A survey of common bacterial blight in Central Brazil reveals a third Xanthomonas species infecting common bean

Abstract

A survey of common bacterial blight (CBB) was conducted in common bean fields in Brazil, in the state of Goiás and in the Federal District. Isolation from symptomatic leaves on a semi-selective medium yielded yellow, mucoid colonies typical of Xanthomonas, and 161 pure cultures were obtained. PCR with specific primers for X. phaseoli pv. phaseoli and X. citri pv. fuscans was performed and confirmed the presence of both fuscans and GL2 strains of X. citri pv. fuscans and the presence of X. phaseoli pv. phaseoli. However, for 81 isolates, PCR results were negative. Housekeeping genes gyrB and rpoD sequences placed these strains in the provisionally named X. cannabis species clade, clustering pv. zinniae, pv. esculenti, and the strain Nyagatare, isolated from beans in Rwanda in 2013. The identification of a subset of 20 strains was complemented by a positive PCR with Xanthomonas-specific primers that amplify a portion of the gumD gene and the induction of hypersensitive reaction in tomato leaves within 24-48 hours. Pathogenicity was confirmed by inoculation on the French common bean cv. Flavert and the Brazilian common bean cv. BRS Ártico. Symptoms such as white spots along the leaf blade, leaf curling, and wilting developed in inoculated plants. Partial gyrB and rpoD sequences analyses revealed identity values ranging from 98.3 to 100% between the Brazilian isolates and strain Nyagatare, proposed as X. cannabis pv. phaseoli.