Threats

Ralstonia bacterial wilt

Ralstonia bacterial wilt
Ralstonia solanacearum and Ralstonia pseudosolanacearum

Pseudomonas solanacearum Smith

Vascular wilt

A rapid wilting followed by defoliation, death of stems, reduced growth and dark discolouration of the wood (Coutinho et al. 2010). Trees between 2 and 4 years of age are the most susceptible to infection (Wardlaw et al. 2010). Infected trees usually die within 6 months.  In South Africa, bacterial wilt is occasionally responsible for tree death. Outbreaks are usually not widespread. Nursery infections have not been reported locally although this is not the case in Brazil (Alfenas et al. 2006).

Bacterial exudation from the cut stem surface. One of the diagnostic methods used to determine the cause of the wilt is to place the cut stem into water and bacterial exudate will be seen streaming from the tissue.

Ralstonia spp. are both soil and waterborne and enter the plant roots through wounds or at the site of rootlet emergence. If the infection is successful they rapidly colonize the xylem tissue leading to the death of the host. Once the host has died, the bacteria then return to the environment and survive in soil, water, reservoir (nonhost) plants (Denny et al. 1994) and other potential hosts such as weeds (Pradhanang et al. 2000). In the case of Eucalyptus, latent infections may occur and when the tree is stressed by either abiotic or biotic factors, the disease develops. These stress factors weaken the defence system of the host allowing the pathogen to proliferate (Coutinho & Wingfield, 2017). Currently only two of the plant pathogenic Ralstonia species have been reported to be associated with bacterial wilt of Eucalyptus and these two species are mostly geographically separated with R. solanacearum found in the Americas and R. pseudosolanacearum in Africa and Asia (Carstensen et al. 2017).

2000 (Coutinho et al. 2000)
KwaZulu-Natal, Limpopo

Gallery

Stem of Eucalyptushybrid infected with Ralstonia solanacearum
Discoloured wood caused by Ralstonia solanacearum
Bacterial exudate of Ralstonia solanacearum
Bacterial wilt caused by Ralstonia solanacearum

Alfenas AC, Mafia RG, Sartorio RC, Binoti DHB, Silva RR, et al. 2006. Ralstonia solanacearum em vevetros clonais de eucalypto no Brasil. Fitopatologia Brasileira 31: 357-366.

Carstensen GD, Venter SN, Wingfield MJ and Coutinho TA. 2017. Two Ralstonia spp. associated with bacterial wilt of Eucalyptus. Plant Pathology 66: 393-403.

Coutinho TA, Wingfield MJ. 2017. Ralstonia solanacearum and R. pseudosolanacearum on Eucalyptus: opportunists or primary pathogens. Frontiers in Plant Science 8: 761.

Coutinho TA., Wingfield MJ, Roux J, Riedel K-H, Terblanche J. 2000. First report of bacterial wilt caused by Ralstonia solanacearum on eucalypts in South Africa. Forest Pathology 30: 205-210.

Denny DP, Brumbley SM, Carney BF, Clough SJ, Schell MA. 1994. Phenotypic conversion of Pseudomonas solanacearum: its molecular basis and potential function. In: Bacterial wilt: the disease and its causative agent, Pseudomonas solanacearum (Ed. By AC Hayward & GL Hartman, CABI: Wallington.) 137-147.

Wardlaw TJ, Kile GA, Dianese JC. 2010. Diseases of eucalypts associated with viruses, phytoplasmas, bacteria and nematodes. In: Diseases and pathogens of eucalypts (Ed. By Keane PJ, Kile GA, Podger FD & Brown BN, CSIRO Publishing: Canberra.) 339-352.