First Report of Crown and Root Rot Caused by Binucleate Rhizoctonia AG-A on Dodonaea viscosa in Italy.

Abstract

Florida hopbush (Dodonaea viscosa (L.) Jacq.) is an evergreen bush or small tree native to Australia and a member of the Sapindaceae. During September of 2008, a crown and root rot of D. viscosa was observed on 1-year-old potted plants in a commercial nursery in eastern Sicily, Italy. More than 15% of the plants showed disease symptoms. Infected plants were characterized by a lack of vigor. Roots and crowns were partially or completely destroyed, and as a consequence, infected plants were often wilted. Early in the disease development, roots and crowns showed brown lesions. Successively, mature crown lesions turned dark brown. Diseased tissues were surface disinfested for 1 min in 1% NaOCl, rinsed in sterile water, plated on potato dextrose agar (PDA) amended with 100 mg/liter of streptomycin sulfate, and then incubated at 25°C. A binucleate Rhizoctonia (BNR) species was consistently isolated from affected tissue of plants. Fungal colonies were white with floccose, aerial hyphae. Hyphal cells were determined to be binucleate when stained with 1% safranin O and 3% KOH solution (1) and examined at ×400. Anastomosis groups were determined by pairing isolates with five different tester isolates of BNR AG-A on 2% water agar in petri plates (3). Anastomosis was observed with all tester isolates. The rDNA-ITS of one isolate of BNR (DISTEF-DV2) was sequenced (GenBank Accession No. AB514569) (2). The sequence from this isolate exhibited 99% homology with BNR AG-A (GenBank Accession No. AY738628). Pathogenicity tests were conducted on potted, healthy, 8-month-old plants of D. viscosa. 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 controls. Plants were kept at 25°C and 95% relative humidity on a 12-h fluorescent light/dark regimen. Root and crown rots, identical to those observed in the nursery, appeared 30 days after inoculation, and all the inoculated plants died within 2 months. Control plants remained healthy. Binucleate Rhizoctonia was reisolated from symptomatic tissues, completing Koch's postulates. To our knowledge, this is the first report in the world of BNR AG-A causing disease on Florida hopbush.