Main content area

First Report of Sclerotinia sclerotiorum (Sclerotinia Blight) on Symphyotrichum dumosum in Poland

Pieczul, K.
Plant disease 2018 v.102 no.5 pp. 1027
DNA primers, Sclerotinia sclerotiorum, Symphyotrichum, agar, aluminum foil, autumn, blight, color, culture media, databases, fungi, genetic analysis, home gardens, host plants, humidity, internal transcribed spacers, leaves, mycelium, necrosis, pathogen identification, pathogenicity, perennials, plastic bags, polymerase chain reaction, protocols, ribosomal DNA, sclerotia, seedlings, sodium hypochlorite, stems, Canada, Poland
Symphyotrichum dumosum (L.) G.L. Nesom, family Asteraceae, is a popular perennial plant, often cultivated in gardens due to attractive autumn blooming. From June to August in 2015, 2016, and 2017, typical symptoms of Sclerotinia blight were observed in four private gardens located in Nowy Tomyśl county (Miedzichowo, Jablonka Stara). The most intense disease development was found during prolonged rainy weather conditions or intensive watering of the gardens. Symptoms began as chlorotic wilt of the foliage, with leaves often changing color from green to reddish-brown. This was followed by a quickly developing necrosis on the stems. In the advanced stages of disease, plants died in circular patches with each patch encompassing approximately a dozen plants in a 30 to 50 cm radius and white, fluffy mycelia and black, irregularly spherical sclerotia (3 to 6 mm; n = 20) were observed at the base of each stem. In this study, isolates obtained from five samples (three S. dumosum varieties) were collected and used for pathogen identification. Sclerotia and 3 mm pieces of infected stems were surface-disinfested for 1 min in 3% NaOCl, rinsed with sterile water, and transferred onto potato dextrose agar (PDA). The isolations consistently yielded white fluffy colonies typical of Sclerotinia sclerotiorum (Lib.) de Bary. After 7 to 14 days of incubation, silvery-dark, globose sclerotia appeared on the surface of the agar plates. Based on the morphology of the mycelium and sclerotia, the fungus was identified as S. sclerotiorum. Pathogenicity testing was conducted on healthy S. dumosum seedlings (two seedlings per isolate) following the method described by Brodal et al. (2017). The plants were inoculated with 5-mm-diameter plugs taken from 5-day-old PDA cultures of S. sclerotiorum. On each plant, two disks were placed on the leaf axils and shielded by a piece of aluminum foil. As a control, additional plants (one per isolate) were inoculated with a pure PDA plug. All plants were covered with plastic bags to maintain high humidity and incubated at 18 to 24°C. After 3 to 5 days, the first symptoms of the disease appeared on the stems as water soaking and necrosis. Ten days after inoculation, typical Sclerotinia blight symptoms, necrosis covered by white mycelium, were found only on plants treated with S. sclerotiorum. Reisolation of the fungus was performed following the protocol described above and yielded colonies characteristic of S. sclerotiorum. For the subsequent genetic analysis, mycelia from all five samples were used. Genomic DNA was extracted from the fresh mycelium of each isolate with Norgen Biotek’s Plant/Fungi DNA Isolation Kit (Thorold, Canada). The internal transcribed spacer (ITS1-5.8S-ITS2) region of ribosomal DNA was PCR amplified with primers ITS1 and ITS4 (White et al. 1990), followed by sequencing of the product. The rDNA sequences of tested isolates were identical. One accession was deposited in the NCBI database (GenBank accession no. MF461640). A BLAST search of the NCBI database revealed that the tested S. sclerotiorum isolates had 100% identity to several previously deposited sequences of S. sclerotiorum (ITS1-5.8S-ITS2) from other host plants. To our knowledge, this is the first report of Sclerotinia blight, caused by S. sclerotiorum, on S. dumosum.