Potato (Solanum tuberosum) is the world's third-largest food crop. It severely suffers from late blight, a devastating disease caused by Phytophthora infestans. This oomycete pathogen secretes host-translocated RXLR effectors that include avirulence (AVR) proteins, which are targeted by resistance (R) proteins from wild Solanum species. Most Solanum R genes appear to have coevolved with P. infestans at its center of origin in central Mexico. Various R and Avr genes were recently cloned, and here we catalog characterized R-AVR pairs. We describe the mechanisms that P. infestans employs for evading R protein recognition and discuss partial resistance and partial virulence phenotypes in the context of our knowledge of effector diversity and activity. Genome-wide catalogs of P. infestans effectors are available, enabling effectoromics approaches that accelerate R gene cloning and specificity profiling. Engineering R genes with expanded pathogen recognition has also become possible. Importantly, monitoring effector allelic diversity in pathogen populations can assist in R gene deployment in agriculture.

Potato is the world's fourth largest food crop yet it continues to endure late blight, a devastating disease caused by the Irish famine pathogen Phytophthora infestans. Breeding broad-spectrum disease resistance (R) genes into potato (Solanum tuberosum) is the best strategy for genetically managing late blight but current approaches are slow and inefficient. We used a repertoire of effector genes predicted computationally from the P. infestans genome to accelerate the identification, functional characterization, and cloning of potentially broad-spectrum R genes. An initial set of 54 effectors containing a signal peptide and a RXLR motif was profiled for activation of innate immunity (avirulence or Avr activity) on wild Solanum species and tentative Avr candidates were identified. The RXLR effector family IpiO induced hypersensitive responses (HR) in S. stoloniferum, S. papita and the more distantly related S. bulbocastanum, the source of the R gene Rpi-blb1. Genetic studies with S. stoloniferum showed cosegregation of resistance to P. infestans and response to IpiO. Transient co-expression of IpiO with Rpi-blb1 in a heterologous Nicotiana benthamiana system identified IpiO as Avr-blb1. A candidate gene approach led to the rapid cloning of S. stoloniferum Rpi-sto1 and S. papita Rpi-pta1, which are functionally equivalent to Rpi-blb1. Our findings indicate that effector genomics enables discovery and functional profiling of late blight R genes and Avr genes at an unprecedented rate and promises to accelerate the engineering of late blight resistant potato varieties.

Pest and pathogen losses jeopardise global food security and ever since the 19^th century Irish famine, potato late blight has exemplified this threat. The causal oomycete pathogen, Phytophthora infestans, undergoes major population shifts in agricultural systems via the successive emergence and migration of asexual lineages. The phenotypic and genotypic bases of these selective sweeps are largely unknown but management strategies need to adapt to reflect the changing pathogen population. Here, we used molecular markers to document the emergence of a lineage, termed 13_A2, in the European P. infestans population, and its rapid displacement of other lineages to exceed 75% of the pathogen population across Great Britain in less than three years. We show that isolates of the 13_A2 lineage are among the most aggressive on cultivated potatoes, outcompete other aggressive lineages in the field, and overcome previously effective forms of plant host resistance. Genome analyses of a 13_A2 isolate revealed extensive genetic and expression polymorphisms particularly in effector genes. Copy number variations, gene gains and losses, amino-acid replacements and changes in expression patterns of disease effector genes within the 13_A2 isolate likely contribute to enhanced virulence and aggressiveness to drive this population displacement. Importantly, 13_A2 isolates carry intact and in planta induced Avrblb1, Avrblb2 and Avrvnt1 effector genes that trigger resistance in potato lines carrying the corresponding R immune receptor genes Rpi-blb1, Rpi-blb2, and Rpi-vnt1.1. These findings point towards a strategy for deploying genetic resistance to mitigate the impact of the 13_A2 lineage and illustrate how pathogen population monitoring, combined with genome analysis, informs the management of devastating disease epidemics. We have documented a dramatic shift in the population of the potato late blight pathogen Phytophthora infestans in northwest Europe in which an invasive and aggressive lineage called 13_A2 has emerged and rapidly displaced other genotypes. The genome of a 13_A2 isolate revealed a high rate of sequence polymorphism and a remarkable level of variation in gene expression during infection, particularly of effector genes with putative roles in pathogenicity. Collectively, these polymorphisms, in combination with an extended biotrophic phase, may explain the aggressiveness of 13_A2 and its ability to cause disease on previously resistant potato cultivars. The genome analysis identified conserved effectors that are sensed by potato resistance genes. These findings provide options for the strategic deployment of host resistance with a positive impact on crop yield and food security. This work stresses the benefits of a crop disease management strategy incorporating knowledge of the geographical structure, evolutionary dynamics, genome sequence diversity and in planta-induced effector complement of pathogen lineages.