First Report of Damping-Off Caused by Rhizoctonia solani AG-4 on Lagunaria patersonii in Italy
Authors: Aiello D. , Parlavecchio G. , Vitale A. , Lahoz E. , Nicoletti R. , Polizzi G. 
Published: April 2008

Lagunaria patersonii (Adr.) G. Don (cow itch tree) is native to Australia and tolerates salted winds. During July 2007, damping-off of cow itch tree was observed on 4-month-old seedlings growing in a commercial nursery in eastern Sicily, Italy. More than 20% of the seedlings showed disease symptoms. First symptoms consisting of water-soaked lesions at the seedling base that expand rapidly girdle the stem and collapse the seedling in a few days. Diseased tissues were disinfested for 1 min in 1% NaOCl, rinsed in sterile water, plated on potato dextrose agar (PDA) amended with streptomycin sulphate at 100 mg/l, and then incubated at 25°C. A fungus with mycelial and morphological characteristics of Rhizoctonia solani Kühn was consistently yielded. Fungal colonies were initially white, turned brown with age, and produced irregularly shaped, brown sclerotia. Microscopic examination revealed that hyphae had a right-angle branching pattern, were constricted at the base of the branch near the union with main hyphae, and septate near the constriction. Basidia were not observed in the greenhouses or on the plates. Hyphal cells were determined to be multinucleate when stained with 0.5% aniline blue solution and examined at ×400 magnification with a microscope. Anastomosis groups were determined by pairing isolates on 2% water agar in petri plates (3). Pairings were made with tester strains of AG-1 IA, AG-2-2-1, AG-2-2IIIB, AG-2-2IV, AG-3, AG-4, AG-5, AG-6, AG-11. Anastomosis was observed only with tester isolates of AG-4 producing both C2 and C3 reactions. The hyphal diameter at the point of anastomosis was reduced, the anastomosis point was obvious, and cell death of adjacent cells was observed. These results were consistent with other reports on anastomosis reactions (1). The identification of group AG-4 within R. solani has been confirmed by electrophoretic patterns of pectic enzymes (polygalacturonases) in vertical pectin-acrylamide gel stained with ruthenium red (2). Pathogenicity tests were conducted on potted, healthy, 3-month-old seedlings of cow itch tree. Twenty plants were inoculated by placing 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 on a 12-h fluorescent light/dark regimen. Wilt symptoms due to basal stem rot, identical to ones observed in the nursery, appeared 10 days after inoculation and all inoculated plants showed symptoms within 1 month. Control plants remained healthy. The pathogen was reisolated from symptomatic tissues, completing Koch's postulates. To our knowledge, this is the first report in the world of R. solani causing disease on L. patersonii.

References: (1) D. E. Carling. Page 37 in: Grouping in Rhizoctonia solani by Hyphal Anastomosis Reactions. Kluwer Academic Publishers, the Netherlands, 1996. (2) R. H. Cruickshank and G. C. Wade. Anal. Biochem. 107:177, 1980. (3) C. C. Tu and J. W. Kimbrough. Mycologia 65:941, 1973.