First Report of White Blister Rust Disease Caused by Albugo occidentalis on Spinach in Turkey

Spinach (Spinacia oleracea L.) is an increasingly important vegetable crop in Turkey as is the case for many parts of the world. In March 2017, detailed field surveys were conducted to investigate the occurrence of leaf diseases in major spinach fields in Hatay Province of Turkey. White blister rust disease was sporadically present in many production fields. The disease incidence on affected crops ranged from 21 to 45% across all the areas surveyed. Initial symptoms were small chlorotic spots on the upper leaf surface. As the disease developed, white pustules (sori) or blisters were frequently produced as irregularly oval, elongated concentric rings, 3 to 4 mm in diameter, mostly on the lower leaf surface but also often on the upper surface. In the advanced stages of the disease, the white pustules often coalesced, and lesions appeared grainy due to the production of numerous oospores, rendering infected leaves unmarketable. A representative specimen (SAL2) was deposited in the Korea University Herbarium (KUS-F30037). Microscopic examination revealed that sporangia were born in basipetal chains, globose to oval, with a smooth wall surface and measured 12.5 to 15.0 × 17.5 to 20.0 μm (n = 100). The small stalked spherical haustoria, which were produced within mesophyll cells, were connected to the intercellular coenocytic hyphae, and measured 5.0 μm in diameter (n = 50). Dark-colored oospores were produced in subepidermal tissue, finely reticulated, 45 to 60 µm in diameter, with a wall thickness of 5 μm (n = 50). The morphological characteristics closely resembled those reported for Albugo occidentalis by Choi and Priest (1995). To confirm the morphological identification, both the internal transcribed spacer (ITS) rDNA and the cytochrome oxidase II (cox2) mtDNA were amplified with ITS1-O and LR0 for ITS and cox2-F and cox2-RC4 and directly sequenced (Choi et al. 2015). The resulting sequences were deposited in GenBank (accession nos. MF991148 for ITS, MF991149 for cox2). A BLASTn search revealed that the Turkish isolate is identical to the ITS and the cox2 sequences (KC676794, KC676795) of A. occidentalis in GenBank. A pathogenicity test was conducted by spraying a sporangial suspension (10⁵ sporangia/ml) onto 10 healthy leaves of 3-week-old spinach plants (cv. Matador). Inoculated plants were incubated in a dew chamber for 72 h at 18°C and then maintained in a greenhouse at 20°C with a 10-h photoperiod. Ten control plants were sprayed with distilled water and maintained under the same conditions as the inoculated plants. After 10 to 12 days, white rust pustules similar to the original symptoms observed in the field developed on inoculated plants, whereas the control plants remained symptomless. The pathogen present on the inoculated plants was morphologically identical to the original one observed on the diseased plants, fulfilling Koch’s postulates. Based on morphological characteristics, molecular sequencing data, and pathogenicity test, it was concluded that the pathogen on spinach in Turkey is A. occidentalis. The causal agent has been reported as an economically important pathogen of spinach in the United States, Mexico, Greece (Farr and Rossman 2017), and Iran (Ebrahimi and Afzali 2000). To our knowledge, this is the first report of A. occidentalis in Turkey. Although prevalence and host range of the causal disease agent is limited, major spinach production areas may be vulnerable to the introduction of this disease due to an increasing demand for this crop.