Nematode Pests of Citrus

Abstract

The plant-parasitic nematodes that parasitise citrus in South Africa are discussed, with the citrus nematode (Tylenchulus semipenetrans) being the most important. Citrus is grown in more than 125 countries with South Africa ranking 12th in terms of production. The cultivars grown in southern Africa include sweet orange, grapefruit, lemon, lime and mandarin, as well as some hybrids. Whereas more than 30 years ago, 90 % of citrus trees in South Africa were budded on rough lemon rootstocks, a vigorous grower but highly susceptible to citrus nematode; now only a very small proportion of new plantings use this rootstock. Also, in the past, nursery trees were often situated between old nematode-infected trees within orchards, which resulted in infected nursery stock being planted into new orchards. The citrus nematode is not indigenous to South Africa, but it has, nevertheless, spread to all citrus-growing areas in the country. The South African Citrus Improvement Scheme was initiated in 1973 to establish a more productive citrus industry by supplying the best available planting material, free from harmful pests and diseases. All budwood is rendered free from graft transmissible diseases through shoot-tip grafting, and irrigation water is kept free of nematodes and other pathogens. Nurseries participating in the scheme changed to growing trees in containers raised above the ground and filled with a nematode-free potting medium. The main effect of the citrus nematode on citrus production is reduced fruit size, resulting in lower prices, especially in the export market. Control using three consecutive applications of the organophosphate nematicide cadusafos can have a significant effect in reducing citrus nematode numbers to undetectable levels. Such treatment should, however, be applied at the commencement of the rainy season to ensure that the nematicide is distributed throughout the root area. Furthermore, the timing between the three applications should be scheduled so that it will not allow second-stage juveniles to hatch and develop into egg-laying adult females before the follow-up application is applied.