Influence of maerua angolensis and tabernaemontana elegans extracts application time on tomato growth and nematode management.

Abstract

Tomato (Solanum lycopersicon L.) is amongst the most common vegetable crops grown in South Africa and an essential part of a healthy diet. Unfortunately, its production is negatively impacted by various pests, including the root-knot nematodes, Meloidogyne javanica and Meloidogyne enterolobii, which cause high yield losses. The use of synthetic chemical nematicides has been very successful as a control method. Unfortunately, there are many challenges associated with it, including the negative impact on the environment, people and animals, hence most have been removed from the agrochemical markets. An alternative method of plant extracts has recently received a lot of research and development, however, the method comes with inconsistent results from several factors, among which is time of application, hence the current study seeks to determine the effect of time of application of the two medicinal plant extracts, M. angolensis and T. elegans on nematode population densities and tomato plant growth under greenhouse and field conditions. Two experiments were established to achieve the objective, a greenhouse experiment of 2 x 2 x 3 + 2 factorial arrangement with the first factor being two plant extracts (M. angolensis and T. elegans) applied at a recommended rate of 5g per plant, and the second factor consisted of two nematodes (M. javanica and M. enterolobii) at 5000 eggs and J2 per plant. The third factor was made up of three application and inoculation times (plant extract applied before nematode inoculation; plant extract applied simultaneously with nematode inoculation and plant extract applied after nematode inoculation) plus positive experiment control (Crop Guard, a.i. furfural, aldehyde 900g/L) and negative control (plants only inoculated with nematodes) with 5 replications. While a field experiment consisted of five treatments namely, the application interval of plant extracts and positive control (Crop Guard) weekly, every second, third and fourth week. Field treatments were laid out in a randomized complete block design (RCBD) with 15 replications. Greenhouse experiment was done over two seasons: summer and autumn, 2023 and 2024, respectively, and field experiment was done once in 2024. At 56 days after the last treatment application, nematode and plant growth variables were measured from both experiments. Under greenhouse conditions, all second order interactions of season, nematode and time of application had no significant (P > 0.05) effects on all measured variables, except on juveniles in soil and juveniles in tomato root. While the first-order interactions of season and plant extracts, season and time of application were only significant for eggs in tomato root, juveniles in tomato root, total number of nematodes in root and final nematode populations. Generally, all three plant extract applications had a significant effect on the two nematodes, more nematodes were observed in summer than in winter across all treatments, also more nematodes were in tomato roots than in soil. Across all other factors, M. angolensis reduced nematode variables more than T. elegans. Applying plant extracts after inoculation yielded the lowest nematode numbers, followed by simultaneous application, with plant extract applied before nematode inoculation having the highest numbers. Across all measured variables, positive control consistently achieved lower nematode numbers than negative control. Under field conditions, nematode suppression was highest when plant extracts were applied weekly, as evidenced by a significantly decreased numbers of eggs in tomato root, juveniles in soil, total nematode in tomato root and final nematode population relative to the negative control. In conclusion, both plant extracts can be used in the suppression of nematodes even though best results can be obtained from M. angolesis applied weekly after nematode infestations