Curtobacterium flaccumfaciens pv. flaccumfaciens (Cff) causes bacterial wilt on beans (Phaseolus vulgaris) and bacterial tan spot on soybeans (Glycine max). Cff was detected on beans in Brazil in 1995. Plants of commercial and experimental fields of soybean with typical symptoms of the disease were collected in the State of Paraná, Brazil, during the 2011/2012 growing season. The causal agent was identified as Cff by isolation from symptomatic leaves on CNS semi-selective medium, artificial inoculation test and re-isolation in soybean and bean, Gram staining test, solubility in KOH, and by PCR. This is the first report of Cff on soybean in Brazil.

Curtobacterium flaccumfaciens pv. flaccumfaciens (Cff) causes bacterial tan spot on soybeans (Glycine max), which was detected in Brazilian crops in 2012. The objective of this study was to determine the Cff transmission rate from plant to seeds in soybean cultivars with different levels of resistance to bacterial tan spot. For this, two assays were conducted under greenhouse and laboratory conditions. Inoculation in soybean plants was performed by the stem puncture method in the first assay and the toothpick method in the second assay. The Cff transmission to seed was checked by plating the suspensions extracted from seeds of the inoculated plants on semi-selective medium. Molecular identification of Cff amongst the obtained isolates was performed by PCR. The susceptible cultivars, BRS 284 and NS 5858 RR, showed zero to 1% Cff seed transmission, while the moderately resistant cultivar, BRS 388RR, showed 0% seed transmission. We concluded that Cff can be transmitted to soybean seeds of susceptible cultivars at low rates. Keywords Glycine max. Tan spot. Bacterium Curtobacterium flaccumfaciens pv. flaccumfaciens (Hedges) Collins & Jones (Cff) was first described on soybean [Glycine max (L.) Merr.] in the United States, in 1975, and the disease was named bacterial tan spot (Dunleavy 1983). The same pathogen also causes bacterial wilt (Harveson 2015). Experiments showed maximum yield losses of 18.8% in tan spot soybean susceptible cultivars, with mean losses of 12.5% (Dunleavy 1984). In the 2011/2012 growing season, the occurrence of Cff in Brazilian soybean crops in the State of Paraná was confirmed (Soares et al. 2013). In addition to Brazil and the United States, Cff was also detected in soybean crops in Canada, Russia (Harveson 2015) and Germany (Sammer & Reiher 2012). Characteristics like long latency period, relatively slow growth on complex media, endophytic nature, and occurrence in few plants have made disease diagnosis and pathogen detection difficult, especially in seed