First report of bacterial leaf and flower spot of Zinnia elegans caused by Xanthomonas campestris pv. zinniae in Hungary

In 2006, a serious outbreak of bacterial leaf and flower spot disease was observed on zinnia plants (Zinnia elegans cv. Capricio and Mondo) grown in several parks in Budapest, Hungary. The disease first appeared on leaves as diffuse, translucent, circular spots surrounded by large chlorotic haloes. The lesions enlarged and became reddish brown in the centre and angular in shape (Fig. 1). On the flowers, at high humidity, small brown spots appeared, and the flower heads were disfigured and decayed completely (Fig. 2). Bacteria were isolated on modified Tween medium (Shaad et al., 2001). All isolates were Gram-negative rods, aerobic, and produced yellow, xanthomonadin pigments identified by thin-layer chromatography (Shaad et al., 2001). Isolates were positive for catalase, negative for oxidase, hydrolised starch, gelatine, casein, and aesculin. Hydrogen sulphide was produced from cysteine. In medium C of Dye (Dye, 1968) acids were produced from arabinose, glucose, maltose and sucrose, but not from sorbitol. The sequence of the 16S-23S rDNA spacer region of the bacterial strain (GenBank Accession No. EF514223) was determined. The sequence shared 99.7% identity with other X. campestris pv. zinniae strains available in GenBank.

Pathogenicity was confirmed by artificial inoculation of healthy, four to six-leaf stage zinnia plants with a suspension of an isolated bacterial strain (10⁷ CFU per ml). Sterile distilled water was used as negative control. The inoculated plants were incubated in a mist chamber (95% relative humidity) for 3 days, and then transferred to a greenhouse at 21-27°C. Characteristic leaf spot symptoms were observed on inoculated zinnia plants 8 days after inoculation. No symptoms were observed on control plants. The original pathogen strain was reisolated from diseased leaves.

Bacterial leaf spot of zinnia was first reported in 1929, in Italy (Nanizzi, 1929), and the pathogen was named later as X. campestris pv. zinniae (Dye, 1978). To our knowledge since 1929 no report describing the disease has been published in Europe. This is the first report on the occurrence of this bacterium on zinnia in Hungary.