Morphological and molecular characterization of Diaporthe (anamorph Phomopsis) complex and pathogenicity of Diaporthe aspalathi isolates causing stem canker in soybean

The complex of Diaporthe (asexual morph) species occurring on soybean constitutes an important pathogenic group associated with diseases such as pod and stem blight, seed decay and stem canker. Stem canker, caused by Diaporthe aspalathi, has been reported as the most aggressive form of canker and its occurrence has limited soybean crop productivity in the southern United States. The main form of pathogen control is the use of stem canker resistant soybean varieties. In this study, strains of Diaporthe and Phomopsis were isolated from stem and seeds of soybean in different locations in South America during the years 1989–2014. Genomic DNA from 26 isolates were analyzed by PCR-restriction fragment length polymorphism (RFLP) and Amplified Fragment Length Polymorphism (AFLP) techniques, and sequencing of internal transcribed spacer (ITS) regions of ribosomal DNA. The molecular analysis of ITS sequences by alignment with those of ex-type strains deposited in GenBank and morphological characteristics allowed the identification of Phomopsis longicolla , D . phaseolorum var. sojae , D . caulivora and D . aspalathi . An analysis of the pathogenicity of 13 isolates of D. aspalathi inoculated in soybean genotypes carrying different resistance genes to stem canker ( Rdm 1, Rdm 2, Rdm 3, Rdm 4, Rdm 5 and Rdm ?) enabled us to identify the occurrence of at least three races of D. aspalathi occurring in Brazil. Among the isolates identified as D. aspalathi , both molecular and phenotypic analyses showed clustering depending on the date of collection and pathogenicity, which revealed the existence of variability of the pathogen.