Department of Plant Pathology, The Ohio State University, OARDC, Wooster, OH 44691, USA

Background and objectives
Bacterial leaf and flower spot of zinnia, caused by Xanthomonas campestris pv. zinniae, was first found in Italy in 1929 [1]. Since then, this disease has been observed in many other countries, including Brazil, Sierra Leone, Malawi, Rhodesia, India, Australia and the USA. The pathogen, first reported in Ohio, USA, in 1972 [2], was widespread on zinnia cultivars in Ohio home gardens in 1997. X. c. pv. zinniae has not been well-studied. No attempts have been made to determine its host range, or the relationship within or between strains of X. c. pv. zinniae and other X. campestris pathovars. The objectives of this study were: (1) to determine the pathogenicity of X. c. pv. zinniae strains on different crops, (2) to analyse the population structure of X. c. pv. zinniae strains using traditional and molecular techniques, and (3) to evaluate methods which could be useful for differentiation of X. c. pv. zinniae strains from other X. campestris pathovars.

Results and discussion
Twenty-two putative X. c. pv. zinniae strains were isolated from contaminated seedlots and diseased zinnia plants grown in several different home gardens in Ohio. All of the strains were identified as X. campestris based on morphological (motile, rod-shaped cells; circular, mucoid yellow colonies), biochemical (Gram-negative, aerobic, catalase positive, oxidase-negative) and physiological (amylolytic and pectolytic) characteristics, and fatty acid methyl ester (FAME) analysis. FAME analysis matched the strains to X. c. pv. raphani (similarity indices (S.I.) = 0.101-0.255) or X. c. pv. campestris (S.I. = 0.099-0.208). However, the strains were identified as X. c. pv. zinniae based on pathogenicity tests performed on zinnia, pepper, tomato, lettuce, cabbage and radish. All of the strains isolated from zinnias were pathogenic on zinnia and tomato, but not on the other crops. Representative strains of other X. campestrispathovars, ncluding X. c. pv. vesicatoria, X. c. pv. campestris, and X. c. pv. vitians (causal agents of bacterial spot of pepper and tomato, black rot of cabbage and radish and bacterial leaf spot of lettuce, respectively) were not pathogenic on zinnia. This is the first report of X. c. pv. zinniae as a pathogen of tomato. The relationship of strains of X. c. pv. zinniae to each other and to strains of other X. campestris pathovars was characterized using indirect ELISA with a set of 16 monoclonal antibodies (mAbs), protein electrophoresis, rep-PCR fingerprinting and sequence analysis of a PCR-amplified 16S-23S rDNA spacer region. The mAbs Xv1, Xv5, Xv6, Xv8, Xv10, Xv15, Xv21 and Xv30, specific for strains of X. c. pv. vesicatoria, and X9, X13, X17, X21, A11 and B35, specific to X. c. pv. armoraciae and X. c. pv. campestris strains (AM Alvarez and JB Jones, pers. comm.) did not react with any strains of X. c. pv. zinniae. However, all of the X. c. pv. zinniae strains reacted positively with X1 and X11, which are Xanthomonas genus-specific mAbs. SDS-protein patterns and rep-PCR fingerprints were similar for all X. c. pv. zinniae strains. No sequence polymorphisms were found in the 16S-23S rDNA spacer regions of the strains tested. However, X. c. pv. zinniae strains differed from those of related X. campestris pathovars by all techniques used in this study. Thus, we conclude that X. c. pv. zinniae strains form a distinct group that is clearly distinguishable by several methods from other X. campestris pathovars.

1. Naizzi A, 1930. Reviews in Applied Mycology 9, 316-317.

2. Sleesman J, White DG, Ellett CW, 1973. Plant Disease Reporter 57, 555-557.