Population dynamics of Meloidogyne spp. and the effect of pre-plant practices on the pest in tomato net houses in South Africa
In South Africa, the largest tomato producer, ZZ2, practices extensive monocropping under net house structures. These net houses allow the farmer prolonged harvest and good quality yields, however, they also provide ideal conditions for the build-up of diseases and pest such as plant-parasitic nematodes (PPN). The aim of this study was to determine nematode population dynamics in net houses of ZZ2 over a period of three years and investigate the effect of pre-plant strategies on root-knot nematode population development under tomato production systems. To achieve this, six net houses at ZZ2 farms, located in three different climatic regions in the Limpopo Province, were suveyed for three consecutive years to investigate the population dynamics of PPN. Nematode densities in soil fluctuated between the years, however, Meloidogyne spp. numbers significantly increased, irrespective of location or climatic conditons, from the first year to the third year for all net houses. Net houses also differed significantly among each other concerning Meloidogyne spp. densities recorded in tomato rhizospheres. Chemical soil analysis indicated that net houses with high potassium, calcium, pH and acid saturation levels had lower Meloidogyne spp. numbers, while those with higher Mg:K and Na:K ratios were associated with higher Meloidogyne spp. densities. Meloidogyne spp. numbers were higher in sandy soils. Planting time, however, had less influence on Meloidogyne spp. numbers, while cultivation of rootstocks RS2, RS3 and RS13 resulted in lower Meloidogyne spp. numbers compared to RS11 and scion ZZX132. Root-knot nematode species associated with tomato crops in six net houses situated in different climatic areas were identified using morphological (perineal-pattern and oesophageal structure) and molecular (SCAR-PCR) approaches. The combination of both techniques (molecular and morphological) confirmed the presence of four Meloidogyne species namely M. arenaria, M. enterolobii, M. incognita and M. javanica and should be used to ensure that the correct information is gathered since both techniques did not present the same results for the different net houses. Pre-plant strategies undertaken by ZZ2 producers to prepare net houses for a next tomato crop cycle were evaluated in the net house to determine their efficacy in delaying infection in tomato roots. The trial layouts were randomized, complete block designs with five treatments representing i) the end of crop cycle, ii) fallow, iii) ridge disking, iv) addition of organic material (OM; compost, compost tea, fermented plant extracts and other organic products), and v) addition of OM combined with inoculation of the soil with ±2000 infective, second-stage juveniles (J2) of M. javanica. Soil from cold semi-arid (BSK) net houses contained both Meloidogyne spp. and Pratylenchus spp. individuals, and those from a warm subtropical (CWA) net houses only Meloidogyne spp. individuals. Addition of OM to the potted BSK and CWA net house soils reduced Meloidogyne spp. densities in tomato roots and soil. Addition of OM to soil inoculated with ±2000 infective J2 of M. javanica and left fallow for both net houses also resulted in a reduction of Meloidogyne spp. denities in roots and soil of tomato plants. Pratylenchus spp. densities were not affected by the treatments.