Evaluation of fruit-based amendments for the management of root-knot nematodes (Meloidgyne spp.) in tomato
Vegetable crops are commonly grown by both commercial and resource-poor farmers in South Africa. They are widely used as fresh and processed vegetables and as a result serve as an income for subsistence farmers. However, vegetables are subject to attack by a large number of pests, such as insects and nematodes. Plant-parasitic nematodes are among the most important pests of vegetables and cause substantial quality and quantity yield losses. Control of nematode pests is largely based on the use of pre-plant fumigants, granular and soluble synthetic chemical nematicides of which some have been withdrawn from the market due to health and environmental concerns. One of the solutions lies in the development of natural strategies for controlling nematode pests in the rural farming sector of South Africa. Soil amendments, constituting of various plant and animal sources, were evaluated in glasshouse and fields for their effects on Meloidogyne spp. infecting tomato in comparison to the synthetic nematicide fenamiphos and untreated control. Under field conditions, the plant-based pit-composting treatments decreased nematode population densities, from 43 to 94%; with the citrus fruit-pit compost being the most effective (reducing nematode densities by up to 94%) and significantly enhanced yield by 100 to 400%. The application of citrus juices (grapefruit, lemon, mandarin and orange), citrus oils (lemon, lime and orange) and orange powder as soil amendments before planting tomato significantly suppressed the M. incognita root population densities compared to the untreated control under glasshouse conditions in tomato. Results from this present study furthermore showed that soil treatment with different dosages of lemon and orange juice significantly affected the Pf of M. incognita with higher rates of the orange juice resulting in lower Pf values. Moreover, results showed that the pre-plant, single-dose application exhibited adequate nematicidal effects since its Pf did not differ significantly from those of the follow-up applications. This is a further indication that farmers may benefit using citrus juices due to its cost-effectivity and less managerial inputs as demonstrated in this particular situation. These treatments were further tested under field conditions to evaluate the suppressive effect of different citrus oils, juices and orange powder applied as soil amendments on root-knot nematode population densities and their effect on free-living nematodes. The results demonstrated that a soil amendment of orange juice consistently resulted in a substantial reduction of the root-knot nematode population densities in the rhizosphere soil compared to the other citrus oil treatments in the first trial. For the 2nd trial, when considering both years of testing, the application of lemon juice resulted in a significant reduction (97%) in Meloidogyne population densities in 2017 in the rhizosphere soil compared to the untreated control. Despite treatments not differing significantly in 2016, the treatment with the juices in 2017 resulted in pronounced reductions of the root-knot nematode numbers, which were comparable to cadusaphos and oxamyl treatments. In 2016, the oils reduced the densities of Meloidogyne spp. and increased the number of free-living nematodes in the rhizosphere soil. With regard to yields, the results showed little and/or inconsistent increases for both the 1st and 2nd trials of the present study. Since the treatments did not differ from each other it may indicate that larger quantities of citrus-based juices, oils and powder should be evaluated under field conditions for improved growth or that it should be used in combination with other organic amendments and fertilizers. The results of the in vitro study clearly demonstrate the effect of the different concentrations and exposure times of the Citrus spp. fruit-based treatments on J2 hatching and motility. All the tested Citrus-fruit-based juices (orange and lemon) and oils (lemon and lime) were found to be effective at all concentrations in reducing J2 hatching. It was observed that the nematicidal potential of all these products was directly proportional to the concentration of the juices or oils because an increase in treatment concetration caused an increase in inhibition of J2 hatching.. The orange juice was the most effective among the tested juices and oils with respect to immotilize J2. However, other Citrus fruit-based amendments were also effective in causing J2 motility with varying degrees at different concentrations and different exposure times. Further experiments are needed to evaluate the economic aspects and nematicidal activity under in vitro and in vivo conditions with other nematode species affecting high value crops and in different soil types. It will also be necessary to identify, charectarize the nematotoxic compounds, determine their mode of action before they can be included in integrated pest management system.