First report of white mold caused by sclerotinia sclerotiorum on watercress (Nasturtium officinale) in Italy

Watercress (Nasturtium officinale), Brassicaceae family, is an herbaceous plant belonging to the native Italian flora that is appreciated for its flavor as a green salad. During the spring of 2018, approximately 2% of 480 2-month-old potted plants of N. officinale grown in a commercial farm located in Albenga (northern Italy) showed symptoms and signs of white mold. Affected leaves yellowed and became pale brown, and within 2 days of yellowing they wilted and withered. Stems rotted starting from the base. A whitish mycelium producing spheroid, sometimes elongated, 2 to 5 (average, 3) mm (n = 50), dark sclerotia covered the affected tissues and the surface of the potted soil. Affected plants died within 7 days of first symptoms. Apothecia, asci, and ascospores were not observed. Small pieces (approximately 2 × 2 × 2 mm) of affected tissue were surface disinfected in a NaOCl solution for 10 s, rinsed with sterilized water and plated on potato dextrose agar (PDA). Plates were incubated at 22 ± 1°C for 6 days. Isolations consistently yielded white, fluffy, cottony colonies that produced spheroid, dark sclerotia primarily near the edges of plates. Sclerotia were 2 to 8 (average, 3) mm (n = 50 from five plates). Based on the morphological features of the isolates, the causal agent was tentatively identified as Sclerotinia sclerotiorum (Lib.) De Bary (Mordue and Holliday 1976). DNA was extracted from a pure culture of a single isolate (DB18APR01) using the E.Z.N.A. Fungal DNA Mini Kit (Omega Bio-Tek, Darmstadt, Germany). A polymerase chain reaction was performed using primers ITS1/ITS4 (White et al. 1990) to amplify the internal transcribed spacer (ITS) region of the rDNA. NCBI BLAST analysis of the 514-bp sequence (GenBank accession no. MH327997) showed 100% identity with the sequence KX184720 of S. sclerotiorum. In the pathogenicity test, a steamed substrate was artificially inoculated (3 g/liter) with wheat kernels colonized by mycelium and sclerotia of S. sclerotiorum isolate DB18APR01. Fifteen 20-day-old plants of N. officinale were transplanted into the artificially inoculated soil, whereas 20 control plants were transplanted into soil infested with sterilized noninoculated kernels. All plants were kept in a humid chamber at 23 ± 1°C. Five days later, all inoculated plants showed first symptoms of basal stem rot and wilted, and a white mycelium developed on affected tissues. A fungus characterized by whitish mycelium with spheroid or elongated, dark sclerotia, identical to S. sclerotiorum, was reisolated from 100% of the plants. Control plants remained asymptomatic. S. sclerotiorum has been reported on N. officinalis in the United States (Farr et al. 1989). To our knowledge, this is the first report of S. sclerotiorum on N. officinalis in Italy and in Europe. The economic importance of the disease detected on N. officinalis is at present limited, but it could spread owing to the increasing use of this species for culinary uses, causing severe losses to the local production made up of several minor crops.