Application of maximum quantum yield, a parameter of chlorophyll fluorescence, for early determination of bacterial wilt in tomato seedlings

Bacterial wilt caused by Ralstonia solanacearum is a devastating disease that limits tomato seedling and fruit production. Monitoring and early detection are required to effectively manage and reduce the spread of this disease. Seedlings of 30 tomato accessions from three Solanum spp. that came from different countries were grown under greenhouse conditions. Seedlings at the 4-leaf stage were inoculated with R. solanacearum. Disease severity was monitored using the chlorophyll fluorescence parameter (maximum quantum yield; Fv/Fm) until 5 days post inoculation (dpi). Visual symptoms were recorded daily until 16 dpi, and compared with non-infected control seedlings. Fv/Fm started to decrease at 2 dpi in most of the accessions and reached the lowest values at 5 dpi. Visual symptoms started to appear at 3 dpi for 16 accessions, and all of the moderately resistant and susceptible accessions showed visual symptoms at 5 dpi excluding four highly resistant accessions showing non visual disease index at 5 dpi. Finally, 17 accessions were susceptible to R. solanacearum, resulting in visual disease indices (DI) at 3–4 dpi and Fv/Fm values between 0.00 and 0.55. These plants died at the end of the experiment. Nine accessions were moderately resistant to R. solanacearum, showing visual DI values of 1–2 and Fv/Fm values between 0.55 and 0.65. However, four accessions were highly resistant to bacterial wilt and had visual DI values of 0 and Fv/Fm values between 0.65 and 0.83. Furthermore, the resistant genotypes did not show any changes in their visual DI until 16 dpi. Collectively, these results indicate that, depending on the genotype, R. solanacearum infection can be precisely predicted 1–2 days before visual symptoms appear in tomato seedlings. The chlorophyll fluorescence parameter can also be used to accurately screen for moderately resistant accessions.