Development of a Toolbox to Study <i>Phytophthora cryptogea</i>–Lettuce Interactions: Characterization and Detection of <i>P. cryptogea</i> in Hydroponic Lettuce

Climate change poses a major threat to crop production, resulting in the emergence of new pests and diseases. <i>Phytophthora cryptogea</i> has recently emerged as a major concern in hydroponic lettuce cultivation, causing substantial yield and economic losses. This oomycete pathogen thrives in elevated water temperatures induced by warmer weather conditions (e.g., heatwaves), facilitating rapid pathogen propagation. Although the disease is already present for several decades in chicory cultivation, where it originates from the field, its origin in lettuce cultivation remains unclear. To get a better understanding of its origin, we conducted a multilocus sequence analysis using five reference genes (ITS, <i>β-tub</i>, <i>COI</i>, <i>EF1α</i>, and <i>HSP90</i>) and 33 <i>P. cryptogea</i> isolates from various hosts, including chicory and lettuce. Results revealed a clear separation between lettuce and chicory isolates. Furthermore, we developed and implemented a robust disease bioassay and qPCR assay to investigate the interaction between <i>P. cryptogea</i> strains and lettuce. Our findings revealed that while lettuce isolates exhibited the highest virulence, some chicory isolates also caused disease in lettuce, suggesting a potential evolutionary link between <i>P. cryptogea</i> in lettuce and chicory. Our experiments also revealed that even a low concentration of zoospores (100 zoospores/liter) can elicit severe symptoms, underscoring the pathogen's high virulence. Therefore, effective disease management strategies are needed for controlling (the spread of) the disease. Together, this research provides several tools that can be used to enhance our understanding of the interaction between <i>P. cryptogea</i> and its host plants, including the development of proper disease management strategies.