Zakheleni P. Dube

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.

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.

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.

Mediterranean fruit fly on Mimusops zeyheri indigenous to South Africa: a threat to the horticulture industry

BACKGROUND Claims abound that the Transvaal red milkwood, Mimusops zeyheri, indigenous to areas with tropical and subtropical commercial fruit trees and fruiting vegetables in South Africa, is relatively pest free owing to its copious concentrations of latex in the above-ground organs. On account of observed fruit fly damage symptoms, a study was conducted to determine whether M. zeyheri was a host to the notorious quarantined Mediterranean fruit fly (Ceratitis capitata). RESULTS Fruit samples were kept for 16-21 days in plastic pots containing moist steam-pasteurised growing medium with tops covered with a mesh sheath capable of retaining emerging flies. Microscopic diagnosis of the trapped flies suggested that the morphological characteristics were congruent with those of C. capitata, which was confirmed through cytochrome c oxidase I (COI) gene sequence alignment with a 100% bootstrap value and 99% confidence probability when compared with those from the National Centre for Biotechnology Information database. CONCLUSION This study demonstrated that M. zeyheri is a host of C. capitata. Therefore, C. capitata from infestation reservoirs of M. zeyheri fruit trees could be a major threat to the tropical and subtropical fruit industries in South Africa owing to the fruit-bearing nature of the new host.

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.

Identification of spider mites from Moringa oleifera using molecular techniques

Moringa oleifera plants are highly nutritious and had since attracted much attention for serving as alternative crops in marginal communities in developing countries. Generally, claims abound that moringa plants are pest-free, with ISO precluding the use of unregistered products. However, moringa seedlings are often attacked by an unidentified aggressive spider mite (Tetranychus species) under humid greenhouse conditions, resulting in outright withering of seedlings, with the potential of decimating an entire moringa orchard. Thus, molecular techniques were used to identify the spider mites on moringa to the species level in order to allow for the development of management strategies. Spider mite strains from moringa and adjacent sweet stem sorghum (Sorghum bicolor) seedlings were collected and mitochondrial cytochrome c oxidase subunit I (COI) gene used for molecular identification. DNA sequences were generated and subjected to Blastn search on the National Center for Biotechnology Information database, with the neighbour-joining method used to establish the relative closeness of the test strains to Tetranychus species. Phylogenetically, the test spider mite was, Tetranychus urticae, closely related to T. urticae strains in Lineage I, which is dominated by the Mediterranean haplotypes. The derived information would allow for the development of appropriate management strategies of this pest on moringa seedlings using various products for eventual registration.

Sensitivity of Meloidogyne incognita second-stage juvenile hatch, motility and viability to pure cucurbitacins and cucurbitacin-containing phytonematicides

Previous claims suggested that pure active ingredients from botanicals were less effective in pest management than their fermented crude-extracts. The objective of this study was to compare the toxicity of pure (98%) cucurbitacin A and B on nematode bioactivities with those of their fermented crude-extracts, the Nemarioc-AL and Nemafric-BL phytonematicides, respectively. Purified active ingredients were each diluted to 0.00, 0.25, 0.50, 0.75, 1.00, 1.25, 1.50, 1.75, 2.00, 2.25 and 2.50 μg ml‒1 and their fermented crude-extracts to 0.0%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5% and 5.0% phytonematicide. The exposure time for second-stage juveniles (J2) hatch, motility and viability of the J2 to each concentration was 24, 48 and 72 h. The overall sensitivities (Σk) of J2 hatch to Nemarioc-AL, cucurbitacin A, Nemafric-BL and cucurbitacin B were 1, 30, 5 and 2 units, respectively, for motility 7, 12, 2 and 12 units, respectively, and for viability 2, 4, 1 and 4 units, respectively. Generally, nematodes displayed high sensitivities to cucurbitacin-containing phytonematicides than to their respective purified active ingredients (cucurbitacins). In conclusion, the cucurbitacin-containing phytonematicides were more effective in nematode suppression than their purified active ingredients.

Confirmation of bioactivities of active ingredients of nemarioc-AL and nemafric-BL phytonematicides

ABSTRACT Nemarioc-AL and Nemafric-BL phytonematicides consistently reduced populations of plant-parasitic nematodes. The contribution of juvenile hatch inhibition to the overall reduction of the nematode numbers by the two phytonematicides, with cucurbitacin A and B as active ingredients, respectively, remains undocumented. The objectives of this study were to examine (i) the response of Meloidogyne incognita second-stage juveniles (J2) hatch to increasing concentration of cucurbitacins A and B, (ii) the potential saturation of J2 hatch when exposed to cucurbitacins for extended incubation periods, (iii) the minimum inhibition concentration for J2 hatch and (iv) the reversibility of J2 hatch inhibition.. Eggs of M. incognita were exposed to a series of purified cucurbitacin A and B concentrations over five incubation periods of 24, 48, 72 h and extended incubation periods of 7 and 10 days. Methanol-dissolved cucurbitacin A and B were each diluted and pipetted into well-plates making 11 concentrations, ranging from 0.0 to 2.5 µg.ml−1 water solvent. Juvenile counts were made after 24, 48 and 72 h, with those for saturation assessed at 7 and 10 days. Thereafter, treatments were diluted five times, incubated again for 5 days and counted to establish reversibility of J2 hatch inhibition. In all incubation periods, treatment effects were highly significant (P ≤ 0.01), with J2 hatch and cucurbitacin concentrations exhibiting quadratic relations. Minimum inhibition concentrations of the two cucurbitacins were between 1.13 and 1.40 µg.ml−1. Treatment effects for reversibility to J2 hatch inhibition were not significant (P > 0.05). In conclusion, J2 hatch inhibition could be one of the waysthrough which the two phytonematicides reduced population densities of Meloidogyne species.

Potential existence of multiple Tylenchulus semipenetrans biotypes in South Africa

The existence of multiple Tylenchulus semipenetrans biotypes within a citrus-producing region complicates the selection of nematode-resistant rootstocks for managing the slow decline of citrus. The potential existence of multiple T. semipenetrans biotypes in South Africa was investigated under greenhouse conditions on differential host plants using nematode isolates from 18 citrus-producing districts with at least six diverse soil groups and three different climates. After three nematode generations, the regional effects had no influence on final nematode population densities, whereas the differential host plants significantly segregated the nematode reproductive potentials. On Olea europaea reproductive potentials (eggs and juveniles/g) were zero, whereas those on Citrus jambhiri and Poncirus trifoliata ranged from 79 to 482 and 86 to 433, respectively. All T. semipenetrans isolates, regardless of the soil group and climate type, behaved similarly to the Poncirus biotype, which therefore rules out the existence of multiple T. semipenetrans biotypes in South Africa.