Invasive properties of Ralstonia solanacearum virulent and avirulent strains in tomato roots.

Ralstonia solanacearum causes bacterial wilt disease in a wide range of host plants, including tomato. To elucidate the invasion pattern of R. solanacearum, one avirulent strain, FJAT-1458, and one virulent strain, FJAT-91, were characterized according to their colonization and interaction with host in tomato roots. Both strains colonized in tomato roots. The highest colonization numbers were 1.66 × 10 cfu g by FJAT-91 at 5 days (d) after inoculation and 1.09 × 10 cfu g by FJAT-1458 at 6 d after inoculation. Infection with FJAT-91 caused tomato plant wilt with a disease index of 23.65% at 3 d after inoculation and 100% at 6 d after inoculation, and infection with FJAT-1458 did not cause plant wilt. Compared with FJAT-1458, infection with FJAT-91 reduced elongation of tomato roots, induced serious browning, and overflowed bacteria during the late stages of infection. Examination of cellular structure showed that infection with FJAT-1458 did not have obvious destructive effects on plant cells, while FJAT-91 induced a series of cytopathological changes, including swelling of mitochondria, degeneration of cytoplasm and nuclear heterochromatin, and collapse of host cells, which eventually resulted in the death of the host plant. The cytopathological changes appeared from the second to the fourth disease stages.