Resistance to whitefly
The results of the analysis of variance at the 5% level indicate that the genotypes had a significant effect (p<0.05) on the number of eggs which had an F-Value of 2.2953 and a very significant effect (p<0.01) on the percentage of imago survival rate, the number of nymphs, and the number of offspring population which had F values of 2.4345, 2.7591, and 2.4911, respectively. In addition, the genotypes possessed a highly significant effect (p<0.001) with an F value of 4.4296 on the whitefly attack intensity parameter, but had no significant effect on the number of pupae (See Table 2).
The results of the whitefly resistance tests on 19 chili pepper genotypes prove that several genotypes, namely Bonita, CR2, and JT1, can be categorized as resistant genotypes due to lower attack intensity values compared to the other genotypes. These genotypes had an attack intensity value of 0%. Meanwhile, the highest attack intensity values were found in CR3, CR4, and KD3 genotypes with values of 17.2%, 16.3%, and 17.9%, respectively. Therefore, the three genotypes can be categorized as susceptible to whitefly attacks (See Table 3).
The Honest Significant Difference (HSD) test at α= 5% level. SI = survival rate of imago (%). E = number of eggs. N = number of nymphs. P = number of pupae. OP = number of offspring population. I = whitefly attack intensity (%).
The number of whitefly offspring in each genotype also showed significant differences, where in Bonita, CR2, and JT1 genotypes there was a smaller average number of offspring, ranging from 2-5 per genotype. Genotypes with high attack intensity value also had a higher average number of offspring, around 31-50 per genotype. Table 3 indicates that in line with the value of pest attack intensity, Bonita, CR2, and JT1 genotypes also showed a lower percentage of imago survival, namely 10%, 20%, and 0%, respectively. Meanwhile, the surviving imago value for CR3, CR4, and KD3 genotypes are 70%, 100%, and 40%, respectively.
As seen in Figure 1, the whitefly-resistant genotypes showed no sign of significant damage, either on the upper or the lower surfaces of the leaves. Meanwhile, the susceptible genotypes exhibited signs of leaf damage, such as chlorosis spots on the upper leaf surfaces and sooty mold covering the lower leaf surfaces of the affected plants. The leaves of susceptible genotypes appeared to be more yellow, lacking nutrients, and wilted more than the leaves of whitefly-resistant genotypes.
Resistance to geminivirus
The results of the analysis of variance at the 5% level prove that the genotypes had a very significant effect (p<0.01) on the percentage of disease incidence and disease severity (Table 2).
Symptoms found in the field (Figure 2) due to geminivirus infection differed from one genotype to another. Several genotypes displayed symptoms such as the appearance of yellow mosaics on the leaves, the occurrence of ven clearing on young leaves, leaves curling up and/or down, curly and thickened leaves, yellow leaves, curled up and stunted top leaves, and stunted plants. In addition, sooty mold developed on the underside of the leaves. These symptoms appeared simultaneously in one genotype. However, several genotypes only presented one or two of these symptoms. Some other genotypes even appeared to grow normally without any symptoms of viral attack when the observations were made.
Based on the symptoms emerged, the resistance level of the chili pepper genotypes can be seen from the values of disease incidence and experienced disease severity. Based on the results of this study (Table 4), two genotypes were found to have a lower disease incidence value than the other genotypes, i.e. CR9 genotype with a value of 10.71% and JT5 genotype with a value of 19.23%. In addition, these two genotypes had low disease severity values; CR9 had a disease severity value of 7.14% and was included in the moderately resistant category, while JT5 had a disease severity value of 5.19% and fell under in the resistant category.
Genotypes with a high percentage of disease incidences were CR15, CR13, Bonita, and KD4 genotypes, whose values ranging from 80.00-92.86%. Based on the disease severity values, these four genotypes belonged to the medium and high categories. The highest disease severity value was found in CR15 genotype, which was 76.36%, meaning that this genotype fell under the very susceptible category. CR13 and KD4 genotypes were both included in the susceptible category with a disease severity value of 27.69% and 34.05%, respectively. Meanwhile, Bonita genotype was in the moderately susceptible category with a disease severity value of 18.86%. All tested genotypes, both those belonging to the resistant and susceptible categories based on the values listed in Table 4, confirmed the presence of the geminivirus strands in plants molecularly by PCR method.
Confirmation of the presence of geminivirus in plants was obtained 30 days after the inoculation. The PCR results indicate that viral DNA was present in all tested genotypes, both those included in the resistant and susceptible categories. Figure 3 signifies that all genotypes were infected by the virus although they showed phenotypic differences in symptoms and resistance responses.
The connection between plant resistance to vectors and the yellow disease can be seen through the correlation between observed characters as displayed in the following diagram (Figure 4).
The correlation diagram reveals that the number of whitefly pupae has a highly significant correlation value (64%) to the severity of the disease caused by geminivirus. The observed character which was also significantly related to disease severity is the number of imago, with a correlation value of 39%. The disease severity possessed a highly significant correlation with the incidence of disease in plants, with a correlation value of 49%. Furthermore, the intensity of vector attack on plants had a highly significant correlation with the number of surviving imago (74%), the number of offspring (82%), and the number of eggs (83%), and significantly related to the number of nymphs, with a correlation value of 52%. The number of nymphs and the pest attack intensity parameters also had a significant correlation with the percentage of disease incidence, with values of 44% and 43%, respectively.
