MSC

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.