| Summary: | Dry rot of potato tubers is a harmful disease caused by species of the <i>Fusarium</i> genus. Studies on the composition and features of <i>Fusarium</i> spp. that cause the disease in Russia are limited. Thirty-one <i>Fusarium</i> strains belonging to the <i>F. sambucinum</i> species complex (FSAMSC) and <i>F. solani</i> species complex (FSSC) were accurately identified using multilocus phylogenetic analysis of the <i>tef</i> and <i>rpb2</i> loci, and their physiological characteristics were studied in detail. As a result, 21 strains of <i>F. sambucinum</i> s. str. and 1 strain of <i>F. venenatum</i> within the FSAMSC were identified. Among the analyzed strains within the FSSC, one strain of <i>F. mori</i>, four strains of <i>F. noneumartii</i>, and two strains of both <i>F. stercicola</i> and <i>F. vanettenii</i> species were identified. This is the first record of <i>F. mori</i> on potato as a novel host plant, and the first detection of <i>F. noneumartii</i> and <i>F. stercicola</i> species in Russia. The clear optimal temperature for the growth of the strains belonging to FSAMSC was noted to be 25 °C, with a growth rate of 11.6–15.0 mm/day, whereas, for the strains belonging to FSSC, the optimal temperature range was between 25 and 30 °C, with a growth rate of 5.5–14.1 mm/day. The distinctive ability of <i>F. sambucinum</i> strains to grow at 5 °C has been demonstrated. All analyzed <i>Fusarium</i> strains were pathogenic to potato cv. Gala and caused extensive damage of the tuber tissue at an incubation temperature of 23 °C for one month. Among the fungi belonging to the FSAMSC, the <i>F. sambucinum</i> strains were more aggressive and caused 23.9 ± 2.2 mm of necrosis in the tubers on average compared to the <i>F. venenatum</i> strain—17.7 ± 1.2 mm. Among the fungi belonging to the FSSC, the <i>F. noneumartii</i> strains were the most aggressive and caused 32.2 ± 0.8 mm of necrosis on average. The aggressiveness of the <i>F. mori</i>, <i>F. stercicola</i>, and especially the <i>F. vanettenii</i> strains was significantly lower: the average sizes of damage were 17.5 ± 0.5 mm, 17.2 ± 0.2 mm, and 12.5 ± 1.7 mm, respectively. At an incubation temperature of 5 °C, only the <i>F. sambucinum</i> strains caused tuber necroses in the range of 6.7 ± 0.5–15.9 ± 0.8 mm.
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