Determination of Races and Biovars of Ralstonia solanacearum Causing Tomato Wilt in Iraq

Abstract

Eight bacterial isolates were isolated from tomato plants that showed symptoms of wilt disease from eight districts within Salah Al-Din Province, Iraq. These isolates were molecularly identified according to the nucleotide sequence analysis method of the 16S rRNA gene to the species level Ralstonia solanacearum. The results revealed genetic clustering between isolates Al-Alam (R-3) and Dejail (R-6), as well as between Dhuluiya (R-7) and Ishaqi (R-8). Additionally, isolates Tikrit (R-2) and Dur (R-4) shared the shortest branch with a zero genetic distance, indicating complete genetic similarity. Conversely, isolate Shirqat (R-1) was positioned on a relatively long branch (0.1153), reflecting more genetic divergence from other isolates. This genetic relatedness was consistent with pathogenicity. The highest disease severity was recorded for isolates R.s-2-Ti (Tikrit) and R.s-4-Do (Dur) on tomato cultivar Maysarah, with infection severity of 0.88 and 0.85, respectively. In contrast, the lowest infection severity was 0.12 observed for isolates R.s-7-Dh (Dhuluiya), R.s-1-Sh (Shirqat), and R.s-8-Is (Ishaqi) on the tomato cultivar Barcelona. Based on biochemical reactions and host specificity, the bacterial isolates R.s-1-Sh, R.s-2-Ti, R.s-4-Do, R.s-5-Sm, R.s-7-Dh, and R.s-8-Is were classified into Biovar 3, Race 1, as they infect tomatoes. Meanwhile, isolates R.s-3-Al and R.s-6-Dj were classified as Biovar 2, Race 3. To the best of our knowledge, this study represents the first report characterizing R. solanacearum isolates causing tomato wilt disease in Iraq.

Keywords:

Bacterial diseases, wilt, genetic diversity, Ralstonia solanacearum, Iraq.