Seedling resistance among cultivated eggplant and wild relatives against Kanzawa spider mite, Tetranychus kanzawai Kishida (Acari: Tetranychidae)

The Kanzawa spider mite (KSM), Tetranychus kanzawai Kishida is one of the most critical injurious arthropod pests of eggplant (Solanum melongena L.) in the field, especially during the dry season. The KSM sucks the plant sap resulting in speckled or stippled leaves, thus reducing the plant's photosynthetic ability. Severe feeding damage may result in drying-up of leaves, stunted growth, and even death of the plant. In this study, eggplant germplasm consisting of two wild crop species (S. mammosum and S. aethiopicum), and four accessions of cultivated eggplant (S. melongena) were evaluated for KSM resistance in four-week-old seedlings based on leaf damage, trichome types, density, and length. Whole plant assays of seedlings aged 32 days after sowing were conducted under greenhouse conditions to assess the reaction of the test entries to feeding damage of KSM for 10 days. The leaf trichome is an important physical defense against sucking arthropods. Detailed imaging of leaf trichomes using scanning electron microscopy determined glandular and non-glandular trichomes. Both S. melongena and S. aethiopicum possess Type 8 non-glandular trichomes with 2.45+0.22 and 3.67+0.21 trichomes/mm2, respectively. The S. mammosum is provided with both glandular and capitate or globular (types 4, 6, and 7) and non-glandular trichomes (types 2, 3, and 5). Among the glandular trichomes, Type 6 was observed to be the most abundant (9.30+0.67 trichomes/mm2), while type 5 non-glandular trichome has the highest number among non-glandular trichome (2.90+0.42 trichome/mm2). The mean percent KSM leaf damage area was 8.44%+0.64 in S. mammosum, which was significantly lower than those of S. aethiopicum with a mean of 47.91%+5.57 and a range from 46.93-48.88%. Similarly, the mean % damaged leaf area in S. melongena was 51.19%+3.26 with a range of 42.34-62.14%. The density of glandular trichomes is moderately and negatively correlated with the %KSM damaged leaf area with a Spearman's rank correlation coefficient of -0.60 suggesting that as the density of glandular trichome increases the %KSM leaf damaged area decreases. Similarly, the non-glandular leaf trichomes also has a significant negative correlation with the %KSM damaged leaf area but with weak correlation coefficient of -0.36 implying that the density of the non-glandular trichomes has less effect on the host acceptance and feeding preference of KSM. In terms of trichome length, type 2 non-glandular trichome hairs of S. mammosum has the longest, ranging from 1.06 to 1.60 mm with a mean of 1.38+0.05 mm while the Type 7 glandular trichomes has the shortest with a range of 0.04 to 0.06 mm and a mean of 0.05 mm. Based on the correlation analysis, the length of trichomes (glandular and non-glandular) is significantly and negatively correlated with the %KSM damaged leaf area but with a weak correlation coefficient of -0.33. Overall, the results suggest that the presence of glandular trichomes in S. mammosum is a resistance factor against the Kanzawa spider mite, T. kanzawai.