In the last years Conyza bonariensis has become an important weed and control is difficult with the use of current technology in Argentinean pampas region. The increasing prevalence of herbicide-resistant weed species, public concern related to pesticide use and the introduction of government policies for pesticide reduction, is driving the search for alternative methods to chemical control. The aims of the present study were to detect fungal diseases associated with C. bonariensis, to identify fungal isolates from the symptomatic leaves and to confirm through Koch's postulates the isolates pathogenicity. Mycological analysis of symptomatic leaves showed the presence of twelve genera of filamentous fungi. Among 116 isolates, Colletotrichum spp. was the most prevalent genus followed by Nigrospora spp. and Septoria spp. In the pathogenicity assays, 22 out of 116 isolates were able to comply with the Koch's postulates. The pathogenic isolates were included into three genera Alternaria spp., Colletotrichum spp. and Septoria spp. This study provides the first report that demonstrates pathogenicity of fungal isolates on C. bonariensis in Argentina and represents the first step in a future biocontrol program.

The present study evaluates the possible modes of action of antagonistic bacteria and their tolerance to UV radiation, temperature and osmotic stress. The partial 16S-23S rRNA gene sequencing of eight antagonistic bacteria had a high match with three bacterial genera: Curtobacterium, Pantoea and Bacillus. In this study, the three Bacillus isolates showed the most relevant production of enzymes, volatile organic compounds and antibiosis against Exserohilum turcicum. Respect to UV radiation and temperature Pantoea and Bacillus isolates were more tolerant; whereas the eight isolates were tolerant to osmotic stress in varying degree. The three Bacillus isolates have the greatest potential as biocontrol agents for foliar diseases in maize. The antagonistic action could be explained through different modes of action such as enzymes, volatile organic compounds and/or direct antibiosis by other secondary metabolites. Bacillus isolates tolerance to environmental stresses including UV radiation, temperature and osmotic stress is relevant for survival and persistence on the leaf surface. This work provides new information about the mode of action of antagonistic bacteria with proven efficacy against maize leaf pathogens. In addition, it provides information about the tolerance of antagonistic bacteria against different stress conditions. The data of the present study could contribute to the development of a successful foliar biofungicide.

Aims This study investigated the diversity of Colletotrichum isolates recovered from Conyza bonariensis leaves through the use of morphological characteristics, growth rate, carbon sources utilization and phylogenetic analysis. Methods and Results In all, 30 Colletotrichum isolates recovered from C. bonariensis leaves showing symptoms of disease were included in the present study. Based on the analysis of morphology and sequences, the isolates were distributed into six Colletotrichum species complexes. The concatenated alignment of GAPDH and ITS sequences showed that 20 out of 30 isolates were included in four species complexes which comprise the most important pathogens causing anthracnose in soybean or anthracnose and stalk rot in maize: C. truncatum, C. orchidearum, C. gloeosporioides and C. graminicola. The remaining 10 isolates were included in the C. boninense and C. destructivum species complexes or could not be assigned to any complex with the available information. Conclusion Weeds belonging to genus Conyza are host to soybean and maize potential pathogenic species of Colletotrichum and could have a role as inoculum reservoir for cross contamination in the agroecosystem. Significance and Impact of the Study The combined use of morphological, kinetics and physiological parameters of growth and phylogenetic analysis in Colletotrichum isolates from Conyza leaves allowed the detection of species complexes previously not identified in Argentina.