Rhazya stricta Decne (Apocynaceae) "Harmal": A medicinal plant with biocontrol potential against Meloidogyne incognita in tomato

Abstract

Root-knot nematodes, Meloidogyne species are top-ranking polyphagous pests in the world affecting a wide range of crops in Pakistan. Rapid climatic changes and the extensive use of nematicides have strongly impacted human health, the environment, beneficial microbial communities and the soil food web. In the current study the efficacy of crude aqueous and MeOH (methanolic) extracts of the whole plant of Rhazya stricta Decne (Apocynaceae), a medicinal Harmal bush was assessed at three different concentrations and exposure periods in vitro and in planta against M. incognita infesting tomato crops in 2020 and 2021. Besides Carbofuran, MeOH extracts @ 75 % caused maximum inhibition of egg hatching (79.5 and 80.0 %) and J₂s mortality (93.4 and 92.7 %) at 24 and 72 h of exposure in both year trials, respectively. Phytochemical screening of R. stricta revealed alkaloids, flavonoids, phenols, saponins, tannins, steroids, and terpenoids. Thin layer chromatography of aqueous and MeOH extracts detected 6 and 12 bioactive compounds, respectively. Quantitative detection of MeOH extracts produced higher amounts of total phenols (30.56 ± 0.16 mg/g), flavonoids (21.85 ± 0.03 mg/g), and alkaloids (96.7 ± 0.17 mg/g). In planta greenhouse and field assays on tomatoes, MeOH extracts @ 75 % produced highly significant (P = 0.00) results and exhibited no phytotoxic effects. It markedly suppressed a number of galls; galling indices, a number of J₂s 100 cm⁻³ soil, adult females, egg masses and reproduction factor. MeOH extracts substantially increased plant growth and yield parameters under controlled and open field conditions. This research demonstrates the potential of R. stricta as a novel sustainable and eco-friendly strategy for M. incognita in tomato crops, providing a viable alternative to chemical nematicides with low cost and low environmental risks.