Abstract:
Root-knot nematodes (RKNs) are economically important polyphagous group of highly adapted obligate plant parasites. They pose a substantial threat to crop production globally due to losses caused in a wide range of agricultural crops. To curb infestations of RKNs, several mitigation measures have been deployed to control these parasites but with minimal success. Alternative integrated strategies are therefore needed for management of these pests. A new strategy being explored at icipe focuses on understanding the mechanisms of host location in order to contribute to development of alternative environmentally friendly methods. In this study, it was hypothesized that infective second stage juveniles (J2s) of Meloidogyne incognita use volatile and non-volatile chemical signals to locate the roots of the solanaceous plant, Capsicum spp. The interactions between the root-knot nematode, M. incognita, and three pepper cultivars (California Wonder, Yolo Wonder and Long Red Cayenne) and one accession (AVDRC accession number: PP0237) were studied. Dual choice olfactometer assays to test chemotactic responses of J2s to root odors of the pepper plants were used. In addition, the responses of J2s when in contact with root exudates of the three pepper cultivars and the AVDRC accession were studied by observing the number of stylet thrusts per minute. Using a modified dual choice set up, the chemotactic responses of J2s to root exudates of the three pepper cultivars and the AVDRC accession were tested. Root volatiles were trapped on Super Q adsorbent, analyzed by Gas Chromatography linked Mass Spectrometry (GC/MS), and quantified using authentic standards. Root exudates were collected on XAD-4 amberlite adsorbent and analyzed using Liquid Chromatography coupled to Quadrupole Time of Flight Mass Spectrometry (LC-QToF-MS) to tentatively identify the components. The results indicated that the J2s preferred root odors (70-82%) of the three pepper cultivars than the AVDRC accession (60%) over sand controls. In stylet thrusting assays,the three pepper cultivars were observed to elicit thrusts five times more than the control, while the AVDRC accession elicited two times fewer thrusts compared to the three cultivars. Based on the chemotaxis assays with the pepper root exudates, nematodes were found to prefer (70-82%) the three pepper cultivars than the AVDRC accession (77%) over the control. GC/MS analysis of the root volatiles showed that the three pepper cultivars and the AVDRC accession shared six common components, of which five were confirmed using synthetic standards as α-pinene (1),limonene (3), 2-methoxy-3-(1-methylpropyl)-pyrazine (8), methyl salicylate (10) and tridecane (12). On the other hand, thymol (11) was identified as a component specific to the root odors emitted by the AVDRC accession. In olfactometer assays, J2s chose (90%) the arm permeated with different doses of a 5-component synthetic blend, but preferred less (74-93%) the arm permeated with thymol alone, and thymol combined with either the preferred natural plant root odors or the 5-component synthetic blend. These results provide new insights into the host finding and differential selection behavior of J2s to different cultivars of pepper plants, showing that the composition of root chemical signals determine J2 host choices. These results open up opportunities for the management of root-knot nematodes using semiochemical-based tools. Breeding programmes that exploit allomonal signals can also be explored to control RKNs