Screening of 55 pinto bean lines for resistance to the two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae)

Two-spotted spider mite (TSSM) (Tetranychus urticae Koch) is considered as the most important pest of bean in different parts of the world. In this study, 54 lines selected from different germplasms of pinto bean, were screened for resistance to the TSSM in two stages. In the first stage, they were classified into five groups based on the intensity of damage to their leaf disks (from 0, without damage to 6, feeding patches more than 80% of leaf area). Twenty-one lines presented the highest leaf damage index (LDI = 5.30 ± 0.27), therefore, they were excluded from subsequent experiments, except line ‘D521’, which was used as the susceptible check. In the second stage, 34 lines with LDI less than 4, along with line ‘D521’ (as susceptible), were studied using leaf disk bioassay. Based on the results, line ‘D521’ (7.52 ± 1.23 eggs/female/day) supported the highest, whereas, lines ‘L1’ (1.46 ± 0.23), ‘J29’ (1.60 ± 0.26) and ‘L19’ (1.69 ± 0.59 eggs/female/day) supported the lowest level of oviposition. The highest and lowest level of damage was observed on leaf disk of ‘D521’ (5.5 ± 0.40) and ‘L19’ (1.8 ± 0.37), respectively. The studied lines were classified into five groups, based on the mite response to the host (oviposition and mortality) and host response to the mite (damage score). In the cluster analysis, lines ‘D521’, ‘J633’, ‘D524’ and ‘D532’ appeared to be highly susceptible and susceptible, whereas, lines ‘L31’, ‘L329’, ‘L321’, ‘L16’, ‘B417’, ‘B425’, ‘L328’, ‘J29’, ‘J67’, ‘L19’, ‘D3’, ‘L25’ and ‘L1’ were resistant to TSSM, which can be used as the source of resistance in future plant breeding programs.