Characterization of resistance in tomato (Solanum lycopersicum L.) genotypes against whitefly (Bemisia tabaci genn.)

Abstract

: Tomato, Solanum lycopersicum (Mill.), is one of the most popular vegetables cultivated in tropical and subtropical regions of the world. In India, it has been cultivated across an area of 841 thousand ha, with a production of 20.38 metric tonnes (Statista, 2022). However, tomato production is hindered by various abiotic and biotic factors and among the biotic factors, the polyphagous sucking insect pest, whitefly, Bemisia tabaci (Genn.) causes both direct and indirect damage and yield loss to the tune of 25-100 per cent (Mutisya et al., 2016). Both nymphs and adults of B. tabaci feed on phloem sap and devitalizes the tomato plants and also serves as a vector for the causal organism of tomato leaf curl virus disease. To manage the whitefly menace in tomato, farmers rely heavily on chemical insecticides. However, the polyphagous nature of the insect as well as its innate ability to develop resistance to insecticides makes the management of the pest all the more difficult. Exploiting host plant resistance could be an alternative tool to manage whitefly infestation in tomatoes. Thus, the present study entitled “Characterization of resistance in tomato (Solanum lycopersicum L.) genotypes against whitefly (Bemisia tabaci Genn)” was undertaken in the Department of Entomology, College of Agriculture, Vellanikkara, Kerala Agricultural University, Thrissur during 2021-2022. 50 tomato genotypes obtained from different institutes viz., IARI, IIHR, NBPGR, KAU, TNAU and local collections were screened for whitefly resistance under polyhouse conditions. The tomato genotypes showed significant variation with respect to eggs, nymphal and adult populations of whitefly. The mean (pooled) number of eggs per plant varied between 1.63 and 7.28 eggs/cm2 and within the plants, the highest mean number of eggs was recorded on the top leaves (8.20 eggs/cm2), followed by the middle leaves (7.40 eggs/cm2), and the bottom leaves (6.30 eggs/cm2). Whereas, the mean (pooled) number of nymphs per plant varied from 1.45 to 7.41 nymphs/cm2 and within the plant, the highest number of nymphs was observed on the middle canopy (6.53 nymphs/cm2) followed by upper (5.84 nymphs/cm2) and bottom (4.62 nymphs/ cm2) parts of the canopy. However, the mean (pooled) number of adult whiteflies per plant ranged from 1.68 to 7.19 adults/ cm2 and within the plant, the highest number of adults were recorded on the upper canopy (8.20 adults/ cm2), followed by middle (7.20 adults/ cm2) and the bottom (5.90 adults/ cm2). Further, the genotypes were categorized based on scale given by Pradhan (1964). Three genotypes LC Idukki, LC Palakkad and EC 519806 which recorded the mean population of active stages of whitefly <3.67/ cm2 come under the resistant category, whereas eleven genotypes with a mean population ranging from 3.67 to 5.57 were categorised as moderately resistant. Sixteen genotypes were classified under the moderately susceptible category with a mean population ranging from 5.57 to 7.46, whereas 18 genotypes with a mean population >7.46 were considered as the highly susceptible category. The leaf area damage due to the feeding activity of whitefly, which results in the degradation of chlorophyll was measured indirectly in terms of the hue value of scanned photographic images of the leaves represented as integrated densities. The tomato genotypes classified under the resistant category recorded a low hue value ranging from 47 to 53, whereas in the susceptible genotypes, the hue value reached up to 99.5. Morphological characters like the type, length and density of trichome, and leaf lamina thickness were analyzed. Observation of the trichome type revealed that LC Idukki and LC Palakkad possessed three types of trichome i.e., type IV (glandular), type V (nonglandular), and VI (glandular). The length of the non-glandular trichome (Type V) present in the tomato genotypes varied from 513.10 μm to 1475.05 μm. The non-glandular trichome and glandular trichome densities recorded in the tomato genotypes ranged between 30.5 to 74.5 per mm2 and 5.50 to 98.00 per mm2, respectively. Leaf lamina thickness was measured and it ranged from 233.20 μm to 440.5 μm. There was a significant positive correlation between the whitefly population and the parameters such as nonglandular trichome density, leaf lamina thickness and length of trichome. However, a significant negative correlation exists between and glandular trichome density and whitefly population and it is presumed that the trichome type IV and VI present in the genotypes confers resistance to whitefly infestation. The biochemical parameters such as relative leaf water content (91.16 %), and total amino acid content (3.58 mg g-1) were found to be low, whereas, the total phenol (4.56 mg/ g), total flavonoid (1.82 𝜇𝑔 gˉ¹), and total alkaloid content (0.59 mg g-1) were significantly higher in resistant genotype LC Idukki. It was found that there was a significant positive correlation between the whitefly population and parameters such as relative water content and total amino acid content, whereas a significant negative correlation was observed between the whitefly population and parameters such as total phenol, flavonoid and alkaloid contents. Based on the present investigation, LC Idukki, LC Palakkad and EC 519806 may be rated as resistant to whitefly. The studies also show that resistance could be mediated by the type, density and length of leaf trichomes, along with leaf lamina thickness. It also indicated that the resistance in tomato to whitefly could be related to biochemical constituents of the plant, which, however, need to be confirmed. Sustained efforts could lead to the development of whitefly resistant tomato genotypes, providing the muchneeded edge to whitefly management in tomatoes.