Common millet (Panicum miliaceum L.) is a niche crop cultivated in Italy for its high nutritional and gluten-free characteristics. These aspects combined with its sustainability and adaptability to paddy soils make it relevant in crop rotation with rice in some organic farms in Northwestern Italy. In September 2021, in a farm producing organic rice located in Rovasenda (VC), Italy (GPS: 45°54'82"N, 8°.31'96"E), plants of millet (var. San Marino), sown on July 1st and almost at physiological maturity, were observed with gray leaf spot symptoms with an incidence of 50% in a 12.5 ha field cultivated in a yearly rotation with rice. Oval shape lesions (between 1 and 5 cm long and 0.5 to1 cm wide) with a light gray colored centers and dark reddish-brown edges were observed on the length of the leaf blade. Gray colonies producing conidia morphologically like Pyricularia developed from 140 symptomatic leaf fragments with a 2% frequency. Monosporic cultures were obtained from five colonies and were grown on Potato Dextrose Agar for morphological and molecular identification. The colonies showed gray, fluffy mycelium, with a smooth margin, with a radius of 28 to 30 mm after 10 days at 28°C. The isolates produced hyaline, pyriform, and 2-septate conidia 17.9 to 33.5 µm (average 25.7 µm) long and 8.3 to 11.7 µm (average 10. µm) wide (n=50) on PDA. Conidiophores were solitary, erect, straight, or curved, septate, and measured 80.2 to 221.3 µm (average 150.7 µm) long and 3.6 to 5.8 µm (average 4.7 µm) wide (n=50). The molecular identification of one single-conidia isolate was confirmed by PCR and sequence comparison of the region of the transposon Pot2 (Harmon et al., 2003). BLASTn searches of GenBank using transposon Pot2 (ON843711) representative isolate 21-03-C, revealed 99.79% identity to P. oryzae (Syn. Magnaporthe oryzae) isolate MD1 (MK608664). Pathogenicity tests were carried out by suspending conidia from a 14-days old culture on PDA in sterile water at a concentration of 1×105 CFU/ml. Twenty seeds of P. miliaceum were sown in pots filled with a steamed mix of white peat and perlite, 80:20 v/v, and maintained at 28°C under a 12 hours day/night light cycle. Diseased leaves of 10 plants were inoculated by aspersion of 10 ml of the conidial suspension of isolate 21-03-C 2 weeks after the sowing (Harmon et al.; 2003). Plants were incubated in a near moisture-saturated environment in plastic bags at 28°C for 72h, after which symptoms were visually assessed. Ten control plants, inoculated with 10 ml of sterile distilled water remained healthy. P. oryzae was reisolated from leaves showing oblong, whitish lesions, surrounded by light brown margins and identified by resequencing of transposon Pot2. P. oryzae has been already reported on millet in South Korea as causal agent of blast (Klaubauf et al.; 2014). To our knowledge, this is the first report of P. oryzae as a pathogen of millet in Italy and in Europe. Further studies should be conducted to characterize these millet isolates by assessing their virulence on Oryza sativa and the implications in the adoption of P. miliaceum in rotation with rice, to prevent possible carry over of the pathogen on the main crop.
Keywords: Panicum miliaceum; blast; rice.