P. W. Mashela

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.

Principal agronomic and seed oil traits in the industrial oil crop vernonia (Centrapalus pauciflorus var. ethiopica)

Vernonia (Centrapalus pauciflorus) is potentially a new crop that could offer naturally epoxidised industrial oil. The objective of this study was to apply principal component analyses on agronomic traits, seed oil and fatty acid compositions in vernonia to identify the most influential and representative characters for effective breeding. Field evaluations, and oil and fatty acid analyses were conducted using 36 diverse accessions of C. pauciflorus var. ethiopica. Two principal components (PC) were determined contributing 79.26% of the variation among the agronomic traits. PC1 explained 59.71% of the variation represented by the numbers of productive primary seed heads and seed yield. PC2 contributed at 19.55% highly correlated with the number of productive secondary heads. The principal component analysis on seed oil and fatty acid compositions allocated three components explaining 75% of the variation. PC1, PC2 and PC3 contributed 41%, 19% and 15%, respectively, to the total variance. The principal traits were seed oil content, palmitic acid and stearic acid in PC1, oleic acid in PC2 and arachidic acid in PC3. The selected traits may help in developing appropriate and effective breeding strategies for the improvement, large-scale production and germplasm conservation of vernonia.

Optimizing application frequency of diluted (3%) fermented Cucumis africanus fruit in tomato production and nematode management

Abstract A microplot experiment was conducted to determine application interval of fermented crude extracts of wild watermelon (Cucumis africanus) fruit for optimum growth of tomato (Solanum lycopersicum) plants and suppression of the southern root-knot (Meloidogyne incognita) nematode. Four-week-old tomato cultivar ‘Floradade’ seedlings were each inoculated with 5000 second-stage juveniles of M. incognita race 2 with 3% dilution of fermented crude extracts applied through drip irrigation at 1, 2, 3 and 4 times per 30-day month. At 56 days after the treatment, plant variables and increasing application frequencies had quadratic relationships, with the model explaining 63–99% total treatment variation in variables. Mean integrated optimum application frequency was 2.4 weeks, which translated to an optimum 17-day application interval. At this interval, final nematode population densities were low. In conclusion, at 3% dilution fermented crude extracts from C. africanus fruit could be applied at 17-day interval for interruption of the nematode life cycle and improvement of tomato plant growth.

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.

Nemafric-BL phytonematicide induces egg hatch inhibition in Meloidogyne incognita

ABSTRACT Contribution of egg hatch inhibition in the overall reduction of Meloidogyne incognita numbers by Nemafric-BL phytonematicide was determined in vitro using a series of water-diluted phytonematicide solutions. Hatched juveniles were assessed at four different incubation periods. Thereafter, each phytonematicide solution was water-diluted 5 times and incubated for 5 days to evaluate the reversibility of egg hatch inhibition. At all exposure periods, except the shortest (24 h), concentration effects on egg hatch were highly significant with juvenile numbers versus increasing concentrations exhibiting quadratic relations. The models for the three consecutive exposure periods explain the quadratic relations by 95%, 94% and 98%, respectively. Results suggested that egg hatch inhibition was one of the mechanisms involved in reducing M. incognita population densities by Nemafric-BL phytonematicide.

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.

Soil allelochemical residue effects in a tomato cowpea rotation – nodulation and productivity of cowpea and nematode suppression

Soil allelochemical residue (SAR) effects from nemarioc-AL phytonematicide in a tomato–cowpea (Vigna unguiculata) succession system were investigated on nodulation of Bradyrhizobium japonicum, productivity of cowpea and suppressive capabilities of Meloidogyne species. Following a tomato (Solanum lycopersicum) crop which was subjected to five concentrations (CCs) and five application interval (AI) levels of nemarioc-AL, the cowpea var. Eureka was raised on spots previously occupied by tomato plants, without reapplying the treatments. SAR effects due to CC × AI consistently inhibited B. japonicum nodulation in 96% factorial pairwise combinations, while CC alone increased soil pH and seed yield, but reduced Meloidogyne incognita numbers. Similarly, AI alone increased dry shoot mass and reduced Meloidogyne species. Overall, at high CCs and repeated AIs, cowpea var. Eureka was not suitable for immediate use as successor crop in botinemagation systems where Meloidogyne species were managed using nemarioc-AL phytonematicide in tomato production.

