Lychnis coronaria (syn. Silene coronaria), rose campion, is a perennial in the Caryophyllaceae used in gardens. In the summer of 2014, a web blight was observed in a private garden located near Biella (northern Italy), approximately 45°39′N 8°00′E, on 40% of 100 5-month-old plants grown in sandy soil. In the days preceding the outbreak of the disease, daytime temperatures ranged from 18 to 24°C and relative humidity from 45 to 83%. Affected plants showed pale brown discoloration of stems, starting from the base, and eventually collapsed. Under conditions of high relative humidity, a scant amount of whitish mycelium developed on leaves of about 50% of diseased plants. Eventually, infected plants died about 10 days after symptoms appeared. Symptomatic tissues of stems and leaves were disinfected for 10 s in 1% NaOCl, rinsed in sterile water, and plated on potato dextrose agar (PDA). A fungus with the morphological characters of Rhizoctonia solani (3) was consistently recovered. Three representative isolates were paired with tester strains of R. solani (AG 1, AG 2-2-IIIB, AG 4, AG 7, and AG 11) (2) and examined microscopically. The Lychnis isolates anastomosed only with the AG 1 tester strain, with low fusion frequency. The anastomosis point was obvious: the hyphal diameter at the point of anastomosis was reduced and death of adjacent cells was observed, indicating an anastomosis reaction (1). Mycelium maintained on PDA at 23 ± 1°C was coarse and reddish brown. After 5 days of growth, mycelium started differentiating numerous sclerotia, often aggregated. Mature sclerotia were dark, spheroidal, with diameters ranging from 0.2 to 1.6 (average 0.6) mm. The internal transcribed spacer (ITS) region of rDNA was amplified using the primers ITS1/ITS4 and sequenced. BLASTn analysis of the 609-bp amplicon (GenBank Accession No. KM596899) showed a 98% homology with the sequence of the R. solani isolate FJ746937 obtained from Zoysiagrass. On the basis of molecular and cultural characteristics and anastomosis tests, the isolates from L. coronaria were identified as R. solani AG 1-IB (4). For pathogenicity tests performed in August, mycelial disks (8 mm diam.) from 10-day-old PDA cultures of an isolate of the fungus were placed on four healthy 6-month-old L. coronaria plants (four stem and six leaf disks per plant). Four plants inoculated with disks of PDA served as controls. Plants were covered with plastic bags for 4 days and maintained in a garden located in the same area in which the disease appeared, at field temperatures ranging from 15 to 28°C. The first symptoms developed 4 days after inoculation, and 15 days after the artificial inoculation, all inoculated plants were dead. R. solani was re-isolated from the stem of symptomatic plants, whereas no colonies developed from controls, which all remained healthy. This is the first report of blight of L. coronaria caused by R. solani in Italy or anywhere else in the world. The impact of this disease may become a significant problem for L. coronaria, a very common species in Italian gardens.
References: (1) D. E. Carling. Page 37 in: Rhizoctonia Species: Taxonomy, Molecular Biology, Ecology, Pathology and Disease Control. Kluwer Academic Publishers, The Netherlands, 1996. (2) A. Ogoshi. Ann. Rev. Phytopathol. 25:125, 1987. (3) B. Sneh et al. Identification of Rhizoctonia species. APS Press, St. Paul, MN, 1991. (4) R. T. Sherwood. Phytopathology 59:1924, 1969.