Identification of Pseudomonas Isolates Associated With Bacterial Canker of Stone Fruit Trees in the Western Cape, South Africa

Abstract

Bacterial canker is a common bacterial disease of stone fruit trees. The causal agents responsible for the disease include several pathovars in Pseudomonas syringae sensu lato and newly described Pseudomonas species. Pseudomonad strains were isolated from symptomatic stone fruit trees, namely apricot, peach, and plum trees cultivated in spatially separated orchards in the Western Cape. A polyphasic approach was used to identify and characterize these strains. Using a multilocus sequence typing approach of four housekeeping loci, namely cts, gapA, gyrB, and rpoD, the pseudomonad strains were delineated into two phylogenetic groups within P. syringae sensu lato: P. syringae sensu stricto and Pseudomonas viridiflava. These results were further supported by LOPAT diagnostic assays and analysis of clades in the rep-PCR dendrogram. The pseudomonad strains were pathogenic on both apricot and plum seedlings, indicative of a lack of host specificity between Pseudomonas strains infecting Prunus spp. This is a first report of P. viridiflava isolated from plum trees showing symptoms of bacterial canker. P. viridiflava is considered to be an opportunistic pathogen that causes foliar diseases of vegetable crops, fruit trees, and aromatic herbs, and thus the isolation of pathogenic P. viridiflava from twigs of plum trees showing symptoms of bacterial canker suggests that this bacterial species is a potentially emerging stem canker pathogen of stone fruit trees in South Africa.

TaqMan qPCR assays improve Pseudomonas syringae pv. actinidiae biovar 3 and P. viridiflava (PG07) detection within the Pseudomonas sp. community of kiwifruit

“…The present study gave similar results using BIO-PCRPsa3 and qPCRPsa3 at 77 dap (June). In the case of Pv7, which was recently found associated with canker tissues of plum and apricot (Parisi et al, 2019;Bophela et al, 2020), the qPCRPv7 and BIO-PCR-Pv7 approaches gave conflicting results at pruning. With qPCRPv7, 75% of the cane sections were infected by the bacterium, while according to BIO-PCRPv7, culturable Pv7 was nearly absent (5%), in accordance with isolate typing results.…”

Genetic characterization and prevalence of Pseudomonas syringae strains from sweet cherry orchards in New Zealand

“…In this study, only one strain per sample was selected for gltA /MLSA, and therefore it was not possible to establish if strains also coexisted within the same tissue. The coexistence of pathogenic and/or nonpathogenic strains, of the same or different genera, within the same tissue may indicate an interaction between the organisms, which might affect disease expression (Bophela et al, 2020). It also poses questions regarding the dynamics of host tissue colonization including exclusion, coexistence or succession, and should be further studied.…”

“…Approximately 12 cherry cultivars are grown commercially: 13S2009 (marketed as Staccato), Sweetheart and Lapins are the most important, followed by Samba, Santina, Sonnet, Stella, Sandra Rose, Kordia, Regina, Molmac (marketed as Romance) and SPC103 (marketed as Sentennial). Bacterial canker is one of the most important bacterial diseases affecting sweet cherry orchards worldwide (Bophela et al, 2020; Hulin et al, 2020; Kennelly et al, 2007; Ruinelli et al, 2019; Vicente et al, 2004). The disease causes direct losses by reducing plant vigour and growth, numbers of buds, blossoms and branches and causing plant death, all of which in turn reduce yields from orchards.…”

Pseudomonas viridiflava: An internal outsider of the Pseudomonas syringae species complex

“…Also, a recent report of P . viridiflava causing disease on plum ( Prunus domestica ) cultivars Sapphire and Songold in South Africa (Bophela et al, 2020 ) contradicts the result of a “no disease” outcome in a host range test with plum cultivar Marina GF8‐1 by Morris et al ( 2019 ); this may be an example of cultivar‐specific resistance. These examples of varied capacity for causing disease are similar to the observations that different isolates within the same species of P .…”