Responses of Tylenchulus Semipenetrans to crude extracts of indiginous cucumis fruits with and without effective micro-organisms in citrus production

Abstract

The ground leaching technology (GLT) system, using crude extracts of wild cucumber (Cucumis myriocarpus) and wild watermelon (Cucumis africanus) fruits, had been widely researched and developed in management of the root-knot (Meloidogyne species) nematodes in tomato (Solanum lycopersicon) production. In the GLT system, experiments were harvested at 56 days after inoculation with nematodes, which was approximately three generations of Meloidogyne species. Also, studies in GLT systems demonstrated that effective micro-organisms (EM) were not essential in the release of chemicals from crude extracts for nematode suppression, with suggestions that the system exclusively relied upon irrigation or rainwater for leaching out chemicals. However, the system had hardly been tested on other nematode species with longer life cycles and crops. The objective of this study was to investigate the influence of crude extracts of C. myriocarpus (cucurbitacin A-containing phytonematicide) and C. africanus (cucurbitacin B-containing phytonematicide) with and without EM on suppression of population densities of the citrus nematode (Tylenchulus semipenetrans) on rough lemon (Citrus jambhiri) over three generations of the nematode. Two studies, one on C. myriocarpus and the other on C. africanus, with and without EM, were conducted separately using the GLT system under greenhouse conditions with three generations of T. semipenetrans as the standard of application interval of the materials. Citrus seedlings were transplanted in 7-L plastic pots contain 6.5-L pasteurised river sand and Hygromix (3:1 v/v) and inoculated with approximately 25 000 second-stage juveniles (J2s) in 2 × 2 × 2 factorial experiment, where treatments were arranged in a randomised complete block design (RCBD), with six replications. At 150 days after treatment, nematode and plant variables were collected and subjected to factorial analysis of variance. Under C. myriocarpus (Cm), EM × Cm interaction was not significant for nematodes (juveniles + eggs) in roots and juveniles in soil, while under C. africanus (Ca), EM × Ca interaction was highly significant for nematodes, but not for juveniles. Crude extracts of C. myriocarpus and C. africanus fruits contributed 21-36% and 38-59% to total treatment variation in nematodes, respectively. Relative to untreated control, crude extracts of C. myriocarpus fruit reduced nematodes by 22% in roots, but increased juveniles in soil by 93%. Similarly, C. africanus fruit reduced nematodes in roots by 80%, but increased juveniles in soil by 178%. The increase of juveniles in the soil was explained on the basis of opposing forces on nematode population densities under crude extracts of Cucumis and untreated control, along with the inherent nature of cyclic population growth in plant-parasitic nematodes. In plant variables, certain significant (P ≤ 0.05) interactions consistently occurred under both Cucumis species. However, effects of the interactions were not consistent under the two Cucumis species. In most of the variables, the non-significant effects of EM × Cm interactions supported the view that the GLT systems were independent of microbial activities, while significant (P ≤ 0.05) EM × Ca interactions suggested that the systems under C. africanus fruit could be viewed as being dependent upon microbial degradation activities. Growth of rough lemon rootstock was, to a certain extent, suppressed by application of crude extracts from Cucumis fruits, suggesting that the material were phytotoxic to this citrus rootstock. Under low nematode population densities, T. semipenetrans infection supported the view that nematode numbers below the damage threshold levels have

stimulatory effects on growth of plants as observed in plant height under conditions of this study. In conclusion, the approximately three nematode-generation-application interval of 150 days for crude extracts of fruits in Cucumis species was rather too long for the efficacies of the materials on suppression of the population densities of T. semipenetrans in rough lemon seedlings. Consequently, shorter application intervals, as demonstrated for Meloidogyne species would be appropriate, although caution has to be taken to ensure that phytotoxicity to the rootstock was avoided.