This bacterial pathogen undergoes two interconnected growth phases, i.e. the epiphytic phase - whereby pseudomonad strains are found on the phyllosphere as naturally occurring residents – and the endophytic phase – when these strains enter the leaf via the stomata to colonize the apoplast; whose proliferation thereof, causes the characteristic water-soak symptom appearance. P. syringae strains cause symptoms such as flower blast, necrotic leaf spots, spots and blisters on fruit, shoot tip dieback, and stem cankers on plant hosts. For example, P. syringae pv. actinidiae (Psa) almost single-handedly wiped out the kiwifruit industry! This particular pathovar is responsible for kiwifruit bacterial canker, i.e. an epidemic that still poses a serious threat on kiwifruit production areas in New Zealand.

1. Mansfield, J., Genin, S., Magori, S., Citovsky, V., Sriariyanum, M., Ronald, P. et al., (2012). Top 10 plant pathogenic bacteria in molecular plant pathology. Molecular plant pathology, 13(6), 614-629

Figure 1 legend:

Characteristic symptoms of bacterial canker on stone fruit trees. A, Blighting of dormant buds resulting in the formation of small cankers at the base of the bud; B, dark brown to black sunken lesions develop on the fruit with irregularly shaped margins; C, branch and stem cankers accompanied by gummosis; D, watersoaked leaf spots that later become necrotic and fall out, resulting in a characteristic shot-hole appearance. Adapted from Bophela et al. 2020. Identification of Pseudomonas isolates associated with bacterial canker of stone fruit trees in the Western Cape, South Africa. Plant Disease 104(3): 882-892.