First Report of Crown and Root Rot Caused by Rhizoctonia solani AG-4 on Orange Jessamine in Italy

Abstract

Murraya paniculata (L.) Jack, commonly called orange jessamine or orange jasmine (Rutaceae), is a small tropical tree that is native to Asia. This species, closely related to Citrus, is grown as an ornamental tree or hedge. During October of 2007, crown and root rot was observed on approximately 12,000 pot-grown, 4-month-old plants in a nursery in eastern Sicily, Italy. Basal leaves turned yellow and gradually became necrotic, and infected plants often died. Disease symptoms were observed on 1,800 (15%) plants. Isolations from affected tissues on potato dextrose agar (PDA) amended with streptomycin sulfate at 100 mg/liter recovered a fungus with mycelial and morphological characteristics consistent with Rhizoctonia solani Kühn. 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 were septate near the constriction. The nuclear condition of hyphal cells was determined on cultures grown at 25°C on 2% water agar (WA) when stained with 3% safranin O solution and examined at ×400. Anastomosis groups were determined by pairing isolates on 2% WA in petri plates (4). Pairings were made with tester strains AG-1 IA, AG-2-2-1, AG-2-2IIIB, AG-2-2IV, AG-3, AG-4, AG-5, AG-6, and 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, 6-month-old seedlings of orange jessamine. Twenty-five plants were inoculated by placing 1-cm2 PDA plugs from 5-day-old mycelial cultures near the base of the stem. The same number of plants inoculated with PDA plugs served as controls. Plants were maintained at 25°C and 95% relative humidity on a 12-h fluorescent light/dark regimen. Wilt symptoms, identical to ones observed in the nursery, developed 3 months after inoculation because of crown and root rot. Control plants remained disease free. The pathogen was reisolated from symptomatic tissues, completing Koch's postulates. Collar rot due to R. solani was previously detected on M. koenigii (3). To our knowledge, this is the first report of R. solani causing disease on M. paniculata. 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) A. C. Jain and K. A. Mahmud. Rev. Appl. Mycol. 32:460, 1953. (4) C. C. Tu and J. W. Kimbrough. Mycologia 65:941, 1973.