Heart rot of Australian pineapples caused by Dickeya zeae

Pineapple plants (hybrid MD2) with bacterial heart rot were detected in a commercial plantation at Glasshouse Mountains, Queensland, in November 2015. The bacterial strain BRIP64263 isolated from infected tissue was shown to be a Gram negative soft-rotting bacterium capable of growth at 41 ºC, and based on its culture properties was provisionally identified as Dickeya. This strain was compared with other putative Dickeya strains affecting banana (BRIP64262) and potato (BRIP29490). Sequence analysis of the recombinase A genes of the pineapple strain placed it in phylotype I of D. zeae, whereas the banana strain was placed in phylotype II. This was confirmed by sequence comparisons for the phosphofructose kinase, RNA polymerase and aconitase genes which showed that the pineapple strain BRIP64263 is distinct from other strains that infect pineapples and other hosts in Australia and overseas. Furthermore, phylogenetic analysis of the replication initiation factor gene showed that strains affecting pineapples were distributed among both phylotypes of D. zeae, indicating multiple acquisitions or opportunistic infections of pineapple from this group of pathogens. The potato isolate, BRIP29490, was shown to be Rahnella aquatica, and is not likely to be pathogenic. It is not known whether the new isolate represents an incursion or whether it has long been associated with pineapples in Australia. Further study is required to determine the epidemiological characteristics of this strain, and what threat it poses to Australian pineapple production