Bacterial spot of Chinese taro (Alocasia cucullata) in Brazil induced by Pantoea agglomerans

Alocasia cucullata (common name: Chinese taro) is a common ornamental species in Brazil. Diseased plants were often found in domestic gardens in Viçosa, Minas Gerais. The leaves had discrete necrotic spots on the leaf lamina and margins (Fig. 1). Typically, spots on the lamina were circular, with a grey centre and dark-brown margins, while on the leaf-margins their appearance was wedge-shaped. Spots initially appear as small pale irregular lesions which enlarge to form necrotic areas.

Pieces (3 x 3 mm) of necrotic tissue were transferred to a drop of sterile water and examined by light microscopy. Bacterial streams diffused into the water from the edges of the affected leaf pieces. The bacteria were isolated by streaking on nutrient agar (Schaad et al., 2001). Small, 2-4 mm diameter, elevated, shining, smooth colonies, with regular margins, were visible within 24 h. Ten isolates were inoculated into single plants of the original host species and tobacco. Leaves were inoculated with bacteria suspended in sterile saline (OD540=0.1) by injecting, pricking, or cutting leaf margins with scissor blades dipped into the inoculum. Water was used as a negative control. No hypersensitive response resulted from the inoculation of tobacco, while the leaves of the original host developed typical symptoms. Bacterial cells observed by light microscopy were single straight, regular rods, Gram negative and non-endosporogenic. For genus and species determination the strategies described by Schaad et al. (2001) were adopted. Data from biochemical, physiological and staining tests, indicated that the isolates belonged to the genus Pantoea. Tests to distinguish the Pantoea species suggested that the isolates were most closely related to Pantoea agglomerans, rather than P. ananatis for which biochemical and physiological similarities were also found. Additionally, cell cultures were submitted to 16S rDNA sequencing and phylogenetic analysis (Maidak et al., 2001) and confirmed P. agglomerans as the causative agent of the disease.

This is thought to be first report of a ‘bacterial spot’ disease on Chinese taro (Alocasia cucullata) caused by Pantoea agglomerans, in Brazil (Romeiro, 2005) or worldwide (Bradbury, 1986). A representative isolate has been deposited in the IBSBF Phytobacteria Culture Collection (Accession No. 2042), Laboratório de Bacteriololgia Vegetal, P.O. Box 70, 13001-970, Campinas, São Paulo, Brazil.