Black mold of stored onion bulbs caused by Aspergillus welwitschiae

Black mold of onion bulbs is a postharvest disease that can cause significant economic losses when onions are inappropriately stored. Among several species of Aspergillus, section Nigri, reported as causal agents of black mold, recent studies have shown a dominant presence of A. welwitschiae on onion bulbs. In the present study, onion bulbs 'Vuelta' were collected in 2015 in a storage facility in Stara Pazova, Serbia. Areas of sporulation of the pathogen that were black in appearance were present on the outer scales of the collected onion bulbs and inside, light brown soft and watery decay was spreading from the neck throughout the fleshy scales. Two fungal isolates were obtained using standard laboratory procedures and pathogenicity of the isolates was tested by wound inoculation of healthy onion bulbs 'Stuttgarter riesen'. Three weeks after inoculation lesions developed and sporulation was visible. The isolates were preliminarily identified based on morphological characteristic as members of Aspergillus section Nigri. Species level identification was completed by analysis of a partial sequence of the calmodulin gene. Based on the molecular and morphological properties, the isolates were identified as A. welwitschiae. This is the first report of A. welwitschiae as the causal agent of black mold of stored onion bulbs in Serbia. Since onion bulbs are widely used in human consumption, the presence of toxigenic A. welwitschiae (produces ochratoxin and fumonisin) indicates the need for further investigation and evaluation of the toxigenic potential of isolates present in Serbia.