First report of Didymella americana on baby lima bean (Phaseolus lunatus)

Abstract

A new foliar disease was observed on baby lima bean (Phaseolus lunatus) in fields across western New York State, USA. The disease occurred in 10 fields with variable incidence and severity. Symptoms were initially necrotic, tan spots on leaves with red to reddish brown irregular margins that coalesced to encompass the entire leaf and cause abscission. Pycnidia were observed within the lesions. Isolations from diseased leaves yielded several pycnidial fungi, including a Didymella species. These isolates were characterized by morphology and sequencing of multiple reference genes (internal transcribed spacer (ITS), partial actin, β-tubulin (tub2), translation elongation factor 1-α (TEF), 28S rDNA large subunit (LSU), rpb2 and calmodulin). A four gene phylogeny (ITS, tub2, LSU and rpb2) showed that the isolates from baby lima bean belonged to a well-supported clade that contained the type culture of Didymella americana. Pathogenicity of the isolates on three commonly grown cultivars of baby lima bean was confirmed. Symptoms that developed on inoculated plants were similar to those observed on diseased plants in the field. This is the first report of D. americana on baby lima bean.