Bacterial wilt, caused by Ralstonia solanacearum, is a major disease affecting potato (Solanum tuberosum) production worldwide. Although local reports suggest that the disease is widespread in Uruguay, characterization of prevalent R. solanacearum strains in that country has not been done. In all, 28 strains of R. solanacearum isolated from major potato-growing areas in Uruguay were evaluated, including 26 strains isolated from potato tubers and 2 from soil samples. All strains belonged to phylotype IIB, sequevar 1 (race 3, biovar 2). Genetic diversity of strains was assessed by repetitive-sequence polymerase chain reaction, which showed that the Uruguayan strains constituted a homogeneous group. In contrast, inoculation of the strains on tomato and potato plants showed, for the first time, different levels of aggressiveness among R. solanacearum strains belonging to phylotype IIB, sequevar 1. Aggressiveness assays were also performed on accessions of S. commersonii, a wild species native to Uruguay that is a source of resistance for potato breeding. No significant interactions were found between bacterial strains and potato and S. commersonii genotypes, and differences in aggressiveness among R. solanacearum strains were consistent with previously identified groups based on tomato and potato inoculations. Moreover, variation in responses to R. solanacearum was observed among the S. commersonii accessions tested.

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This study provides insights into the pathogenesis of Ralstonia solanacearum, in particular with regards to strains belonging to phylotype IIB, sequevar 1 (IIB-1) and their interaction with potato, its natural host. We performed a comparative genomic analysis among IIB-1 R. solanacearum strains with different levels of virulence in order to identify candidate virulence genes. With this approach, we identified a 33.7-kb deletion in a strain showing reduced virulence on potato. This region contains a cluster of six genes putatively involved in type IV pili (Tfp) biogenesis. Functional analysis suggests that these proteins contribute to several Tfp-related functions such as twitching motility and biofilm formation. In addition, this genetic cluster was found to contribute to early bacterial wilt pathogenesis and colonization fitness of potato roots.

Wheat blast of wheat (Triticum aestivum), caused by Magnaporthe oryzae pathotype triticum (MoT; anamorph Pyricularia oryzae) is a destructive disease in the South American countries of Brazil, Paraguay and Bolivia. In Argentina, the fungus was recently recorded on wheat and barley plants in the northeast part of the country, Buenos Aires and Corrientes Provinces, with a potential for spreading. This work aimed to study, for the first time, the morphocultural and pathogenic characteristics of Magnaporthe isolates collected from wheat and other herbaceous species in Argentina and three neighbouring countries (Paraguay, Brazil and Bolivia) and determine their aggressiveness on wheat varieties. Statistical differences among isolates, culture media, and development conditions were found for conidia colour, growth rate, size and sporulation rate. Pathogenicity tests performed on seedlings with 19 isolates of Magnaporthe spp. under greenhouse conditions showed a maximum disease severity of 55.3% and 66.7% for varieties BIOINTA 3004 and Baguette 18, respectively. Weed and grass isolates were infectious on wheat, demonstrating their potential epidemiological role on the disease. Spike disease severity was 34.6% for the host × pathogen interaction of BIOINTA 3004 × PY22. Observed symptoms included partial or total spike bleaching, and glume and rachis discolouration. The 1000‐grain weight was significantly reduced to 38.5% and 63.1% for cultivars BIOINTA 3004 and Baguette 18, respectively. The disease affected grain germination, which fell to 65.9% for seeds infected with the PYAR22 isolate. Symptoms observed in infected grains were partial spotting, grain softening, and rot symptoms with the presence of a greyish mould.