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First Report of an Aerial Blight of Chrysogonum virginianum (Green and Gold) Caused by Sclerotinia sclerotiorum in the United States

Trigiano, R. N., Boggess, S. L., Ownley, B. H.
Plant disease 2018 v.102 no.2 pp. 450
Chrysogonum virginianum, DNA, DNA primers, Sclerotinia sclerotiorum, agar, ambient temperature, blight, culture media, ethanol, flowers, fungal spores, fungi, gold, humidity, hyphae, internal transcribed spacers, microscopy, mycelium, ornamental plants, pathogens, petioles, plant growth, plastic bags, rifampicin, root systems, sclerotia, sodium hypochlorite, soil, spring, tissues, woodlands, Tennessee
Green and gold, also called goldenstar (Chrysogonum virginianum L.: Asteraceae), is native to the eastern woodlands of the United States and commonly used as an ornamental plant in shady, moist areas. The plants grow slowly, forming a dense mat of leaves, and produce small yellow flowers beginning in the spring. Even in dry weather, the area beneath the foliage mat is usually moist to wet. An aerial blight of C. virginianum was observed in an ornamental planting in Knoxville, Tennessee, in May 2016 and 2017. Symptoms included large coalescing areas of black, necrotic lamina tissue and dark black, water-soaked, and softened petioles, as well as necrotic areas on nonadjacent leaves. Necrotic petioles were lightly covered by white, fluffy mycelia. Microscopic examination revealed the presence of hyphae only; no fungal spores or other structures were discovered. Petioles and laminae were surface disinfected for 30 s in 70% ethanol, followed by 7 min in 1.31% NaOCl, and finally rinsed with three, 2-min washes in sterile, deionized water. All tissues were blotted to remove excess water and placed onto half-strength potato dextrose agar (PDA) augmented with 5 mg/liter of rifampicin in Petri dishes, which were incubated at room temperature. Fast-growing mycelium was transferred to new medium after 2 days. After 4 to 5 days, hyaline-to-white mycelium, appressed to the medium, covered the surface of the agar. Tufts of hyphae were formed near the perimeter of the colonies, but occasionally in other locations, and slightly appressed to the sides of the Petri dishes. Small (4 to 8 mm diameter, n = 15), black, rounded on the top and flat on the bottom, sclerotia were formed from the tufts of mycelium after 8 to 10 days, mostly along the perimeter and more rarely from other areas of the colonies. Sclerotia had black rinds and tan interiors. ITS1 and ITS4 primers (White et al. 1990) were used to amplify genomic DNA isolated from mycelium, and the sequence entered in GenBank (accession no. MF497424) was 100% identical to many accessions (e.g., KY750530.1) for Sclerotinia sclerotiorum. Our observations are similar to the growth of colonies and formation of sclerotia on culture medium reported by Ordóñez-Valencia et al. (2015) and to descriptions of disease development and symptomology (Bolton et al. 2006) of S. sclerotiorum. Koch’s postulates were completed by inoculating the base of petioles at the soil interface of seven healthy C. virginianum plants with 5-mm diameter plugs of mycelium grown on half-strength PDA for 4 days; three plants were inoculated with 5-mm diameter plugs of the same medium and served as controls. All plants were placed in plastic bags to maintain high humidity. After 10 days, all inoculated plants had symptoms of the disease and signs of the pathogen; the controls were symptom-free. A fungus with morphology closely resembling S. sclerotiorum was reisolated from diseased leaf petioles and laminae and its identity confirmed with internal transcribed spacer sequences. We concluded on the basis of symptoms presented, morphology of the fungus, and molecular evidence that the aerial blight of C. virginianum was caused by S. sclerotiorum. Although this disease destroys some foliage each year, the crown and root system appear to be unaffected, and the plant refoliates in the spring of the following year.