First report of latent infection of Malva nicaeensis caused by Pectobacterium carotovorum subsp. brasiliense in Israel

Potato blackleg and tuber soft rots in Israel, caused by Pectobacterium and Dickeya spp., originating from seed tubers imported from Europe, are of a great concern due to the warm climatic conditions during the growing season that favour disease expression and may result in the establishment of the pathogens in potato fields and their spread to weeds and other crops (Tsror et al., 2009). In a previous survey in Israel, Dickeya solani was isolated only from Cyperus rotundus, out of symptomless plants of 12 weed species (Tsror et al., 2010). Recently, Pectobacterium carotovorum subsp. brasiliense (Pcb) was reported as an emerging threat in Western Europe and the primary blackleg-causing pathogen for some countries in the region (van der Wolf et al., 2017). The pathogen is also responsible for considerable disease in Israel (personal information).

To study the dissemination of Pcb to weeds, a survey was conducted in potato fields where Pcb-infected plants were detected during the spring of 2018. Symptomless weed plants from 13 genera and 10 families, namely Solanum nigrum (Solanaceae), Polygonum equisetiforme (Polygonaceae), Centaurea procurrens, Sonchus oleraceus (Asteraceae), Lolium rigidum, Phalaris brachystachys, Avena sterilis (Poaceae), Malva nicaeensis (Malvaceae), Amaranthus blitoides (Amaranthaceae), Chenopodium murale (Chenopodiaceae), Chrozophora tinctoria (Euphorbiaceae), Orobanche aegyptiaca (Orobanchaceae) and Erucaria rostrate (Brassicaceae), were randomly collected from areas where potato plants infected by Pcb or D. solani had been identified. Roots or stems (in the case of O. aegyptiaca) of 6-15 plants of each weed were washed, surface sterilised, macerated in sterile distilled water and the suspensions were plated on crystal violet pectate medium. Cavity forming bacteria were transferred to nutrient agar for further characterisation. Pcb was isolated only from latently infected Malva nicaeensis plants and with an incidence of 16.7%. DNA extracted from these colonies reacted positively in a PCR assay using BR1f/L1r specific primers (Duarte et al., 2004) and also in a TaqMan assay based on araC sequence (van der Wolf et al., 2017). Sequencing the gapA gene (Cigna et al., 2017) identified the isolate as Pcb (GenBank Accession No. MK086015). A maceration assay on potato tubers at 30°C (Tsror et al., 2013) was positive.

This is the first report of latent infection of Pectobacterium carotovorum subsp. brasiliense in one of the most prevalent weeds in potato fields in Israel. Malva nicaeensis may serve as an alternative host for Pcb allowing the pathogen to survive in the absence of the host crop.