Evaluation of the Trichoderma harzianum BI alone and in Combination with Thiamine, Riboflavin and Hexanoic Acid Effects on Resistance Induction against Meloidogyne javanica in Tomato Plants

One of the most important problems facing modern agriculture is soil born plant pathogens control, especially root-knot nematodes. Chemical control, crop rotation and using resistance cultivar are common methods in control of most plant pathogens; however, there are low efficacy in control of root-knot nematodes regarding host wide range, long-term survival ability in soil and plant residual even in absence of host. One of the new methods in the management of root-knot nematodes is resistance induction in host using chemical compounds and microorganisms.  In this method there are not any side effect. The aim of this study was to investigate the effectiveness and mechanisms involved in induced resistance by three chemical compounds (Hexanoic acid, Thiamin and Riboflavin) accompanied by Trichoderma harzianum BI in tomato plants against Meloidogyne javanica. The objectives of this study were addressed by monitoring the activity of peroxidase, polyphenol oxidase, catalase, phenylalanine ammonia lyase, accumulation of total phenolic compounds and nematode disease indexes.

This study was conducted in the Faculty of Agriculture, Ferdowsi University of Mashhad and was performed with susceptible cultivar to root knot nematode (Early Urbana). 2-4 leaf stage tomato seedlings were transferred to 3 Liter pots filled with 2 liter sterile soil and maintained under greenhouse condition to the end of experiment. 2000 Meloidogyne javanica J2 larvae (N) were added to each pot. Tomato plants were treated with Trichoderma harzianum BI (TH) with a population of 1×107 spores/mL, Hexanoic acid (HX), thiamine (TI) and riboflavin (RB) with a concentration of 20 mM. Distilled water was used as control treatment. Sampling was performed at specific time points from each pot and the samples were transferred to the laboratory. Enzyme extract was extracted from tomato roots to assay the Peroxidase (POD), Poly phenol oxidase (PPO), Catalase (PAL) and Phenylalanine ammonia lyase (PAL) enzyme activity. Also total phenol content and nematode pathogenicity indexes were measured at the end of the growing season.

 Based on the results, the highest POD and PPO enzyme activities were measured in TH + HX + N, TH + RB + N and TH + TI + N treatments, respectively, at a time point of 72 hours after treatment. The highest CAT enzyme activity was also recorded in the treatments TH + HX + N, TH + RB + N and TH + TI + N, respectively, at 96 hours after treatment application, the highest PAL enzyme activity was recorded in TH + HX + N, TH + RB + N and TH + TI + N treatments, respectively, at a time point of 120 hours after treatment. The highest total phenol content of roots  were measured, respectively,  in TH + RB + N, TH + HX + N and TH + TI + N treatments with values of 125.33, 114.67 and 109.33 mg gallic acid per gram roots. Also, nematode pathogenicity indexes including gall index, number of egg sacs and number of eggs in each egg sac were measured 45 days after the start of the experiment and a significant difference was observed in all treatments compared to the control treatment. The lowest values of pathogenicity indices were recorded in TH + HX + N, TH + RB + N and TH + TI + N treatments, respectively. Systemic resistance in plants can be induced with biotic and abiotic agents. The effects of chemical and microorganisms on plants resistance induction were investigated in many studies. However, effect of these agents on plant parasitic nematodes were considered less than other pathogens. Capability of different Trichoderma isolates on nematodes damage reduction are reported in many different studies. Several biocontrol mechanisms are reported for Trichoderma. As biocontrol ability of an isolate can be different mechanisms consequence. Effect of different chemical such as Hexanoic Acid, Thiamin and Riboflavin on plant disease have been investigated, however, there are little known about the effects of these chemicals on plant parasitic nematodes. The use of these chemicals as an enhancer of plant resistance to pathogens, due to their low risk to humans and the environment has been increasingly welcomed by researchers.

The results showed significant effect of used treatments on the reduction of root-knot nematode damage in tomato plants. These results justify further studies to investigate and identify the mechanisms of these compounds mode of action on increasing host resistance against this pathogen.