First Report of Downy Mildew Caused by Peronospora arthurii on Common Evening Primrose (Oenothera biennis) in Italy

Abstract

Common evening primrose (Oenothera biennis L.) belonging to the family Onagraceae is a low-maintenance biennial plant that is widely used as a border for blooming in the late spring and summer in parks and gardens in northern Italy. During spring 2017, symptoms of a downy mildew were observed on about 400 plants of O. biennis growing in mixed borders as well as in pots, in a private garden near the city of Biella (northern Italy). Plants were originated by seeds produced in Italy, and they were reproduced spontaneously in this garden for about 10 years. Infected leaves showed yellow, vein-limited lesions on the upper surface. As the disease progressed, affected leaves turned necrotic, and plants lost their aesthetic value. The abaxial surface of affected leaves was covered by gray conidiophores and conidia in areas delimited by veins. Conidiophores measured 218 to 374 µm (average, 295 µm). Conidiophores branched dichotomously four to six times with terminal branches arranged at right angles and with curved ultimate branchlets that measured 7.6 to 20.2 µm (average 12.8 µm). Conidia were light gray, elliptic to ovoid, and measured 21.1 to 27.4 × 16.1 to 20.2 µm (average, 24.4 × 18.1 µm). These morphological characteristics corresponded to those described by Mułenko and Gosztyła (1997) for Peronospora arthurii. The internal transcribed spacer (ITS) region of rDNA was amplified using primers ITS1/ITS4 (White et al. 1990) and sequenced. BLAST analysis (Altschul et al. 1997) of a 730-bp sequence (GenBank accession no. MF370230) revealed 99% similarity to that of P. arthurii AY198284. Pathogenicity was confirmed by gently pressing affected leaves of O. biennis with conidia of P. arthurii onto moistened leaves of three 4-month-old healthy plants of the same host. Nine affected leaves (three for each inoculated plant) were used. Leaves of three control plants were moistened with sterilized water. All plants were kept in a humid chamber located outside, at temperatures ranging from 9 to 27°C (17 h daylight, 7 h dark) for 7 days. First yellowing appeared after 10 days only on inoculated plants. As the disease progressed, the same conidiophores and conidia described above were observed on affected leaves. P. arthurii on O. biennis has been reported in Austria, Canada, the Czech Republic, the United States (Farr and Rossman 2017), and Poland (Mułenko and Gosztyła 1997). This is the first report of P. arthurii on O. biennis in Italy. The economic importance of this disease is at present limited, although it could spread owing to the increased planting of O. biennis in low-maintenance gardens.