E. Mangwende and
J. B. Kalonji Kabengele, Department of Microbiology and Plant Pathology, Forestry and Agricultural Biotechnology Institute, University of Pretoria, Pretoria 0002, South Africa;
M. Truter, Plant Protection Research Institute, Agricultural Research Council, Queenswood 0121, South Africa; and
T. A. S. Aveling, Department of Microbiology and Plant Pathology, Forestry and Agricultural Biotechnology Institute, University of Pretoria, Pretoria 0002, South Africa
Garden rocket (Eruca sativa syn.: E. vesicaria subsp. sativa (Mill) Thell.) is an annual plant of the Brassicaceae grown for fresh consumption as a salad vegetable. During winter (May to July) of 2013 and 2014 in South Africa, typical symptoms of white rust were observed in two commercial crops (each ~0.5 ha) of the garden rocket cv. Rucola coltivata in Centurion, Gauteng Province, at 33 and 80% incidence, respectively. Symptomatic leaves were deposited in the National Collection of Fungi, Plant Protection Research Institute, Agricultural Research Council, Pretoria, South Africa (PREM 61073). Early infections appeared as white to cream, blister-like sori on the lower leaf surfaces, and pale yellow lesions on the corresponding upper leaf surfaces. Later stages of infection were characterized by coalescing of lesions into large, irregular, necrotic blotches and development of additional sori on the petioles and stems. Sporangiophores were hyaline, clavate or cylindrical, and measured 24 to 30 × 11 to 14 μm (n = 50). Sporangia developed in basipetal chains and were hyaline, globose or polyangular, and 15 to 20 μm (n = 100). Based on these morphological characters and the host plant, the pathogen was identified as Albugo candida (Pers.) Kunze (2). Genomic DNA was extracted using the DNeasy Plant Mini DNA extraction kit (Qiagen) from sori containing sporangia collected from naturally infected leaves, according to the manufacturer's specifications. The internal transcribed spacer (ITS) region of ribosomonal DNA (rDNA) and the cytochrome c oxidase subunit II (COX2) region were amplified and sequenced (1). The ITS (GenBank Accession No. KM588081) and COX2 (KM588082) sequences confirmed identity of the pathogen as A. candida with 100% homology to the corresponding sequences of several A. candida isolates, including DQ418503 for the ITS sequence and DQ418514 for the COX2 sequence, of a voucher specimen of A. candida on E. sativa (BPI 184870) from Pakistan. Inoculum was prepared by scraping sporangia from infected leaves of the cv. Rucola coltivata collected from the 2014 field and placing the material in sterilized, distilled water (SDW) for 12 h at 5°C to induce zoospore formation. Pathogenicity tests were performed by spraying a suspension of 1 × 105 sporangia/ml onto each of 10 5-week-old rocket seedlings of the cv. Rucola coltivata. Ten additional seedlings were inoculated similarly with SDW to serve as a control treatment. The plants were maintained at 12 to 15°C and 95% RH for 72 h (3) before being moved to a shaded greenhouse at 20 to 24°C and 90% RH. Control plants remained symptomless, whereas white rust symptoms similar to those observed in the original fields developed on leaves of inoculated seedlings 10 to 14 days later, demonstrating that A. candida was the causal agent of the disease on E. sativa. To our knowledge, this is the first report of A. candida infecting garden rocket in South Africa.
References: (1) Y.-J. Choi et al. Mol. Phylogenet. Evol. 40:400, 2006. (2) K. Mukerji. Descriptions of Pathogenic Fungi and Bacteria No. 458. CMI, Kew, Surrey, UK, 1975. (3) M. J. Sullivan et al. Plant Dis. 86:753, 2002.