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First Report of Damping-Off Caused by Rhizoctonia solani AG-4 on Pink Ipê (Tabebuia impetiginosa) in Italy

Pink ipê or pink lapacho (Tabebuia impetiginosa Martius ex DC., family Bignoniaceae) is one of the most attractive blooming trees in the world. In Europe, pink ipê is widely used as an ornamental tree in landscaped gardens and public areas. In August 2010, a widespread damping-off was observed in a stock of approximately 100,000 potted 2-month-old seedlings in a nursery in eastern Sicily (Italy). The seedlings were being watered with overhead irrigation. More than 5% of the seedlings showed disease symptoms. Initial symptoms were black lesions at the seedling crown that expanded rapidly to girdle the stem. On infected seedlings, leaves turned black and gradually died. Black extended stem lesions were followed by death of the entire seedling in a few days. A fungus with mycelial and morphological characteristics of Rhizoctonia solani Kühn was consistently isolated from crown and stem lesions when plated on potato dextrose agar (PDA) amended with streptomycin sulfate at 100 μg/ml. Fungal colonies were initially white, turned brown with age, and produced irregularly shaped, brown sclerotia. Mycelium was branched at right angles with a septum near the branch and a slight constriction at the branch base. Hyphal cells removed from cultures grown at 25°C on 2% water agar were determined to be multinucleate when stained with 1% safranin O and 3% KOH solution (1) and examined at ×400. Anastomosis groups were determined by pairing isolates 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 on 2% water agar in petri plates (4). Anastomosis was observed only with tester isolates of AG-4, giving both C2 and C3 reactions (2). Pathogenicity tests were performed on container-grown, healthy, 3-month-old seedlings. Forty seedlings of T. impetiginosa were inoculated near the base of the stem with two 1-cm² PDA plugs from 5-day-old mycelial cultures. The same number of plants only inoculated with PDA plugs served as controls. Plants were incubated in a growth chamber and maintained at 25°C and 95% relative humidity on a 12-h fluorescent light/dark regimen. Crown and stem lesions identical to those observed in the nursery appeared 5 days after inoculation and all plants died within 25 days. No disease was observed on control plants. R. solani AG-4 was reisolated from symptomatic tissues and identified as previously described. R. solani AG-4 was previously detected in the same nursery on Chamaerops humilis (3). To our knowledge, this is the first report of R. solani causing damping-off on T. impetiginosa.

References: (1) R. J. Bandoni. Mycologia 71:873, 1979. (2) D. E. Carling. Page 37 in: Grouping in Rhizoctonia solani by Hyphal Anastomosis Reactions. Kluwer Academic Publishers, the Netherlands, 1996. (3) G. Polizzi et al. Plant Dis. 94:125, 2010. (4) C. C. Tu and J. W. Kimbrough. Mycologia 65:941, 1973.