Six Species of Diaporthe Associated with Phomopsis Stem Canker of Sunflower in Southern Pampean Region of Argentina

A survey of 67 commercial fields in 19 locations was conducted in the southern Pampean region of Argentina for Phomopsis stem canker of sunflower (Helianthus annuus L.) between 2014 and 2019. A total of 210 plants having typical symptoms of Phomopsis stem canker were randomly sampled, and fungal isolation was performed. Of the 187 isolates of Diaporthe that were recovered, 94% of the isolates showed morphological characteristics similar to D. helianthi, 3% to D. gulyae, 1% to D. caulivora, 1% to D. sojae, 0.5% to D. kongii, and 0.5% to D. longicolla. Following morphological characterization, the identity of the six morphospecies was confirmed by phylogenetic analyses of beta-tubulin, translation elongation factor 1-alpha, and internal transcribed spacer gene regions. Koch's postulates were completed for the six fungi by inoculating one susceptible sunflower hybrid with one isolate each of the six species of Diaporthe using the stem-wound inoculation method. Seven days postinoculation, significant differences in disease severity were observed between the six isolates (P < 0.0001), with D. helianthi and D. gulyae isolates causing significantly greater disease severity. To our knowledge, this is the first report of D. kongii, D. longicolla, D. caulivora, and D. sojae associated with Phomopsis stem canker of sunflower in Argentina.