Physoderma, not Olpidium, is the true cause of faba bean gall disease of Vicia faba in Ethiopia

Abstract

Faba bean gall (FBG) is a devastating disease of faba bean (Vicia faba) in Ethiopia. Studies were undertaken first to compare and contrast similarities between FBG disease symptoms and morphology in Ethiopia with those reported earlier in China and, secondly, to identify definitively the FBG causal agent, previously considered as Olpidium viciae, through molecular studies. Morphological studies confirmed an epibiotic phase of zoosporangia for dispersing zoospores, characteristic of Physoderma but not Olpidium, and did not show critical diagnostic characteristics of Olpidium such as presence of numerous short zoosporangial discharging tubes, or binucleate resting sporangia. Recognizing this epibiotic phase is a foundation for comprehending FBG epidemiology and will allow forecasting of zoospore release to highlight best timings for applications of chemical sprays to reduce reinfection cycles. Sequences of partial ITS1‐5.8S‐partial ITS2, the 18S‐ITS1‐5.8S‐ITS2‐part of 28S rRNA, and LSU (28S rRNA) derived from tissue with symptoms confirmed Physoderma, and not Olpidium, as the causal agent. Sample sequences were either close to Physoderma or the contaminant ascochyta pathogen Didymella. From symptom, morphological, and molecular data, the causal agent of FBG disease in Ethiopia is Physoderma. From observations of symptoms that Physoderma can cause, it was determined that this Physoderma crosses over between different legume host genera (e.g., Vicia, Pisum, Trifolium), highlighting the significant biosecurity risk for countries currently free of FBG.