Soil allelochemical residue effects of nemafric-BL and nemarioc-AL phytonematicides on soil health, growth of sweet sorghum and Meloidogyne species

A field trial was initiated to investigate the post-treatment effects of soil allelochemical residue (SAR) of nemafric-BL and nemarioc-AL phytonematicides on soil health, growth of sweet sorghum (Sorghum bicolor) and population densities of Meloidogyne species. SAR conditions were created in tomato (Solanum lycopersicum) trials, with treatments comprising untreated control, nemafric-BL 3%, nemafric-BL 6%, nemarioc-AL 3% and nemarioc-AL 6%, which were applied at empirically established intervals for a 56-day crop cycle. Soil preparation was followed by a 30-day fallow to allow for microbial decomposition of root remnants. Sweet sorghum seeds were sown on spots where tomato plants grew and soil samples were collected for determining the soil health status at two-leaf stage. At 80 days after sowing, SAR effects improved soil health and contributed 8% in total treatment variation (TTV) of chlorophyll and sugar contents each in sweet sorghum, with nemarioc-AL 3 and 6% phytonematicides stimulating improvement of the two variables. Nemafric-BL SAR effects had no effect on growth of sweet sorghum. In contrast, SAR effects contributed 76 and 39% in TTV of nematode population densities in root and soil, respectively. SAR effects of nemafric-BL and nemarioc-AL inhibited nematode numbers by 67–97% and 64–80%, respectively. In conclusion, SAR effects on soil health, plant growth and nematode population densities were desirable, suggesting that the two products were each still active in the soil.

Alternative nematode management strategies

In South Africa, about 48 % of the people live in rural areas. A large portion (35 %) of this rural population lives below the poverty line. Most of these communities depend on the production of grain, leguminous and vegetable crops, mainly in household or communal gardens, as their main food source. Available land is often limited and, therefore, frequently reused, which aggravates soil disease and pest problems and soil degradation. In agricultural production more than 10 % of the crop yields can be lost due to diseases and pests. However, in rural areas this percentage is much higher. Diseases and pests, including plant-parasitic nematodes, can even cause crop failures. In general, root-knot nematodes (Meloidogyne spp.) are the most abundant and damaging nematode pests in local smallholding farming. To alleviate the nematode problem and secure food production in such a way that it is affordable for smallholding farmers, alternative low-input, cost-effective and environmentally friendly nematode management strategies need to be developed. In South Africa, research related to such strategies for smallholding farming is mainly focused on the discovery of local botanical nematicides (phytonematicides) and their use as soil amendments to manage root-knot nematodes. Also the use of natural sources of resistance or tolerance, the application of crop rotation and intercropping, the use of organic amendments and the use of cover crops as biofumigants are being investigated.

Suitable organ(s) in wild Cucumis africanus for the production of nemafric-BL phytonematicide

ABSTRACT Cucurbitacin B in fruit of wild watermelon (Cucumis africanus) is an active ingredient in products that range from pesticides to various medicines. Reports suggested that cucurbitacins occur in all parts of C. africanus, without specifying whether they are equally distributed or not. Should the active ingredient not be equally distributed in all organs, it would not be prudent to combine the organs for those products that are produced from raw organs of C. africanus plants, like the phytonematicides. A study was, therefore, conducted to determine whether cucurbitacin B was equally distributed in all organs of C. africanus plants. Cucurbitacin B was quantified from oven-dried organs (52°C for 72 h) using isocratic elution Shimadzu HPLC Prominence. Concentration of cucurbitacin B was higher in fruit than in other organs, where the concentrations were not different. Relative to cucurbitacin B concentration in fruit, leaves, roots and vines had 94%, 70% and 67% less cucurbitacin B concentrations, respectively. In conclusion, leaves, roots and vines would introduce the dilution effect when combined with fruits in product development and should therefore not be included.