First Report of Crown Rot and Stem Rot Caused by Rhizoctonia solani AG-4 on Marmalade Bush in Italy.

Abstract

Marmalade bush (Streptosolen jamesonii (Benth.) Miers), also known as fire bush, is an evergreen, perennial shrub in the family Solanaceae, which is native to South America (Colombia, Ecuador, and Peru). In Italy, this species is cultivated as an ornamental creeper or bush. During September 2009, a new disease was observed in a stock of ~10,000 pot-grown, 2-month-old plants of marmalade bush in a nursery in eastern Sicily, Italy. More than 50% of the plants exhibited symptoms of disease. Disease symptoms consisted of extensive water-soaked, dark brown lesions at the crown level that girdled entire stems and an internal brown discoloration of cortical tissue. Infected plants died within a few days. Diseased tissue was disinfested for 10 s in 1% NaOCl, rinsed with sterile water, and plated on potato dextrose agar (PDA) amended with streptomycin sulfate at 100 mg/liter. Fungal colonies were initially white, turned brown after 2 to 3 days, and produced irregularly shaped, brown sclerotia. Microscopic examination showed mycelium consistent with Rhizoctonia solani Kühn that branched at right angles, constricted at the base of the branch originating from primary 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 by staining with 1% safranin O and 3% KOH solution (1) and examined at ×400. The hyphal cells were all multinucleate. Anastomosis group was determined by pairing isolates on 2% water agar in petri plates (2). 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. Pathogenicity tests were performed by placing 1-cm2 plugs of PDA from 5-day-old mycelial cultures near the base of the stem on 25 potted, healthy, 2-month-old rooted cuttings of marmalade bush. The same number of plants treated with 1-cm2 PDA plugs served as controls. Following inoculation, all plants were maintained for 20 days at 25°C and 95% relative humidity under a 12-h fluorescent light/dark regimen. Crown and stem symptoms, identical to those observed in the nursery, developed 5 days after inoculation on all inoculated plants. Control plants remained symptomless. R. solani was consistently reisolated from symptomatic tissues and identified as previously described. To our knowledge, this is the first report of R. solani causing disease on marmalade bush.