First Report of Stem Rot on Cereus peruvianus monstruosus Caused by Bipolaris cactivora (Petr.) Alcorn in Italy.

Abstract

Cereus peruvianus monstruosus, known as "monster cactus," family Cactaceae, is grown as a potted plant. In the winter of 2013, a stem rot was observed on a farm located near Ventimiglia (northern Italy) on 80% of 4,000 9-month-old plants grown in trays in a peat substrate. Symptoms consisted of a rapid rot of the upper portion of the stem. Affected stems at first showed yellowish spots that became brown irregular necrotic lesions with well-defined margins. The tissues below the affected areas were blackened and dry but became soft in the presence of high relative humidity. Fungal sporulation on rotted tissues consisted of caespitose, non-branched, septate conidiophores, olivaceous to brown at the base, paler above, measuring 89.0 to 196.9 × 6.2 to 8.7 (average 124.8 × 7.0) μm. Single conidia were borne on terminal cells. At maturity, conidia with 2 to 5 (average 3) septa were brownish-olivaceous, varying in shape from obclavate, fusiform, ellipsoid or sometimes furcate, and measuring 23.4 to 48.6 × 8.0 to 12.6 (average 38.8 × 10.3) μm. Symptomatic tissues were immersed in 1% sodium hypochlorite for 2 to 3 s and rinsed in sterile distilled water, then fragments excised from the margin of internal lesions were cultured on potato dextrose agar (PDA) medium amended with 25 mg/l of streptomycin sulfate and incubated at 20 to 23°C under alternating daylight and darkness (10 h light, 14 h dark). A fungus that was consistently isolated was subcultured on PDA. At maturity, dark green floccose colonies comprised of light brown septate hyphae, 4.2 to 8.1 (average 5.6) μm in width, produced non-branched, pale to dark brown, septate conidiophores, measuring 99.6 to 176.1 × 4.5 to 6.5 (average 146.7 × 5.4) μm. The conidia produced on PDA were similar to those observed on infected tissues and measured 20.6 to 40.7 × 7.5 to 11.4 (average 32.0 × 9.7) μm, with 1 to 3 septa (average 2). On the basis of the morphological characteristics, the fungus was identified as Bipolaris cactivora (Petr.) Alcorn [Syn.: Drechslera cactivora (Petr.) M. B. Ellis] (4). The internal transcribed spacer (ITS) region of the ribosomal DNA (rDNA) was amplified for one isolate using ITS1/ITS4 primers and sequenced (GenBank Accession No. KF041822). BLASTn analysis (1) of the 557-bp segment showed a 99% similarity with the ITS sequence of Bipolaris cactivora HM598679. For pathogenicity tests, 8 mm diameter mycelial disks removed from 15-day-old PDA cultures of the fungus were placed at the wounded stem apexes of three 7-month-old healthy plants (three disks per plant). Three plants inoculated with non-inoculated PDA disks served as controls. Plants were covered with plastic bags and maintained in a growth chamber at 23 ± 1°C with 12 h light/dark. By 8 days after inoculation, all the inoculated stems were rotted and 10 colonies of B. cactivora were re-isolated from infected tissues. Control plants remained healthy. The pathogenicity test was carried out twice with similar results. Several hosts are listed for B. cactivora including C. peruvianus, and the pathogen has been reported in the United States (2) and in South Korea (3). To our knowledge, this is the first report of B. cactivora on C. peruvianus monstruosus in Italy.