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First Report of Crown and Root Rot Caused by Binucleate Rhizoctonia AG-A on Thryptomene saxicola in Italy

Thryptomene saxicola (Hook.) Schauer is an evergreen shrub native to Western Australia and a member of the Myrtaceae. In Italy, this species was recently introduced as an ornamental plant from abroad. From July of 2008 to September 2009, a new crown and root rot of T. saxicola was observed on several stocks of approximately 20,000 1- to 3-year-old potted plants. Diseased plants were obtained from a commercial nursery in eastern Sicily, Italy. They were propagated from cuttings and grown under drip irrigation. More than 30% 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 chlorotic and often wilted. Early in the disease development, roots and crowns showed brown lesions. Successively, mature crown lesions turned dark brown. Longitudinal sections of crown tissues revealed a discoloration of the basal stem. 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 tissues of plants. Phytophthora isolates were not recovered from symptomatic tissues plated on BNPRAH (benomyl, nystatin, pentachloronitrobenzene, rifampicin, ampicillin, and hymexazol) selective medium. 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-TS1) was sequenced (GenBank Accession No. AB514570) (2). The sequence from this isolate exhibited 99% homology with BNR AG-A (GenBank Accession No. AY738628). Pathogenicity tests were conducted on potted, healthy, 1-year-old plants of T. saxicola. Forty 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 45 days after inoculation, and 80% of the inoculated plants died within 4 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 causing disease on T. saxicola.

References: (1) R. J. Bandoni. Mycologia 71:873, 1979. (2) M. Hyakumachi et al. Phytopathology 95:784, 2005. (3) C. C. Tu and J. W. Kimbrough. Mycologia 65:941, 1973.