Abstract
The damages observed in Tunisian citrus orchards have prompted studies on the Pseudomonas spp. responsible for blast and black pit. Prospective orchards between 2015 and 2017 showed that the diseases rapidly spread geographically and to new cultivars. A screening of Pseudomonas spp. isolated from symptomatic trees revealed their wide diversity according to phylogenetic analysis of their housekeeping rpoD and cts genes. The majority of strains were affiliated to Pseudomonas syringae pv. syringae (Phylogroup PG02b), previously described in Tunisia. However, they exhibited various BOX-PCR fingerprints and were not clonal. This work demonstrated, for the first time in Tunisia, the involvement of Pseudomonas cerasi (PG02a) and Pseudomonas congelans (PG02c). The latter did not show significant pathogenicity on citrus, but was pathogenic on cantaloupe and active for ice nucleation that could play a role in the disease. A comparative phylogenetic study of citrus pathogens from Iran, Montenegro and Tunisia revealed that P. syringae (PG02b) strains are closely related but again not clonal. Interestingly P. cerasi (PG02a) was isolated in two countries and seems to outspread. However, its role in the diseases is not fully understood and it should be monitored in future studies. The diversity of pathogenic Pseudomonas spp. and the extension of the diseases highlight that they have become complex and synergistic. It opens questions about which factors favor diseases and how to fight against them efficiently and with sustainable means.
Highlights
Pseudomonas syringae has a huge impact on our scientific understanding of microbial pathogenicity, and continues to cause economically important plant diseases to both woody and annual crops (Lamichhane et al 2015)
In Tunisia, P. syringae has been described on tomato (Mensi et al 2018), on citrus (Abdellatif et al 2015) and it was isolated from weeds and plant debris that can be the source of inoculums to trigger citrus blast and black pit diseases (Mougou and BoughallebM’Hamdi 2016)
In order to better understand the situation in Tunisian orchards, Oueslati et al (2019), using a similar approach, identified two new pathogenic Pseudomonas species involved in citrus black pit disease in different Tunisian sites: P. kairouanensis and P. nabeulensis
Summary
Pseudomonas syringae has a huge impact on our scientific understanding of microbial pathogenicity, and continues to cause economically important plant diseases to both woody and annual crops (Lamichhane et al 2015). A new pathogenic species from the P. syringae group, P. caspiana was isolated from citrus leaf and stems symptoms (Busquets et al 2017). It seems that many Pseudomonas species could be involved in citrus blast and black pit and that these diseases are more complex than previously thought. In order to better understand the situation in Tunisian orchards, Oueslati et al (2019), using a similar approach, identified two new pathogenic Pseudomonas species involved in citrus black pit disease in different Tunisian sites: P. kairouanensis and P. nabeulensis. In order to control these diseases, it will be necessary to better describe and understand the presence of the bacterial population involved and the role of their diversity
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