Pseudomonas syringae causing leaf spot and blight on Cucurbits
Plant pathologists, plant breeders, seed professionals, growers, and home gardeners are familiar with Pseudomonas syringae pv. lachrymans (Smith and Bryan 1915) Young Dye & Wilkie 1978, the causal agent of angular leaf-spot of cucumber (Cucumis sativus). This pathogen was first isolated and described by Erwin F. Smith and Mary Katherine Bryan, as Bacterium lachrymans and was further reclassified as Phytomonas lachrymans (Smith & Bryan) Bergey et al. 1923 and eventually Pseudomonas lachrymans (Smith & Bryan) Carsner 1918. As with many other plant pathogenic fluorescent Pseudomonas species Pseudomonas syringae pv. lachrymans (Smith and Bryan 1915) Young Dye & Wilkie 1978 was proposed in response to the overhaul of the bacteriological code in 1980 (see Bull et al, 2008 for a description of the process and consequences). In 1999, Gardan et al separated Pseudomonas syringae into 9 genomospecies, many of which have now been formally proposed and validated (Fig. 1; Bull and Koike, 2015; Dutta et al 2018).
Figure 1. The Pseudomonas syringae species complex. The cucurbit pathogens we are working with belong to P. syringae sensu stricto.
Garden et al., (1999) further showed that Pseudomonas syringae pv. lachrymans belongs to the species P. amygdali (Genomospecies 2; phylogroup 3; see Bull and Koike 2015 for placement of additional pathovars into these species).
More recently, leaf spot and wart pathogens isolated from cucurbits were identified as members of Pseudomonas syringae sensu stricto (meaning in the strictest sense). Newberry et al., 2016 and others demonstrated that they belong to Genomospecies 1 or phylogroup 2, otherwise known as Pseudomonas syringae sensu stricto (Fig 2). Figure 2 also shows the diversity of strains shown to cause disease on a variety of cucurbits.
Figure 2. Diversity of some strains causing leaf spot diseases on cucurbits.