First Report of Damping-Off on African Daisy Caused by Rhizoctonia solani AG-4 in Italy

Abstract

Osteospermum (African Daisy or Cape Daisy) is a genus belonging to the Calendulae with a large number of perennial plant species. In February of 2007, a severe damping-off occurred on 3- to 4-month-old potted cuttings of Osteospermum ‘Impassion Rose Purple’, ‘Impassion White’, ‘Impassion Purple’, and ‘Impassion White Rose’ cultivated in a nursery in eastern Sicily. More than 30% of the plants were infected. Disease symptoms consisted of extensive water-soaked lesions at the base of the stem followed by wilt and collapse of the plant. Isolations from diseased tissues on potato dextrose agar (PDA) amended with streptomycin sulfate at 100 mg/l consistently recovered a fungus with morphological characteristics of Rhizoctonia solani. Fungal colonies were initially white, turned brown after 2 to 3 days, and produced irregularly shaped, brown sclerotia after 1 week. Microscopic examination revealed that hyphae had right angle branching patterns, were constricted at the base of the branch near the union with main hyphae, and septate near the constriction. The number of nuclei per hyphal cell was determined on cultures grown at 25°C on 2% water agar in petri plates. Mycelium was stained with 0.5% aniline blue solution (4) and examined with a microscope at ×400. The hyphal cells were all multinucleate. Anastomosis groups were determined by pairing isolates (3) with tester isolates of 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, giving C2 reactions (1) at a high frequency. The identification of group AG-4 within R. solani had been obtained by electrophoretic patterns of pectic enzymes (polygalacturonases) in vertical pectin-acrylamide gel stained with ruthenium red (2). All isolates of R. solani collected from infected plants were paired in all combinations on PDA plus 1% activated charcoal and examined for somatic interaction. All paired colonies merged without producing visible tufts of aerial mycelium. Absence of tufts and the lack of formation of heterokaryon at the hyphal interaction zone indicated that all isolates belonged to the same mating type with the same mating alleles (3). Pathogenicity tests were performed by placing plugs of PDA from 5-day-old mycelial cultures in the soil near the base of the stem on 20 potted, healthy, 2-month-old cuttings of Osteospermum cv. Impassion Rose Purple. The same number of plants treated with 1/cm2 PDA plugs served as controls. Following inoculation, all plants were maintained in a growth chamber at 25°C and 95% relative humidity on a 12-h fluorescent light/dark regimen. Wilt symptoms and lesions at the base of stem identical to those observed in the nursery developed 7 days after inoculation, and all inoculated plants died within 20 days. Control plants remained symptomless. R. solani AG-4 was consistently reisolated from symptomatic tissues, completing Koch's postulates. To our knowledge, this is the first report of damping-off on the genus Osteospermum caused by R. solani. 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) M. C. Juliàn et al. Phytopathology 86:566, 1996. (4) C. C. Tu and J. W. Kimbrough. Mycologia 65:941, 1973.