First report of Aspergillus welwitschiae causing maize ear rot in Serbia

In recent years, countries in Southeast Europe are facing climate changes characterized by extreme hot weather, which contribute to the increased frequency of Aspergillus species. Because of these changes, Aspergillus parasiticus was isolated, for the first time, from maize grain in Serbia (Nikolic et al, 2018). The presence of black powdery mycelia on maize ears indicated occurrence of species of the genus Aspergillus section Nigri, which led to the need for detailed identification of these fungi. Disease incidence ranged from 10 and 15% in August 2013. Maize ears with black powdery symptoms were collected from field in Zemun Polje, Serbia. Symptomatic kernels were surface sterilized with 1% sodium hypochlorite solution for 3 min, rinsed three times with sterilized water, then incubated at 25°C in the dark for 7 days on potato dextrose agar (PDA). Twenty isolates were identified as genus Aspergillus section Nigri. Monospore cultures formed black cottony colonies with a yellowish border on PDA. The average colony diameter was 50 mm. In order to reliably identify, isolates were transferred to Malt Extract agar (MEA) and Czapek Yeast Autolysate agar (CYA) (Samson et al, 2014). On CYA fungal colonies consisted of a white mycelium, covered by a layer of black conidiophores. On MEA fungal colonies were dense, black, with yellowish border. The reverse side was colorless to pale yellow, with a yellow ring in the middle. The average size of conidia was 4.3 µm. The conidia were globose to sub-globose, smooth to roughened, which coincides with previous research (Silva et al, 2020). Given that the fungi Aspergillus niger and Aspergillus welwitschiae are morphologically indistinguishable (Susca et al, 2016), species level identification was completed by analysis of a partial sequence of the internal transcribed spacer (ITS) region (ITS1/ITS4 primers) and calmodulin gene (CMD5/CMD6 primers) (Samson et al., 2014). The sequences were compared with the sequences of A. welwitschiae strains registered in the GenBank database based on nucleotide similarity, and results showed 99,64 and 100% similarity with ITS (OL711714) and calmodulin (KX894585), respectively. The sequence was deposited in GenBank with accession numbers OQ456471 (ITS) and OQ426518 (calmodulin). We also confirmed the presence of this species with specific primers (AWEL1/AWEL2) designed by Susca et al. 2020. Pathogenicity test was performed in Zemun Polje on the same maize hybrid from which the fungal species was isolated. Using artificial inoculations by the injecting conidial suspension into the silk channel, three days after 50% of plants reached the silking stage. Twenty ears were inoculated with each isolate, in four replicates (Reid et al, 1996). Inoculum was prepared from 7-day-old colonies on PDA, and 2 ml of a conidial suspension (1×106 spores/ml) was used. Control plants were inoculated with sterile water. All inoculated ears showed symptoms, similar to those from field infections. Control ears were symptomless. The fungus was reisolated and was morphologically identical to the original isolates, thus completing Koch's postulates. Based on molecular, morphological and pathogenic properties, the isolates were identified as A. welwitschiae. This is the first report of A. welwitschiae as the causal agent of black maize ear rot not only in Serbia, but also in the other countries of the Western Balkans. Given that the fungus A. welwitschiae synthesizes both ochratoxin A (OTA) (Battilani et al, 2006) and fumonisin (FB) (Frisvad et al, 2011), further studies should be focused on assessment its aggressiveness and toxicological profile.