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First Report of Downy Mildew Disease Caused by Peronospora effusa on Spinach in Turkey

Soylu, E. M., Kara, M., Kurt, Ş., Uysal, A., Shin, H. D., Choi, Y. J., Soylu, S.
Plant disease 2018 v.102 no.9 pp. 1854
Peronospora effusa, Spinacia oleracea, branches, conidia, conidiophores, cultivars, cytochrome-c oxidase, dew, disease incidence, downy mildew, greenhouses, herbaria, host range, internal transcribed spacers, leaves, light microscopes, microscopy, mitochondrial DNA, pathogenicity, pathogens, photoperiod, rain, ribosomal DNA, spinach, sporulation, spraying, spring, stomata, temperature, vegetable crops, winter, Korean Peninsula, Turkey (country)
Spinach (Spinacia oleracea L.) is an increasingly important vegetable crop in Turkey, as is the case for many other parts of the world. In March 2017, spinach plants (cultivar Matador) showing typical symptoms of downy mildew were found in four of 48 fields in the Hatay province of Turkey. Disease incidence was relatively low, with 5 to 10% of the plants exhibiting symptoms. Initial symptoms appeared as bright chlorotic spots, ranging from 3 to 5 cm in diameter on the upper surfaces of the leaves. On the corresponding lower surfaces, greyish-violet to brownish sporulation was present. A representative specimen (SPp3) was deposited in the Korea University Herbarium (accession no. KUS-F29732). Microscopic examination of fresh material was performed under a light microscope. Conidiophores emerging through stomata, up to seven in a fascicle, were hyaline, straight to slightly curved, monopodially branched four to six orders, and 175 to 400 µm long (n = 100). Trunks were 110 to 300 µm long and 7 to 10 µm (n = 50) wide below the first branch. Each of the branches, ending in single or in pairs of ultimate branchlets, were straight to slightly curved. Conidia were olivaceous brown, broadly ellipsoidal to ellipsoidal, 22.5 to 35 µm long and 20 to 25 µm wide, with a length/width ratio of 1.20 to 1.70 (n = 100). All morphological characteristics closely resembled those reported for Peronospora effusa by Choi et al. (2007). To confirm the morphological identification, the internal transcribed spacer (ITS) rDNA and the cytochrome oxidase II (cox2) mtDNA regions were amplified and directly sequenced (Choi et al. 2015). The resulting sequences were deposited in GenBank (accession nos. MG793197 for ITS and MG793196 for cox2). A BLASTn search revealed that the sequences of the Turkish isolate were identical to the ITS and cox2 sequences of P. effusa available in GenBank (DQ643847 and KP330686, respectively). A pathogenicity test was performed twice by spraying a conidial suspension (10⁵ conidia/ml) onto the leaves of 10 healthy 3-week-old spinach plants (cultivar Matador). Inoculated plants were incubated in a dew chamber for 24 h at 18°C and then maintained in a greenhouse at 20°C with a 10-h photoperiod. Ten control plants sprayed with distilled water were maintained under identical conditions. After 10 to 15 days, downy mildew disease symptoms appeared and were similar to the original symptoms observed in the field, whereas the control plants remained symptomless. The pathogen present on the inoculated plants was morphologically identified as P. effusa, fulfilling Koch’s postulates. P. effusa has been reported as an economically important pathogen of spinach worldwide (Farr and Rossman 2017), but to our knowledge, this is the first peer-reviewed report of P. effusa in Turkey. It is most likely that spinach downy mildew has been present in the region for a few years, based on unconfirmed reports from local growers and a non-peer-reviewed report from the central Anatolian region in Turkey that only identified the pathogen morphologically (Unlu and Boyraz 2010). Both mild winter temperatures and frequent spring rains during 2017 may have encouraged the disease incidence. Although prevalence of the causal disease agent is low, because the pathogen has a narrow host range, major spinach production areas may be vulnerable to the introduction of the disease owing to an increasing demand for this crop.