Genome analyses of an aggressive and invasive lineage of the Irish potato famine pathogen.

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.

Effects of water potential on mycelial growth, sclerotial production, and germination of Rhizoctonia solani from potato.

The effects of osmotic and matric potential on mycelial growth, sclerotial production and germination of isolates of Rhizoctonia solani [anastomosis groups (AGs) 2-1 and 3] from potato were studied on potato dextrose agar (PDA) adjusted osmotically with sodium chloride, potassium chloride, glycerol, and matrically with polyethylene glycol (PEG) 6000. All isolates from AGs 2-1 and AG-3 exhibited fastest mycelial growth on unamended PDA (-0.4MPa), and growth generally declined with decreasing osmotic and matric potentials. Growth ceased between -3.5 and -4.0MPa on osmotically adjusted media, and at -2.0MPa on matrically adjusted media, with slight differences between isolates and osmotica. Sclerotium yield declined with decreasing osmotic potential, and formation by AG 2-1 and AG-3 isolates ceased between -1.5 and -3.0MPa and -2.5 and -3.5MPa, respectively. On matrically adjusted media, sclerotial formation by AG 2-1 isolates ceased at -0.8MPa, whereas formation by AG-3 isolates ceased at the lower matric potential of -1.5MPa. Sclerotial germination also declined with decreasing osmotic and matric potential, with total inhibition occurring over the range -3.0 to -4.0MPa on osmotically adjusted media, and at -2.0MPa on matrically adjusted media. In soil, mycelial growth and sclerotial germination of AG-3 isolates declined with decreasing total water potential, with a minimum potential of -6.3MPa permitting both growth and germination. The relevance of these results to the behaviour of R. solani AGs in soil and their pathogenicity on potato is discussed.