Survey of Soft Rot Pectobacteriaceae Infecting Potatoes in South Africa

Pectobacterium and Dickeya species are pathogens of increasing importance on potatoes worldwide. These Soft Rotting Pectobacteriaceae (SRP) cause soft rot, blackleg, and aerial stem rot of tubers and plants, which can lead to extensive yield losses under conditions favourable for disease development. High priority is placed on continual, regular surveillance of Pectobacterium and Dickeya populations internationally, to ensure that new pathogens are not introduced into potato production areas. A survey was therefore undertaken to identify the primary SRP on potatoes in South Africa. Representative isolates were tested for pathogenicity and symptom expression, singly and in combination, in tuber slice assays and pot trials. Results showed that Pectobacterium brasiliense (Pb) remains the dominant and most pathogenic species in the country. The presence of Pectobacterium carotovorum (Pc) and Pectobacterium parmentieri was confirmed, although these species were far less common than Pb. Pectobacterium versatile (Pv) was reported for the first time on potatoes in South Africa. Pv caused soft rot in tuber slice assays and blackleg symptoms in pathogenicity pot trials, albeit at low incidences. Some strains identified as Pc were recently reclassified as a novel species, Pv. Pectobacterium carotovorum is more often associated with aerial stem rot than blackleg, which might explain low blackleg incidence in the trials, as well as the aerial stem rot symptoms from which the initial isolations were made. The lack of detection of Dickeya solani in any of the samples suggests that it remains absent from the country.