First Report of Crown Rot, Stem Rot, and Root Rot Caused by Binucleate Rhizoctonia AG-G on Viburnum tinus in Italy.

Abstract

Laurustinus (Viburnum tinus L.), native to the Mediterranean Region, is an evergreen shrub belonging to the Caprifoliaceae that is commonly cultured as an ornamental shrub or small tree. During the summer and autumn of 2007 and 2008, a widespread yellowing, partial foliar necrosis, or death of the whole plant was observed on 3- to 4-year-old potted plants of V. tinus in a commercial nursery in eastern Sicily (Italy). More than 20% of the plants showed disease symptoms. Infected roots, crowns, and stems turned dark brown, leaves gradually became necrotic, and infected plants were often killed. Diseased tissues were disinfested for 1 min in 1% NaOCl, rinsed in sterile water, plated on potato dextrose agar (PDA) amended with streptomycin sulfate at 100 mg/liter, and then incubated at 25°C. A binucleate Rhizoctonia (BNR) species was consistently isolated from affected tissue of Laurustinus. Fungal colonies were initially white, then turned light brown or brown with age, and formed irregularly shaped, light brown sclerotia after 10 days. Hyphal cells were determined to be binucleate when stained with 1% safranin O and 3% KOH solution (1) and examined with a microscope at ×400. Anastomosis groups were determined by pairing isolates on 2% water agar in petri plates (4). Pairings were made with tester strains of binucleate Rhizoctonia AG-A through AG-S, except AG-J and AG-M. Anastomosis was observed only with tester isolates of AG-G. The rDNA-ITS of two isolates of BNR (DISTEF-Vt 31 and DISTEF-Vt 32) was sequenced (GenBank Accession Nos. AB478783 and AB478784, respectively) (3). The sequence from these two isolates exhibited 100% homology with BNR AG-G (GenBank Accession No. AY927334). Pathogenicity tests were conducted on potted, healthy, 6-month-old laurustinus. Twenty plants were inoculated by placing 1-cm2 plugs of PDA from 5-day-old mycelial cultures near the base of the stem. The same number of plants was treated with 1-cm2 PDA plugs as control. Plants were kept at 25°C and 95% relative humidity with a 12-h fluorescent light/dark regimen. Stem, crown, and root rot symptoms, identical to ones observed in nursery, appeared 20 days after inoculation, and all the inoculated plants showed symptoms within 1 month. Control plants remained healthy. Binucleate Rhizoctonia was reisolated from symptomatic tissues, completing Koch's postulates. R. solani was previously reported on Viburnum sp. in the United States (2). To our knowledge, this is the first report of binucleate Rhizoctonia causing disease on V. tinus.