-The objective of this work was to evaluate the effect of a bacterial filtrate containing pyoverdine on the population dynamics of Xanthomonas axonopodis pv. passiflorae, and the severity of passion fruit bacterial blight. The treatments were: King's B medium solution; King's B medium solution with 2 μmol L -1 Fe 2+ supplementation; and King's B medium filtrate containing two concentrations of the siderophore pyoverdine produced by Pseudomonas sp. The filtrate containing pyoverdine at Abs 363 = 0.231 (highest concentration) reduces the number of cells of X. axonopodis pv. passiflorae, and at Abs 363 = 0.115 and Abs = 0.231 significantly reduces the severity of bacterial blight.Index terms: Passiflora edulis, Pseudomonas, Xanthomonas axonopodis pv. passiflorae, phylloplane, siderophore. Passion fruit, Passiflora edulis, is native to Brazil and is widely cultivated for therapeutic use, cosmetic products and, particularly, for juice production. Most passion fruit species are native to Central and South America. The primary biodiversity areas are located in Colombia and Brazil (Ramaiya et al., 2014). Uso de pioverdina no controle da mancha bacteriana do maracujazeiroOne of the most important passion fruit diseases is bacterial blight, which is caused by Xanthomonas axonopodis pv. passiflorae, and represents severe economic losses to farmers (Meletti, 2011).Genetic improvement is one of the most promising methods for controlling the disease in commercial crops (Nakatani et al., 2009;Meletti, 2011); however, it is not efficient to prevent bacterial development (Munhoz et al., 2011).Therefore, biological control should be investigated. Iron is an essential micronutrient for both plants and microorganisms, including bacteria. In ironlimited environments, some bacteria can overcome iron restriction using siderophores, which are ironchelating compounds. Pyoverdines are water-soluble siderophores produced by fluorescent Pseudomonas spp. with a high affinity for iron (Expert & O'Brian, 2012), which limits iron availability to pathogens. Halfeld-Vieira et al. (2015) observed the potential of pyoverdine-producing phylloplane bacteria in disease management. However, the effect of these compounds on the population density of X. axonopodis pv. passiflorae on the phylloplane, and how this potential alteration could affect disease severity remains unknown.The objective of this work was to evaluate the effect of a bacterial filtrate, containing pyoverdine, on the populational dynamics of X. axonopodis pv. The in vivo experiment was carried out using the antagonist isolate Pseudomonas sp. 29RR, which was used in a previous study to control passion fruit bacterial blight. This antagonist strain produces pyoverdine that inhibits the Xap-SP growth by restricting iron availability in the culture medium, instead of restricting it by antibiosis (Halfeld-Vieira et al., 2015).First, the 29RR isolate was cultured in test tubes with King's B medium for 48 hours at 27°C (King et al., 1954). Three treatments (T) were performed: T1, Kin...