First Report of Synchytrium endobioticum (Potato Wart Disease) Pathotype 18(T) in Poland.

Synchytrium endobioticum (Schilb.) Perc. is a soil-borne biotrophic fungus causing potato wart disease (PWD) of cultivated potato, one of the most important crops in Poland. S. endobioticum infects epidermal cells of young potato organs, such as eyes, sprouts, young tubers, stolons, stems, leaves, and even flowers, but never roots. S. endobioticum survives in the soil as winter (resting) spores, which germinate, infect the plant, and produce secondary sporangia (summer spores); infection results in galls on the stolons and tubers, in which the pathogen multiplies. Its long persistence in soil and the severe losses it inflicts to potato crops have prompted its inclusion into the A2 quarantine list. The fungus originates from the Andean zones of South America, from where it spread to North America and Europe at the end of the 19th century. S. endobioticum was first reported in the United Kingdom in 1876. This pathogen is quite variable. The first discovery of a new pathotype in Europe occurred in former East Germany in 1941, and there have been 38 S. endobioticum pathotypes identified so far. Pathotypes 1(D), 2(G), 6(O), 8(F), and 18(T) are of greatest relevance in Europe. In 2008, the Polish Inspectorate of Plant Health and Seed Inspection (PIORiN) collected soil samples from Mazowieckie Region in Central Poland. Microscopic examinations revealed the presence of viable resting spores of S. endobioticum in a soil sample collected from a crop plantation of ornamentals intended for export. One kilogram of soil contained an average of 300 viable spores. A bioassay (pot tests), recommended by the EPPO standard PM 3/59 (1), showed no wart symptoms because of the very low sporangium density of S. endobioticum. However, concentrating S. endobioticum inoculum by centrifugation and using for a bioassay modified Potocek's tube test (1) allowed us to obtain fresh galls with summer sporangia. The first symptoms of PWD were visible on sprouts of extremely susceptible potato genotypes 7 weeks after inoculation. To identify the pathotype of S. endobioticum, 10 differential potato cultivars (Deodara, Tomensa, Eersteling, Producent, Combi, Saphir, Delcora, Miriam, Karolin, and Ulme) were inoculated with fresh galls of S. endobioticum using the Glynne-Lemmerzahl method (2), according to EPPO standard PM 7/28. Galls were formed on all cultivars except Saphir, Karolin, and Ulme (resistant cultivars). This virulence profile was identical to that of European pathotype 18(T) of S. endobioticum. This is the first detection in Poland of pathotype 18(T), which is one of the most virulent pathotypes of this fungus. It should be noted that in 2004, prior to planting, the field was investigated by PIORiN and found to be free of S. endobioticum. The winter sporangia were found on the field with ornamental plants originating from Western Europe, where pathotype 18(T) is still occurring. S. endobioticum is a classic example of the distribution of plant pathogens by man. Although an infection source has not been determined, the field was probably infested by soil connected with roots of the plantlets. This is an example of alternative ways for S. endobioticum spreading without potato as a main host. References: (1) EPPO. Bull. OEPP/EPPO Bull. 29:225, 1999. (2) EPPO. Bull. OEPP/EPPO Bull. 34:213, 2004.