Variation of the resistance to whitefly and geminivirus in pepper
This study finds that there is considerable variation of chili pepper genotypes for their resistance to whitefly and geminivirus. The values of the intensity and the number of whitefly population in the observed genotypes reflect the resistance level of the genotypes to whitefly attacks. Chiang and Talekar (1980) argued that the higher the whitefly population on a plant, the lower the resistance of the plant to whitefly. Plant resistance to whitefly can also be seen from the ability of the imago to survive (as shown in the imago survival rate) in the plant (Firdaus et al. 2011). The high value of attack intensity based on the symptoms that appear on the surface of the leaves, as well as the high population of whitefly imagoes, eggs, nymphs, and pupae, indicate the low level of resistance of the plant to whitefly attacks. This signifies that the genotype has characteristics favored by whiteflies as a place to live and breed as well as a source of food. There are several factors that influence the attraction of whiteflies to host plants, namely leaf morphological characteristics such as leaf thickness, leaf color, and trichome density (Hasanuzzaman et al. 2016), as well as the content of compounds in these plants (Darshanee et al. 2017).
The difference between resistant and susceptible genotypes to whitefly attack can also be seen from the appearance of plant phenotypes (Figure 1). Plants that are attacked by whiteflies appear to be more wilted and experience symptoms of nutrient deficiency. The macroscopic effects caused by whiteflies on plants include the presence of chlorotic spots due to feeding activity of the whiteflies, resulting in a decrease in the amount of chlorophyll and starch in the leaves and the presence of sooty mold covering the leaf surface and thereby inhibiting the rate of photosynthesis (Pollard 1995). Another impact of whitefly attacks on chili plants is a decline in plant dry weight due to loss of nutrients in plants caused by the whiteflies that feed on leaf phloem (Jeevanandham et al. 2018). This generates physiological disturbances in plants due to the loss of nutrients as well as the closure of the leaf lamina on account of the excretion of sooty mold. The sooty mold that covers the leaf surface gradually develops into a fungus with a blackish color, thus disrupting the photosynthesis process of the plants.
Plants infected with geminivirus will show phenotypic symptoms in response. This is the impact of the virus attack as well as the plant's defense efforts against it. Interactions between viruses, plants, and the environment are characterized by the appearance of phenotypic symptoms as a sign of infection (Bennet and Agbandje 2017). As seen in the field, the common symptoms of Begomovirus attack on chili peppers are discoloration of leaves, distortion, curling, mosaics, and yellowing of leaves, yellowing and purpling of leaf veins, and decreasing crop yields (Bennet and Agbandje 2020; Genefianti et al. 2017). The incidence of diseases in plants marked by these symptoms indicates the high and low level of plant sensitivity to virus attacks. In addition to the indicated symptom score, the percentage of disease incidence also affects the severity of disease experienced by the plants (Ayu et al. 2021). Among all genotypes, JT5 genotype did not display any symptoms of geminivirus infection at the time of observation, despite the molecular confirmation of geminivirus DNA (PepYLCV) in plant DNA. Meanwhile, the other genotypes showed symptoms of viral infection even before the time of observation. Resistant genotypes are thought to have a protein-based resistance mechanism or certain genes that inhibit or even block the expression of viral genes (Lin et al. 2007; Venderschuren et al. 2006).
Disease severity signifies how severe the phenotypic effect of the yellow disease on the plants due to virus attack is. The high activity of viruses in plants can interfere with plant physiological processes by decreasing plant metabolism which can inhibit plant growth. The symptoms, such as the appearance of yellow mosaics and the clearance of leaf veins due to the induction of certain proteins from viral DNA, damage plant chloroplasts and interfere with photosynthesis (Bhattacharyya et al. 2015). Furthermore, disruption of plant physiological mechanisms causes inhibition of plant growth and development, thus affecting its yield.
Resistance to whitefly as a geminivirus vector can potentially reduce disease severity in chili peppers
This study find out that whitefly resistance has the potential to be a useful effort to control geminivirus in chili pepper plants since it can reduce the severity of the disease. The high severity and incidence of the yellow disease in chili pepper plants are suspected to be caused by the large whitefly population and the intensity of their attacks. According to Bennet and Agbandje (2017), vectors play an important role in transporting viral genes to the plant nucleus through the plant phloem, which is a source of food for whiteflies. In addition, the results of this study signify that the number of imagoes was positively correlated with attack intensity, total population, and disease incidence. Furthermore, the number of nymphs also has a significant positive correlation with disease incidence. The correlation diagram shows that a large whitefly population may increase disease severity in chili plants due to the increasing attack intensity. In other words, the large whitefly population is suspected to increase the disease severity in chili plants due to the huge virus load invested by whitefly. A study by Plotnikov et al. (2020) stated that there was a significant relationship between the magnitude of the virus load and the level of manifestation of the cucumber green mottle mosaic virus (CGMMV) infection on cucumber leaves. A similar result has been previously identified in thrips-virus relation by Maris et al. (2003) who found that thrips resistance had a significant positive effect in reducing virus spread in both virus-susceptible accessions, resulting in a low number of infected plants, and virus-resistant accessions, resulting in less cosmetic damage.