Resistance of Russian strains of Phytophthora infestans to fungicides metalaxyl and dimethomorph

In total, 2000 P. infestans isolates collected during 1988–2004 in different regions of Russia were tested for resistance to metalaxyl. In the majority of field populations, the frequency of resistant strains decreased after 1993–1994. This might be related to changes in the potato industry in Russia. Potato production was concentrated in small private gardens. The part of resistant strains in populations from small private patches was less than that in large commercial fields. Small private gardens became a great source of sensitive genotypes. In recent years, the part of resistant strains in the majority of field populations was less than 30%. A small number of resistant strains in a population occurs even if there has been no treatment with metalaxyl-containing preparations for a long time. In some populations, the frequency of resistant strains has increased, depending on treatments. Variation in the level of resistance to metalaxyl in one clonal lineage is shown. Resistant strains occurred in potato leaves and tubers, and in tomato leaves. They were rare in tomato fruits. Probably, the sensitive strains affecting fruits have a selective advantage. More than 370 strains from different regions were tested for resistance to dimethomorph-containing preparations. Resistant strains were not detected.