First report of sexual reproduction of Fusarium solani f. sp. piperis in Bahia, Brazil

Abstract

Black pepper (Piper nigrum L.) is a popular spice native of India, and Brazil is one of its most important producing countries. The main disease of black pepper in Brazil is fusariosis, caused by Fusarium solani f. sp. piperis. Symptoms include leaf chlorosis and defoliation, blight of stems or stem cuttings, and root and foot decay. During surveys conducted in the south of the state of Bahia, municipalities of Taperoá (13°34'S, 39°10'W) and Valencia (13°20'S, 39°14'W), stems of diseased plants covered with red or salmon-colored perithecia were observed, while twigs showed leaf chlorosis, leading to early death of the plants. Ascomata were solitary or in groups, mostly superficial or surrounded by mycelia, globose, subglobose, ovoid, and 122 to 400 μm diameter. Microscopic examination revealed unitunicate, cylindric asci, 60 to 90 × 8.5 to 16 μm, thin-walled, containing eight ascospores arranged obliquely in two rows. Ascospores are hyaline, elliptical to oblong, one-septate, constricted at the central septum, 10 to 16 × 4 to 6.5 μm (means ± S.D.: 13.1 ± 1.4 × 5.1 ± 0.6 μm), length/width (L/W) 1.9 to 3.7. Single-spored cultures were transferred to SNA medium (incubated at 20°C for 7 days with 12-h photoperiod) and on potato dextrose agar (25°C in dark) for characterization. The anamorph is characterized by the presence of chlamydospores, canoe-shaped sporodochial macroconidia with three to four septae, and microconidia formed on long monophialidic conidiophores. Based on morphological markers, isolates were identified as F. solani. The partial fragment of the TEF-1α gene of single-spored isolates (CML 2186, 2187, 2188, 2189, 2190, and 2191) were sequenced. BLAST analysis of the sequence resulted in 94 to 99% identity with a reference strain of F. solani f. sp. piperis (NRRL 22570, CML 1888). For pathogenicity tests, cv. Bragantina was used and two isolates were inoculated as 5-mm diameter mycelial plugs on the stem of four plants each. Four control plants were treated only with sterile culture medium. Plants were maintained in the greenhouse at 25°C and 75 to 85% relative humidity under 70% shade. All inoculated plants showed initial symptoms of stem necrosis in inoculated branches 7 days after inoculation. Symptoms were not observed on stems of control plants. Isolates were successfully reisolated and identified as F. solani f. sp. piperis, fulfilling Koch's postulates. Representative isolates were deposited at the Coleção Micológica de Lavras (CML) at Universidade Federal de Lavras, Brazil. Production of perithecia of the pathogen has been previously reported only in Pará and Espírito Santo States (1,3). It is not yet confirmed if this taxon is homothallic or heterothallic. To our knowledge, this is the first report of the associated teleomorph of F. solani f. sp. piperis infecting and causing black pepper fusariosis in Bahia, Brazil. The results suggest that the spread of ascospores from perithecia is likely to be one of the main inoculum sources of the disease on adjacent vines. There is evidence that this special form of F. solani actually represents a distinct species pathogenic to black pepper (2).