First Report of New Pathotype 39(P) of Synchytrium endobioticum Causing Potato Wart Disease in Poland.

Potato wart disease, caused by Synchytrium endobioticum (Schilb.) Perc., is one of the most dangerous diseases of cultivated potato. S. endobioticum is an obligate soil-borne fungus. The pathogen originated in the Andean zones of South America, from which it spread to North America and Europe at the end of the nineteenth century. The typical symptoms of cauliflower-like galls can develop on all meristematic tissues of potato except roots. The pathogen is on the European and Mediterranean Plant Protection Organization (EPPO) A2 list of quarantine pests. Since the discovery of pathotype 2(G) in Germany, more than 40 pathotypes have been reported in Europe (1). In 2009, warted tubers were collected from Lesser Poland. Direct microscopic examination confirmed the presence of summer and winter sporangia in galls. The galls were multiplied on cv. Eersteling using the Glynne-Lemmerzahl method (GL), according to EPPO standard PM 7/28 (3). To identify the pathotype of S. endobioticum, 10 differential potato cultivars were inoculated with fresh galls following the GL method. The reaction types were evaluated 4 weeks after incubation in sand in a growth chamber. Cultivars Combi, Delcora, Deodara, Eersteling, Miriam, Producent, and Tomensa were extremely susceptible (predominant tumor formation); cv. Karolin was slightly susceptible (small galls with numerous winter sporangia); and cvs. Saphir and Ulme were resistant (early and late defense necrosis). This virulence profile was different from patterns of known European (1), Turkish (2), and Polish local pathotypes (4,5). The virulence of the pathogen to cv. Karolin was unique. Winter sporangia were isolated from galls formed on cv. Karolin and used in a modified Potoček's tube test (5) to obtain fresh galls with summer sporangia that were necessary to reconfirm the virulence profile of the pathotype. The reaction of all cultivars to the new pathotype was the same after the GL was performed. In accordance with the summation of known pathotypes, it is proposed to encode this new pathotype as pathotype 39(P), using an Arabic number to designate the subsequent pathotype (the last one, 38 [Nevşehir], was identified in Turkey) (2) and the first letter of the locality where it was found (Piekielnik). The presence of pathotype 39(P) was confirmed in four out of six districts where the Polish local pathotype 2(Ch) was prevalent. The virulence profiles of both pathotypes were compared using differential set and cv. Asche Sämling, which is differential only for pathotype 2(Ch). In contrast to all other pathotypes (big galls), no galls were observed on A. Sämling inoculated with pathotype 2(Ch) and 39(P). From this observation it appears that the new pathotype probably has been selected from the Polish local pathotype 2(Ch). Pathotype 39(P) was detected in small garden potato plots in the rainy mountainous area, a non-economically important potato-growing region where the old traditional cultivars of potato are cultivated without crop rotations. It seems, therefore, that where climatic conditions are suitable for S. endobioticum and the growing of slightly susceptible cultivars is possible, development of the new pathotype is favored. References: (1) R. P. Baayen et al. Eur. J. Plant Pathol. 116:21, 2006. (5) E. Çakir et al. OEPP/EPPO Bull. 39:175, 2009. (2) EPPO. EPPO Bull. 34:213, 2004. (3) J. Przetakiewicz. Biull. IHAR 257/258:207, 2010. (4) J. Przetakiewicz. Plant Dis. 98:688, 2014.