Plasticity in initial infection traits in response to high temperature for worldwide representative Puccinia striiformis isolates

Limited knowledge is available regarding pathogen adaptation to environmental conditions, despite its importance to disease epidemiology. Wheat yellow rust, caused by the pathogen Puccinia striiformis offers an interesting case study, as the disease was so far considered limited to cool and humid climate, till the report of significant crop losses due to emergence of high temperature adapted lineages. We investigated the variability in temperature response in relationship with their genetic structure for 56 P. striiformis worldwide representative isolates including two isolates from the high temperature adapted strains PstS1/S2. High temperature adaptation was assessed through urediospore germination rate (GR) at 8 °C, 20 °C and 25 °C, and relative infection efficiency (RIE) measured at 20 °C, using two independent replicates of the experiments. Phylogenetic relations between isolates were based on 17 informative SSR markers, coinciding strongly with their geographical sampling origin. Variability was assessed considering populations (defined by sampling areas), where isolates from the pathogen centre of diversity in Himalayan region (Nepal and Pakistan) revealed adaptation to high temperatures in terms of germination rate and RIE, even higher than the reference PstS1/S2 isolates, in comparison to isolates originating from cooler areas. The presence of high-temperature adapted isolates in genetically diverse populations of Asia (Nepal, Middle-East and Pakistan) represented an adaptation to local climate, where high temperature tolerance was correlated to the temperature of the sampling locations. Middle-East also represented variability for temperature responses. Following the genetic variability and plasticity of plant pathogens in reaction norm to temperature is therefore critical to forecast future epidemics in a global warming context.