Temporal changes in the population structure of the pathogen Globisporangium irregulare in a hydroponic Eustoma production system in Japan

Temporal changes in the population structure of Globisporangium irregulare were investigated over three years (2015–2017) in a hydroponic Eustoma production system in Fukushima, Japan. The aims of this study were to determine the phylogenetic affiliation of Fukushima isolates, their potential as a source of contamination, and whether any specific molecular types were associated with pathogenic traits. Phylogenetic analyses based on genetic distances revealed that isolates exhibiting an intermediate RAPD DNA group I/II genotype occupied phylogenetic positions between groups I and II, suggesting recombination across lineages. The 2015 population exhibited relatively high genetic diversity, whereas isolates in 2016–2017 were genetically more uniform and often shared identical genotypes across cultivation cycles. These results indicate that pathogen populations were likely maintained within the hydroponic system, probably via infected plant residues, rather than repeatedly being introduced from the surrounding environment. No clear association was detected between molecular type and pathogenicity-related characteristics. Extracellular enzyme activity analysis revealed increased cellulase and xylanase activities with time while a reverse trend was seen in pectinase activity. However, these year-related trends did not correlate with disease severity. Isolates dominant after 2015 showed lower heat tolerance and reduced zoospore productivity compared with the 2015 population. The observed genotype replacement therefore appears to reflect selection for persistence within the hydroponic environment rather than increased virulence, stress tolerance, or dispersal ability. These results suggest that disease management in hydroponic Eustoma production should consider the potential for long-term survival within cultivation facilities and emphasize the importance of sanitation and residue management.