Role of salicylic acid and phenol in susceptibility of Marigold to Rhizoctonia solani influenced by Meloidogyne incognita

The marigold cv. Yellow dwarf was found resistant to root-knot nematode, Meloidogyne incognita and did not support gall formation or egg mass production, although 3–11% larvae penetrated roots up to cortex but they did not attain maturity. The cultivar was found highly susceptible to Rhizoctonia solani and exhibited significant suppression in the plant growth and leaf chlorophylls at 2 g and 4 g inoculum levels of the fungus, being significantly greater at later. In concomitant inoculations, root-knot nematode aggravated the susceptibility of marigold to the fungus resulting in greater root-rot with corresponding decrease in the plant growth and leaf chlorophylls. The concentration of salicylic acid (SA) and phenol (PH) increased greatly in inoculated plants and the order was M. incognita + R. solani > M. incognita > R. solani. Interaction-wise, PH and SA contents of concomitantly inoculated plants were 8–15% less than the sum of increase induced by both the pathogens alone. This showed that lower increase in PH and SA occurred in the plants which sustained greatest decrease in the plant growth parameters indicating that systemic acquired resistance in marigold cultivar failed because of concomitant invasion by two pathogens.