Effect of Trichoderma harzianum Tr6 in inducing resistance in tomato against Trialeurodes vaporariorum (Hem.: Aleyodidae)

The greenhouse whiteflies, Trialeurodes vaporariorum (Westwood), are one of the major pests of vegetable and ornamental crops in greenhouses in the world. They damage crops through direct feeding, inserting their stylet into leaf veins, and extracting nourishment from the phloem sap. As a by-product of feeding, honeydew is excreted, which is an indirect but yet another substantial source of damage. The third most harmful characteristic is the ability of adults to transmit various plant viruses. Induced resistance is a non-chemical control method that has no negative impact on the environment and human health and can be a desirable approach in plant protection and pest control. Trichoderma spp. has spread in many ecosystems and its various species such as Trichoderma harzianum Rifai have been successful as non-pathogenic symbionts in controlling plant pathogens via different mechanisms. Some of these mechanisms include enzymatic degradation, secretion of antibiotics, and increasing root absorption capacity. Recent findings suggest that T. harzianum is also a potent inducer of Induced Systemic Resistance (ISR) and can stimulate the immune system in plants against plant diseases and pests. In this study, the efficacy of T. harzianum Tr6 was investigated on host preference and egg production of T. vaporariorum.

To evaluate the effect of T. harzianum Tr6, four tests were carried out: 1) Total phenol assay using Folin–Ciocalteu reagent, 2) The effect of time on induced resistance, 3) Free choice, and 4) Non choice. These tests were conducted on tomato plants (Falat cultivar) at 25±2°C, relative humidity of 60-70%, and a photoperiod of 8 h/16 h (day/night). Also, these experiments were performed on one-day-old adults in plots with four or six leaves in shelves equipped with organza mesh.

According to the phenol assay, the highest amount of total plant phenol was observed three and four days after inoculation by Tr6 strain respectively 0.3503±0.001 and 0.3323±0.001mg/g. Therefore, for the next three tests, plants that were inoculated after three days were used as treated samples, and plants watered with distilled water were used as control ones. In these experiments, the lowest number of adults on the back of the leaves after 24 hours was observed two and four days upon inoculation respectively 2.2±  0.86 and 2.0±  0.63. On the other hand, plants inoculated by Tr6 displayed a significant decrease in adults on the back of the leaves (3.4± 0.66), egg-laying (7.4± 1.51), and honeydew droplets (1± 0.28) compared to control plants.

The most important factor in controlling the whitefly population is reducing the reproduction rate. Induced resistance can repel insects from plants and limit their nutrition, and thus, reduce their reproduction. This study implied that the inoculation of tomato by T. harzianum Tr6 strain could induce resistance to whitefly and reduce host preference and egg production, as well as honeydew secretion. Therefore, this strain can be a promising option in preventing the outbreak of this multi-generational pest.