Spot blotch caused by Bipolaris sorokiniana is a destructive disease of wheat in warm and humid wheat-growing regions of the world. This fungus shows a high genetic diversity and morphological and physiologic variability. In this study, 19 polysporic and 57 monosporic isolates of B. sorokiniana were characterized using universal rice primers-URP-PCR. The results obtained when the dendrogram was constructed with all the data produced with the amplification products showed very distinct clusters. However, the similarity among the isolates was low where 37 and 26.3 % of the monosporic and polysporic isolates, respectively, showed similarity above 70 %. All primers amplified multiple DNA fragments of polysporic as well as the monosporic isolates. Isolates fingerprints were constructed based on binary characters revealed by the three primers. An amplified fragment of approximately 750 bp was observed among 40 % of the isolates, when primer URP-1F was used. When primers URP-4R and URP-2R were used, a fragment of 450 and 400 bp was present in 31.5 and 29 % of the isolates, respectively. It was expected a higher similarity among the isolates since the monosporic cultures were originated from the polysporic. The dendrogram did not enable the separation of B. sorokiniana isolates by their geographic origin. This low correlation suggests that gene transfer may have occurred by parasexual combination in this fungus population. However, in spite of the research efforts for that end, it has not been possible to establish patterns that characterize the profile of B. sorokiniana.

Actinobacteria secrete substances that limit or inhibit the growth of plant pathogenic fungi and may be used in the biocontrol of these microorganisms. The aim of this study was to characterize physiological and enzymatic activity of endophytic actinobacteria, evaluate their antifungal activity against Bipolaris sorokiniana root colonization, and evaluate their efficiency in promoting the growth of wheat seedlings. Antibiosis was analyzed using the double-layer method, the agar well diffusion test, and volatile metabolites. Physiological and enzymatic activity was evaluated through chitinase, glucanase, siderophores, indole-3-acetic acid, nitrogen fixation and phosphate solubilization tests. In vivo assays were evaluated by root colonization, biocontrol test and efficiency to promote the growth of wheat seedlings. From all isolates tested, 69.6% of them presented antifungal activity against at least one B. sorokiriana isolate. Among these, 17% of the isolates produced bioactive metabolites in the supernatant when grown in submerging culture. The highest production of bioactive metabolites was at 30°C, between 72 and 96 h of incubation. Three isolates produced volatile compounds, chitinase, glucanase, siderophores and exhibited nitrogen fixation, produced indole-3-acetic acid, efficiently colonized the root system of seedlings of two wheat cultivars. The best isolate [6(2)] showed, under the greenhouse, the capacity to promote an increased biomass and tillers per wheat plant.

Bipolaris sorokiniana may present considerable genetic diversity and highly variable pathogenicity and virulence. The pathogenicity of 99 B. sorokiniana isolates (27 polysporic and 72 monosporic isolates) from Brazil and other countries was assessed. Based on aborted germination, black point of seed, leaf spot, and coleoptile lesion, the principal component analysis (PCA) was used to evaluate the similarity patterns between isolates considering the variables of pathogenicity. Polysporic isolates presented higher virulence (over 60%), when compared with the monosporic isolates (43%) for all variables, except coleoptile injury. Of all isolates used to infect seeds, 8% were highly virulent, and the score obtained was over 75%, for all variables analyzed. The correlation of B. sorokiniana isolates with pathogenicity variables demonstrated that polysporic isolates were more virulent, especially upon seeds, as compared to aerial plant parts.