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Histological and morphological characterization of ‘Cardaba’ and ‘Cavendish’ roots of bananas (Musa x paradisiaca L.) infected with Ralstonia solanacearum (E.F. Smith) Yabuuchi et al. ‘Race 2’
Thesis Abstract:
Ralstonia solanacearum causes bacterial wilt in many plant species worldwide. Race 2 affects mostly Musa and Heliconia sp. but its invasion in root tissues and multiplication site in ‘Cardaba’ and ‘Cavendish’ remains unclear. This study provides data on morphological and histological symptoms in the two cultivars of banana.
In the screenhouse, four-month-old tissue cultured ‘Cardaba’ and ‘Cavendish’ bananas were inoculated with moko and bugtok isolates of R. solanacearum Race 2. Inoculation was done by drenching 100 ml bacterial suspension standardized at 1 × 10<sup>9</sup> CFU/ml around wounded and intact roots of bananas grown in 6 × 9 in<sup>2</sup> clear plastic bag.
Susceptibility of ‘Cavendish’ to wilting was manifested by 100 percent wilt incidence at 28 days after inoculation (DAI) with bugtok isolate. ‘Cardaba’ wilted only when the roots were wounded before inoculation with the same isolate. The moko isolate used in this study failed to cause wilting in both cultivars, whether the roots were wounded or kept intact. The moko isolate was not able to establish in roots as efficiently as the bugtok isolate as evidenced by a lower population of bacteria recovered. Wounding of roots provided entry points for the bacteria especially in ‘Cardaba’. Intact root of ‘Cavendish’, however, was easily penetrated by the bugtok isolate. Wilted bananas showed long root discoloration which varied from dirty white to black. Only slight discoloration was observed in roots of plants that were inoculated with the moko isolate.
The bacteria colonized and degraded the cell walls of the xylem vessels and intercellular spaces of ‘Cavendish’ and ‘Cardaba’ cultivars. They were abundant particularly in protoxylem vessels. The ‘Cavendish’ cultivar showed more colonized xylem vessels than ‘Cardaba’. Regardless of root condition, the bugtok isolate was more aggressive as shown by 1.84–6.03 xylem vessels invaded per cross-section root at 14 DAI compared with moko with only 0.38–2.30 vessels invaded per cross-section root. The bugtok-inoculated ‘Cavendish’ with intact roots had the higher xylem colonization of 2.77–9.33 xylem vessels per root section at 14 DAI; however, at 28 DAI, the wounded ‘Cardaba’ inoculated with the same bacterial isolate had the higher xylem colonization (3.07–7.57 vessels per root). The moko isolate poorly invaded the xylem vessels and walls of both cultivars. Tyloses were, however, detected in both cultivars with wounded roots suggesting that wounding induced tyloses formation. In intact roots, there were no tyloses observed. Results also suggested that wilting was not only due to bacterial occlusion but more on the destruction of cell walls of xylem vessels.