Detection of a New Bacterium Related to Xanthomonas fuscans subsp. aurantifolii Infecting Swingle Citrumelo in Brazil.

In March 2009, in a sweet orange orchard (Citrus sinensis) cv. Valencia grafted on Swingle citrumelo (C. paradisi Macf. × Poncirus trifoliata L. Raf.) rootstock in Severínia County, São Paulo State, Brazil, approximately 40 trees were detected with small, necrotic, dark brown leaf spots. These lesions occurred whether or not citrus leafminer (Phyllocnistis citrella) was present and they were only found on leaves from branches arising from the rootstock. Sweet orange foliage was not affected even when in contact with infected rootstock branches. Symptoms were unusual and distinct from typical citrus canker lesions because the lesions were smaller and did not have erumpent margins. Typical yellow Xanthomonas colonies were isolated from the lesions on nutrient agar. The isolates were aerobic, gram negative, rod shaped, and they produced a dark pigment, which is characteristic of some Xanthomonas fuscans subsp. aurantifolii strains. Two reference strains were tested for pathogenicity on not fully expanded leaves of sweet orange, Swingle citrumelo, and key/Mexican lime (C. aurantifolia) plants by wound inoculation with a sterile needle previously dipped in a bacterial suspension (approximately 106 ml-1). Two plants of each species were used for inoculations in greenhouse conditions and six leaves were inoculated per plant. Each inoculated leaf received six point inoculations. These tests confirmed that the host range of this pathogen was restricted to Swingle citrumelo. Symptoms similar to those in the orchard were observed 3 weeks after inoculation and Koch's postulates were completed by reisolation of the bacterium and comparing it with the original isolates. Molecular fingerprinting with PCR-restriction fragment length polymorphism (RFLP) of the 16S-23S spacer region polymorphism (1) and ERIC- and BOX-PCR (2) was used to compare the new strain with 26 reference strains of Xanthomonas citri subsp. citri types A, A* and Aw, X. fuscans subsp. aurantifolii types B and C, and X. alfalfae subsp. citrumelonis. PCR-RFLP and ERIC-PCR showed that this new pathogen had the same profile as X. fuscans subsp. aurantifolii (B and C types). In BOX-PCR, this new strain had a unique profile, but it was still most similar to X. fuscans subsp. aurantifolii and very distinct from X. citri subsp. citri (A, A*, and Aw) and X. alfalfae subsp. citrumelonis strains. During the rainy season in Brazil, this new Xanthomonas strain is less aggressive than X. citri subsp. citri on Swingle citrumelo, inducing fewer lesions without erumpent margins even in young leaves severely infested by the citrus leafminer. The disease only occurred on trees that were separated from each other by 3 to 20 m, suggesting that the bacterium is spread by windblown rain and/or cultural practices. Xanthomonads pathogenic to citrus are of great importance for regulatory purposes worldwide. X. fuscans subsp. aurantifolii is only known to be pathogenic on lemons and limes in the field, and until now, has only been reported to infect lemons and limes in Argentina and key/Mexican lime in São Paulo (Brazil) (3). To our knowledge, this is the first report of a strain of this subspecies that infects Swingle citrumelo but not key/Mexican lime. References: (1) S. A. L. Destéfano and J. Rodrigues Neto. Summa Phytopathol. 28:167, 2002. (2) F. J. Louws et al. Appl. Environ. Microbiol. 60:2286, 1994. (3) N. W. Schaad et al. Syst. Appl. Microbiol. 28:494, 2005.

Plant-pathogenic bacteria as biological weapons - real threats?

At present, much attention is being given to the potential of plant pathogens, including plant-pathogenic bacteria, as biological weapons/bioterror weapons. These two terms are sometimes used interchangeably and there is need for care in their application. It has been claimed that clandestine introduction of certain plant-pathogenic bacteria could cause such crop losses as to impact so significantly on a national economy and thus constitute a threat to national security. As a separate outcome, it is suggested that they could cause serious public alarm, perhaps constituting a source of terror. Legislation is now in place to regulate selected plant-pathogenic bacteria as potential weapons. However, we consider it highly doubtful that any plant-pathogenic bacterium has the requisite capabilities to justify such a classification. Even if they were so capable, the differentiation of pathogens into a special category with regulations that are even more restrictive than those currently applied in quarantine legislation of most jurisdictions offers no obvious benefit. Moreover, we believe that such regulations are disadvantageous insofar as they limit research on precisely those pathogens most in need of study. Whereas some human and animal pathogens may have potential as biological or bioterror weapons, we conclude that it is unlikely that any plant-pathogenic bacterium realistically falls into this category.

Pathological Variations Within Xanthomonas campestris pv. mangiferaeindicae Support Its Separation Into Three Distinct Pathovars that Can Be Distinguished by Amplified Fragment Length Polymorphism.

ABSTRACT Bacterial black spot, caused by Xanthomonas campestris pv. mangiferaeindicae, is an important disease of mango (Mangifera indica). Several other plant genera of the family Anacardiaceae were described as host species for xanthomonads. We studied pathological variations among strains in a worldwide collection from several Anacardiaceae genera. Strains were classified into three pathogenicity groups. Group I strains (from the Old World) multiplied markedly in leaf tissue of mango and cashew (Anacardium occidentale). Group II strains (from Brazil) multiplied markedly in cashew leaf tissue, but not in mango. Moreover, mango leaves inoculated with group I and group II strains exhibited lesions with different morphologies, consistent with variations in symptomology previously reported on mango under field conditions. Group I strains produced black, raised lesions, consistent with the original description of the pathovar, whereas group II strains produced brownish, flat lesions. Group III strains produced a unique syndrome on ambarella (Spondias dulcis) and mombin (Spondias mombin). Based on evolutionary genome divergence derived from amplified fragment length polymorphism (AFLP) data, the three groups were genetically distinct and were related to groups 9.5, 9.6, and 9.4 of X. axonopodis identified by Rademaker, respectively. As each group was characterized by unique symptomology and/or host range, we propose that X. campestris pv. mangiferaeindicae be split into three pathovars of X. axonopodis: X. axonopodis pv. mangiferaeindicae, X. axonopodis pv. anacardii, and X. axonopodis pv. spondiae. Within pv. mangiferaeindicae sensu novo, AFLP data were consistent with that previously published for restriction fragment length polymorphism groups and suggested long-distance movement of the pathogen, likely through propagative material.