Bacterial leaf streak (BLS), caused by Xanthomonas oryzae pv. oryzicola (Xoc), has emerged as a significant threat to rice production in Africa, ranking as the second most important bacterial disease of rice on the continent. 

BLS presence in Africa was first documented by visual observation in the 1980s in West Africa and in Madagascar. The first confirmed Xoc was from diseased leaves collected between 2003 and 2013 in Mali, Burkina Faso, or Madagascar (Gonzalez et al. 2007; Poulin et al. 2014). Today the disease is found in at least nine African countries, including East African countries such as Burundi, Kenya, and Uganda. Yield losses attributed to BLS range from 20 to 30% (Sileshi and Gebeyehu 2021). In Tanzania, rice is the second most important food crop after maize, yet no data on BLS has been reported, aside from a single strain isolated by Tollenaere et al. (2017), with no details on the disease or field context. 

BLS is a seedborne disease that can spread through trade. In 2013, we found rice plants with typical BLS symptoms in the Morogoro region of Tanzania. These included translucent lesions with yellow-brown to black streaks and occasional exudates on leaf surfaces. Another survey was next conducted in March 2016 in three rice plots owned by KPL, a large rice producer in the Morogoro region. A total of seven symptomatic leaves from both surveys were selected for further analysis. Leaf pieces (4 to 5 cm long) were disinfected with 70% ethanol, rinsed with sterile water, and ground. Then, 10 μl of the leaf powder resuspended in 1 ml of sterile water was plated on a semi-selective PSA medium and incubated at 28°C for 5 days. Nine colonies from seven different leaves (two leaves per plot collected in 2016 and one leaf from the 2013 survey) resembling those of X. oryzae were isolated and subjected to a pathogenicity test and molecular characterization. The multiplex PCR tool developed for the identification of X. oryzae pathovars (Lang et al. 2010) revealed the characteristic amplification pattern of Xoc (two amplicons of 331 and 691 bp) for all isolates. The pathogenicity test was performed on 4-week-old plants of Oryza sativa cultivar Azucena by infiltration with a needleless syringe of a bacterial suspension with an optical density of 0.5. Leaf streak symptoms were observed one week later, and all nine inoculated Xoc strains resulted in the appearance of typical BLS symptoms, similar to those observed after infiltration of the control strain BLS256, while the water-inoculated leaves remained asymptomatic. The isolation of PCR-confirmed Xoc strains from these artificially inoculated plants allowed the completion of Koch’s postulates. After amplification using the primers XgyrB1F and XgyrB1R (Young et al. 2008) and sequencing, the analysis of the 541-bp partial sequence of the gyrB housekeeping gene for these nine strains showed 100% nucleotide sequence identity with the homologous sequence of the Xoc reference strain BLS256 (accession no. CP003057). In the end, each of the four fields studied resulted in the isolation of one strain of Xoc in 2013, as well as 3, 2, and 3 strains in 2016, all of which were deposited in the collection of the French Institute for Research on Sustainable Development (IRD) in Montpellier (France), under the accession numbers CIX26, CIX1043, CIX1044, CIX1045, CIX1046, CIX1047, CIX1048, CIX1049, and CIX1050. This study validates the presence of BLS in Tanzania and provides a foundation for future efforts in rice breeding for locally adapted resistance.