Weeds, as ancillary hosts, pose disproportionate risk for virulent pathogen transfer to crops

Table 1 Predicted and observed Rhynchosporium commune diversity estimates under the contrasting host diversities and pathogen census population size

	Barley			Barley grass
	Large pathogen pop size	Genetically uniform host		Small pathogen population size	Genetically diverse host
R. commune diversity	Predicted	Predicted	Observed	Predicted	Predicted	Observed
Neutral genetic diversity:
No. of SSR alleles	Many	Few	Many	Few	Many	Few
Ne	Large	Small	Large	Small	Large	Small
No. of MLGs	Many	Few	Many	Few	Many	Few
Mode of reproduction/linkage disequilibrium	-	More asexual/high linkage disequilibrium	More sexual/lower linkage disequilibrium	-	More sexual/low linkage disequilibrium	More asexual/higher linkage disequilibrium
Migration	More from barley to barley grass	More from barley grass to barley	More from barley grass to barley	More from barley to barley grass	More from barley grass to barley	More from barley grass to barley
Diversity under selection:
No. of effector alleles	-	Few	Few	-	Many	Many
No. of pathotypes	-	Few	Few	-	Many	Many
Aggressiveness	-	Less	Less	-	More	More

Host dynamics (taken as a given based on literature) for barley is a large acreage/population size with small N _e (few cultivars used), inbred monocultures, assumed few resistance genes/alleles and higher disease (R. commune) incidence (large pathogen census population size). Barley grass is a weedy annual which may have a smaller population size than barley (on average less plants), is genetically diverse and thus has a large Ne, outcrossing, assumed to have many resistance genes/alleles and has lower disease (R. commune) incidence (small pathogen census population size). Observed pathogen genetic diversity and effective population sizes are as observed at neutral loci (SSRs) and traits that may be under selection (effector locus Nip1, pathogenicity). Predictions and observations in bold indicate when results confirmed predictions

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