Heuchera sanguinea (Saxifragaceae), coral bells or alum root, is an herbaceous perennial used in parks and gardens and sometimes grown in pots for its heart-shaped leaves and upright panicles of bright red, tiny flowers produced in late spring. At the end of fall 2006, a leaf blight was observed on 50% of a crop of potted 45-day-old plants grown in a sphagnum peat/clay/perlite (70:20:10) substrate at temperatures ranging between 20 and 25°C in a nursery. Semicircular, water-soaked lesions developed on leaves just above the soil line at the leaf-petiole junction and later along the leaf margins. For several days, lesions expanded along the midvein until the entire leaf was destroyed. Blighted leaves turned brown, withered, clung to the shoots, and matted on the surrounding foliage. Mycelia were often seen on and suspended between leaves. Blight progressed up the plant from the leaves to the shoot tip. Affected plants often died leaving wide empty areas. Diseased tissue was disinfected for 1 min in 1% NaOCl, rinsed with sterile water, and plated on potato dextrose agar (PDA) amended with 100 μg/liter of streptomycin sulfate. A fungus with the morphological characters of Rhizoctonia solani was consistently and readily recovered, then transferred and maintained in pure culture (3). The isolates of R. solani obtained from affected plants were successfully anastomosed with tester isolate AG 1 (ATCC 58946). The hyphal diameter at the point of anastomosis was reduced, the anastomosis point was obvious, and cell death of adjacent cells was observed. Results were consistent with other reports on anastomosis reactions (1). Pairings were also made with tester isolates of AG 2, 3, 4, 5, 6, 7, and 11 with no anastomoses observed between the recovered and tester isolates. Sclerotia were of uniform size with a diameter from 0.4 to 4 mm and sometimes joined laterally. The description of sclerotia was typical for subgroup 1A Type 2 (2). For pathogenicity tests, the inoculum of R. solani was prepared by growing three isolates of the pathogen on PDA for 7 days. Plants of 30-day-old H. sanguinea were grown in 10-liter containers (6 plants per container) on a steam disinfested peat/clay/perlite substrate (70:20:10)). Inoculum consisted of an aqueous suspension of PDA and mycelium disks (1 cm2 of mycelium per plant) and was placed at the base of the plant stems and on leaves. Plants inoculated with water and PDA fragments alone served as control treatments. Three replicates were used. Plants were maintained in a growth chamber at 24°C with 12 h of light/dark. The first symptoms, similar to those observed in the nursery, developed 12 days after the artificial inoculation. R. solani was consistently reisolated from infected leaves and stems. Control plants remained healthy. The pathogenicity test was carried out twice with similar results. This is, to our knowledge, the first report of leaf blight of H. sanguinea caused by R. solani in Italy and probably in the world.
References: (1) D. E. Carling. Grouping in Rhizoctonia solani by hyphal anastomosis reactions. In: Rhizoctonia Species: Taxonomy, Molecular Biology, Ecology, Pathology and Disease control. Kluwer Academic Publishers, the Netherlands, 1996. (2) R. T. Sherwood. Phytopathology, 59:1924, 1969. (3) B. Sneh et al. Identification of Rhizoctonia species. The American Phytopathological Society, St Paul, MN, 1991.
