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Oral: Population Dynamics


Changes in Plasmopara obducens population structure corresponded with the emergence and rapid spread of the impatiens downy mildew epidemics
C. SALGADO-SALAZAR (1), Y. Rivera (1) (1) USDA-ARS, Rutgers University, U.S.A.

Plasmopara obducens is the newly emergent oomycete responsible for impatiens downy mildew disease (IDM) on cultivated Impatiens walleriana. Since IDM outbreaks began in 2004, the disease has reduced the $187 million annual production of impatiens in the U.S. by 40%. In this study, we evaluated if there was a change in P. obducens population structure associated with the U.S. epidemic. Over 900 pre- and post-epidemic P. obducens samples were genotyped using 20 polymorphic microsatellite markers developed from a de novo genome assembly. Population structure analysis identified five populations (P1-P5). Prior to IDM epidemics, P1 dominated, existing as pure P1 or admixed with the other four genotypes (~50%) on North American native Impatiens species. The first epidemic IDM specimen collected on I. walleriana in 2004 from Tennessee showed an almost pure P1 genotype. Approximately 50% of the post-epidemic samples showed admixed genotypes, but admixture between P1 and the other four genotypes was uncommon. Genetic diversity was high for the four most abundant populations P2-P5 responsible for the IDM epidemics (0.56-0.78), but lower for P1 (0.37). These data showed that the recent emergence of IDM in the U.S. corresponded with several major shifts in P. obducens populations, with endemic P1 genotypes supplanted by novel genotypes. The high level of diversity and genetic structure in P. obducens populations may present difficulties for durable disease control