Pea seed priming in cucurbitacin-containing phytomaticides for generating mean concentration point

Abstract

In use of phytonematicides as an alternative to synthetic chemical nematicides, the major challenge had been the development of appropriate application technologies, which are currently limited to the ground leaching technology (GLT) and botinemagation (BNT) systems. The former is labour-intensive, whereas the latter requires infrastructure that could be costly for smallholder farmers. The priming of seeds with hypogenous germination properties in phytonematicide solutions could serve as an alternative method of the application of phytonematicides, where the cotyledons would serve as carriers of the active ingredients that are leached into the rhizosphere for suppression of nematode numbers. However, since germination is a chemical process, it is not known whether the active ingredients in cucurbitacin containing phytonematicides would interfere with germination and the subsequent emergence of the seedlings through the incidence of phytotoxicity as observed in the use of the products in crop production. The objectives of the study, therefore, were (1) to investigate the sensitivity and overall sensitivity of pea (Pisum sativum L.) plants to Nemarioc-AL and Nemafric-BL phytonematicides, and (2) to determine the mean concentration point (MCSP) for pea-inoculated with Meloidogyne incognita under greenhouse and microplot conditions, where seeds were previously primed in phytonematicide solutions. Two separate trials were conducted with seven treatments, namely, 0, 2, 4, 8, 16, 32 and 64% Nemarioc-AL or Nemafric-BL phytonematicide, arranged in completely randomised design (CRD), with 8 replications each. Pea seeds were primed in Nemarioc-AL and Nemafric-BL phytonematicide solutions for two hours and shade dried prior to sowing. In vitro trial, 10 seeds were spread uniformly on a moistened filter paper in sterilised petri-dishes with lids and placed in an incubator at 25oC. In vivo trials were under greenhouse and micro-plot conditions, pea seeds were sown in 25-cm and 30-cm diameter plastic pots, respectively. Pots were filled with pasteurised loam soil. Seedlings were inoculated with 5 000 eggs + second-stage juveniles (J2) of M. incognita. Treatments in each case included priming seeds as explained earlier, arranged in a randomised complete block design (RCBD), with 6 replications under greenhouse conditions and 8 replications under micro-plot conditions. In all cases, plant growth variables were assessed using the Curve-fitting Allelochemical Response Dose (CARD) model to generate biological indices which were used to calculate MCSP and the overall sensitivity (Σk). Nematode variables in inoculated trials were assessed using the regression model. In vitro trials, germination variables had positive quadratic relation versus Nemafric-BL phytonematicide, with MCSP= 0.62 % and ∑k = 34 units. In contrast, tested germination variables exhibited negative quadratic relations versus Nemarioc-AL phytonematicide. In greenhouse trials, MCSP values for Nemarioc-AL and Nemafric-BL phytonematicides were 0.62 and 2.18 %, respectively, with ∑k = 0. Plant height (R2 = 0.86), stem diameter (R2 = 0.93) and chlorophyll content (R2 = 0.85), exhibited positive quadratic relationship against Nemarioc-AL phytonematicide, whereas, plant height (R2 = 0.95), stem diameter (R2 = 0.92), chlorophyll content (R2 = 0.89), number of flowers (R2 = 0.93) and dry shoot mass (R2 = 0.94), exhibited positive quadratic relationship against Nemafric-BL phytonematicide. In micro-plot trials, MCSP values for Nemarioc-AL and Nemafric-BL phytonematicides were 0.71 and 2.45 %, respectively, with ∑k = 0. Plant height (R2 = 0.95), stem diameter (R2 = 0.98), chlorophyll content (R2 = 0.98), and gall ratings (R2 = 0.98), exhibited positive quadratic relationships against Nemarioc-AL phytonematicide, while chlorophyll content (R2 = 0.97) and gall ratings (R2 = 0.96) exhibited positive quadratic relationships against Nemafric-BL phytonematicide. All degrees of Nemarioc-AL and Nemafric-BL phytonematicides profoundly reduced nematode numbers under greenhouse and micro-plot trials. In conclusion, both Nemarioc-AL and Nemafric-BL phytonematicides could be applied through the priming technology on pea seeds which have hypogenous germination properties in suppression of nematode population densities.