Timeframes in Host-status Trials

The timeframe host plants should be exposed to Meloidogyne species prior to nematode sampling for host-status and/or host-sensitivity is indispensable in interpretation of results and the subsequent management decisions. The aim of this study was to use density dependent growth patterns of population density of M. javanicainfecting hemp ( Cannabis

Suitability of Brassica oleracea leaves in managing Meloidogyne incognita through the ground leaching technology system under microplot conditions

Abstract The ground leaching technology (GLT) system was introduced in low-input farming systems in response to the withdrawal of methyl bromide from agrochemical markets and the amelioration of conventional organic amendment drawbacks in managing plant-parasitic nematodes in olericulture. The primary indicator for success in GLT system is that potent nematicidal chemicals be released through leaching by water, while in conventional organic amendments potent chemicals are released through microbial degradation. Due to widespread availability of cabbage (Brassica oleracea) leaves in local markets as a waste, the material would be readily available for use in GLT systems in peri-urban agriculture. The objective of the study was to determine whether crude extracts of B. oleracea leaves were suitable for use in GLT system for managing the root-knot (Meloidogyne spp.) nematodes and improving growth of tomato (Solanum lycorpersicum). A 2×2×2 factorial experiment, with the main factors being Bacillus species, crude extracts of B. oleracea leaves and Meloidogyne incognita, was conducted over two seasons under microplot conditions. At harvest, Bacillus×Brassica×nematode interaction reduced final nematode population densities, fresh fruit yield, dry shoot mass, plant height and stem diameter, while Bacillus× Brassica interaction increased tomato growth variables. Incidentally, Brassica alone reduced the productivity of tomato. Overall, the efficacy of crude extracts of B. oleracea leaves in nematode suppression and tomato productivity was dependent upon microbial activities and therefore, the material was not suitable for use in GLT systems.

Response of mineral malnutrition elements in African ginger pseudo-stems to nematode infection

ABSTRACT Interference of nematode infection with mineral malnutrition (MMN) elements in biofortificated cultigens could devalue the health potential of such crops for humans. A pot trial was conducted to determine the response of selected MMN elements in African ginger (Siphonochilus aethiopicus) pseudo-stems to the southern root-knot (Meloidogyne incognita) nematode. Fifty-six days after inoculation, the sensitivity of the pseudo-stems to nematode infection was confirmed, with 23–45% reduced potassium in pseudo-stem tissues, but had no effect on other MMN elements. In conclusion, population densities of M. incognita race 2 should be managed to enhance the potential uses of S. aethiopicus as a biofortification crop.

Host-status of sweet potato cultivars to South Africa root-knot nematodes

ABSTRACT Globally, most of the crop-related developmental projects collapsed due to the existence of aggressive plant-parasitic nematodes. Intensive South African biofortification efforts using exotic and local sweet potato (Ipomoea batatas) cultivars have hardly considered the inclusion of nematode resistance in plant breeding, despite the withdrawal of most nematicides from agrochemical markets. The objective of this study was to screen 12 selected biofortification sweet potato cultivars for host-status to South Africa tropical root-knot (Meloidogyne species) nematodes. Three exotic orange-fleshed, three local orange-fleshed and six cream-fleshed cultivars were used in three separate experiments of M. javanica and races 2 and 4 of M. incognita. In each experiment, 6000 eggs and second-stage juveniles/plant were used for inoculation, with nematode assessment performed at 56 days after inoculation. All tested biofortification exotic (except for cv. ‘W-119’) and local cultivars were hosts to Meloidogyne species and races, whereas three non-biofortication local cultivars, namely, ‘Bosbok’, ‘Blesbok’ and ‘Mvuvhelo’ were non-hosts. In conclusion, the findings suggested the likelihood of the existence of non-host-status in certain exotic and local sweet potato cultivars for use in plant breeding programmes against South African Meloidogyne species and races.

Influence of cucurbitacin-containing phytonematicides on selected nutrient elements in leaf tissues of green bean under greenhouse conditions

ABSTRACT Cucurbitacin-containing phytonematicides consistently reduce nematode population densities in various crops. However, there is no information on how these products affect the accumulation of essential nutrient elements in crops. The objective of this study was to determine the influence of Nemafric-BL and Nemarioc-AL phytonematicides on accumulation of essential nutrient elements in leaf tissues of green bean (Phaseolus vulgaris L.) under greenhouse conditions. Weekly application of phytonematicides each at 0%, 2%, 4%, 8%, 16% and 32%, were assessed on plant growth variables, nematode suppression and nutrient elements (Ca, K, Na, Fe, Zn) in leaf tissues of green bean at 56 days after initiating the treatments. Phytonematicide concentrations significantly (P ≤ 0.05) affected nematode numbers and nutrient elements, without affecting plant growth. Calcium (R2 = 0.97), K (R2 = 0.93), Na (R2 = 0.94) and Fe (R2 = 0.91) each with increasing Nemafric-BL phytonematicide concentration exhibited positive quadratic relations. In contrast, K (R2 = 0.97) and Fe (R2 = 0.98) each with increasing Nemarioc-AL phytonematicide concentration exhibited positive quadratic relations, whereas Na (R2 = 0.92) and Zn (R2 = 0.72) each with increasing phytonematicide concentration exhibited negative quadratic relations. In conclusion, accumulation of essential nutrient elements in green bean leaf tissues and increasing phytonematicide concentrations exhibited density-dependent growth patterns prior to the eventual expression of plant growth responses.

Nematode resistance in bitter gourd to Meloidogyne incognita

Bitter gourd (Momordica balsamina L.) – indigenous to tropical areas of Africa – has nutritional, ethnomedicinal, neutraceutical and pesticidal attributes which are potentially attractive to various industries with bias towards rural development. However, the southern root-knot nematode (Meloidogyne incognita, Kofoid & White, Chitwood) and its biological races constitute serious limitation to successful introduction of alternative crops with economic attributes, particularly when the target plants have limited degree of nematode resistance. A pot trial was therefore conducted to determine the degree of nematode resistance in M. balsamina seedlings to M. incognita race 2. Seedlings were inoculated at seven levels of M. incognita race 2, which were laid out in a randomised complete block design, with 12 replications. After three nematode generations, the mean reproductive factor (RF = Pf/Pi) values at all levels of inoculation were less than unity, while nematode infection had no effect on growth of M. balsamina seedlings, which suggested that this plant was resistant to the test nematode. In conclusion, due to its resistance to M. incognita race 2, M. balsamina could serve as an alternative crop for use as vegetable, ethnomedicinal and/or neutraceutical plant in rural communities with high populations of M. incognita race 2.

Host response of Capsicum frutens cultivar ‘Capistrano’ to Meloidogyne incognita race 2

Abstract Host resistance in managing the southern root-knot nematode (Meloidogyne incognita) is limited by the existence of races of this nematode species. Pepper (Capistrano frutens) cultivar ‘Capistrano’ released for resistance against M. incognita races 3 and 4 in Central and South America, was evaluated for host-status and host-sensitivity to M. incognita race 2 in Limpopo Province, South Africa. Plants in pots uninoculated or inoculated with nematodes were arranged in a completely randomized design, with 15 replications. At 120 days after inoculation, the reproductive factor was less than unity, while nematode infection had no effect on plant growth. Thus, the cultivar is resistant to M. incognita race 2 and could be included in crop rotation systems intended to suppress population densities of M. incognita race 2.