First Report of White Rust of Rocket (Eruca sativa) Caused by Albugo candida in Montenegro

In the past few years, rocket (Eruca sativa, family Brassicaceae) has increasingly been produced in Montenegro, owing to favorable market prices. It is mainly used for fresh consumption as a component of mixed vegetable salads or toppings. In April 2017, white rust symptoms were observed on rocket plants of cultivar Rucola coltivata in a plastic greenhouse in Podgorica, where the disease incidence reached 60%. Numerous pustules, individual or in groups, were present on leaves and stems under the epidermis. Symptoms appeared as pale-yellow blotches on the upper leaf surface, which coincided with white sori emerging on the lower surface of the leaves. Sporangia were released in powdery masses from the sori. Microscopic examination of lesions revealed the presence of sporangiophores and sporangia, whereas oospores were not observed. The sporangiophores were hyaline, mostly straight, clavate or cylindrical, and measured 26.3 to 32.5 × 11.9 to 14.0 μm (n = 30). Sporangia were oval to spheroidal, hyaline, with a uniformly thin wall. The sporangia measured 14.7 to 24.5 × 14.7 to 19.6 μm (n = 100). The morphological features of the pathogen were in accordance with those described for Albugo candida (Pers.) Kunze (Mukerji 1975). To perform molecular identification, DNA was extracted from pustules of a representative isolate, IHPS-F48. Polymerase chain reaction was carried out using the internal transcribed spacer (ITS) region primers ITS4/DC6 (Bonants et al. 1997) and the cytochrome c oxidase subunit II (COX2) primers COX2f and COX2r (Hudspeth et al. 2000). BLAST analysis of the 1,203-bp ITS (GenBank accession no. MH229360) and 632-bp COX2 (MH234579) sequences confirmed the results of the morphological identification, because the sequences had 99 and 100% identity with Albugo candida GenBank accessions GQ328836, HM587262, DQ418502 and DQ418514, KM588082, GU292095, respectively. A voucher specimen has been deposited in the Plant Pathology Herbarium at the Biotechnical Faculty in Podgorica, Montenegro, and in the phytopathological herbarium of the Slovenian Institute of Hop Research and Brewing. The pathogenicity of isolate IHPS-F48 was determined by inoculating 5-week-old rocket seedlings of the cultivar Rucola coltivata. Inoculum was obtained from naturally infected leaves from the greenhouse, by scraping sporangia from leaves into sterile distilled water (SDW). The sporangial suspension (1 × 10⁵ sporangia/ml) was sprayed onto 10 plants. An additional 10 control plants were sprayed only with SDW. All plants were kept in plastic bags for 48 h at 18°C and maintained in a moist chamber at 20 to 24°C. After 10 to 12 days, typical white rust symptoms appeared on inoculated plants, whereas control plants remained symptomless. A. candida has previously only been recorded on Capsella bursa-pastoris and Sinapis arvensis in Montenegro in 1951 and 1955, respectively (Mijušković 1956). Therefore, to our knowledge, this is the first report of A. candida on rocket in Montenegro. It is likely that the source of pathogen infection in the plastic greenhouse was contaminated seed, because the pathogen can be seedborne (Jacobson et al. 1998). The disease must be carefully monitored because it poses a threat to the production of rocket and potentially other cultivated plants from the Brassicaceae family in Montenegro.