Experimental Evolution of <i>Phytophthora infestans</i> on Tomato Reveals Rapid Genotypic and Phenotypic Adaptation and Dynamic RXLR Genome Variation

The late blight pathogen, <i>Phytophthora infestans</i>, poses a significant threat to tomato crops worldwide. To understand the potential for rapid adaptation of this pathogen, we conducted an in vitro experimental evolution study with four <i>P. infestans</i> US-23 lineages collected from tomato hosts with different combinations of resistance genes (<i>Ph</i> genes). These isolates were passed serially over eight generations on five tomato cultivars. After infection, targeted sequencing of the pathogen's RXLR effector genes was done. In just eight generations, we observed both phenotypic and genotypic changes in the US-23 lineages, with differences in disease severity among pathogen isolates and alterations in the RXLR genome. Our findings suggest rapid mutation even in a clonally reproducing lineage, highlighting the potential for adaptation of <i>P. infestans</i> within a single growing season on tomato. These insights shed light on the adaptability of this devastating pathogen and emphasize the importance of considering tomato host resistance in late blight management strategies.