Brazil and Argentina have a combined soybean area of 53.6 million hectares, which accounts for over half of the total global production. The soybean crop in South America extends from latitude 8–10° S to 32–36° S. Such a vast, almost contiguous area imposes a serious sanitary risk to the crop. Currently, the prevalence of anthracnose is increasing, with recurring reports of severe epidemics and expressive yield losses. Soybean anthracnose is mainly associated with Colletotrichum truncatum, although other Colletotrichum species have also been reported as causal agents of this disease. Knowledge about the morphological, cultural, and molecular variability of C. truncatum in South America is crucial for disease management. Here, we present data on the molecular, morphological, biological, cultural, and pathogenicity of C. truncatum isolates collected in Brazil and Argentina. Light microscopy and randomly-amplified polymorphic DNA (RAPD) analysis were used for estimating the variability of isolates. Colletotrichum truncatum displayed three types of conidiogenesis, viz. conidial formation from conidiogenous cells on hyphal extremities, in conidiomas in acervuli, and directly from fertile setae (a mechanism yet-unreported for C. truncatum). RAPD profiling was effective in revealing the genetic diversity among C. truncatum isolates. The intra-group similarity was greater among the Argentinian isolates when compared to the Brazilian group. Furthermore, the results indicated a strong correlation between geographical origin and molecular grouping, with the exclusive or semi-exclusive assembling of Brazilian and Argentinian isolates in distinct clades. Finally, a preliminary account of the reaction of soybean accessions to C. truncatum is also included.

Soybean anthracnose is a complex disease, comprising isolates of Colletotrichum truncatum and at least six other species. In Brazil, C. cliviae was recently reported as a novel causal agent of soybean anthracnose but very little information is available about the C. cliviae-soybean pathosystem. Here we report the occurrence of C. cliviae in commercial soybean fields in the Tocantins State, TO (Northern Brazil). We also describe its most distinctive morphological, biological, cultural, and pathogenic aspects, particularly in contrast with C. truncatum (the prevalent anthracnose-causing species in Brazil), including symptoms, production of sexual structures, host range, seed infection and aggressiveness to soybean cultivars. The results indicated that the involvement of C. cliviae as an anthracnose-inducing species might impact current control strategies, including crop rotation systems and the replacement of susceptible cultivars. Studies were also carried out to clarify if the Brazilian soybean-infecting C. cliviae isolates were introduced into the country via contaminated seeds or if populations are endemic. The haplotype diversity of Brazilian as well as of a worldwide collection of C. cliviae isolates (available at the GenBank) was investigated based upon polymorphisms in three genomic regions (β-tubulin 2, glyceraldehyde-3-phosphate dehydrogenase, and actin). Nine haplotypes were identified among worldwide C. cliviae isolates from six different hosts. The Brazilian soybean isolates were placed into the closely related haplotypes 5 (composed by isolates from Mato Grosso State) and 6 (which included the isolate from TO), which were discriminated from each other by one single nucleotide polymorphism. Haplotype 6 group was also composed by a Chinese isolate from Camelia sinensis. The results suggest that this cosmopolitan fungal species is more likely endemic to Brazil where it has been reported in association with plant species across distinct botanical families. Therefore, the recent outbreaks of C. cliviae isolates on soybeans can be explained by either previous misdiagnosis or they may represent a recent shift/adaptation of this fungus to this legume crop. Thus far, the geographic occurrence of C. cliviae on soybeans is restricted to northern (warm) tropical regions of Brazil. The present report adds novel and relevant information with potential for the control of anthracnose caused by C. cliviae isolates in Brazil, and possibly in warm regions elsewhere.

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