BIOLOGY, HISTOPATHOLOGY, AND IMPROVEMENTS IN AXENIC CULTURE OF LABYRINTHULA TERRESTRIS, CAUSAL AGENT OF RAPID BLIGHT OF COOL-SEASON TURFGRASSES

Rapid blight is a disease of cool-season turfgrasses caused by Labyrinthula terrestris. Disease symptoms include water-soaked lesions, yellowing and browning of foliage, and coalescing patches of dead turf. L. terrestris belongs to a group of marine microorganisms in the kingdom Chromista (also known as Stramenopila). Morphological characteristics of Labyrinthula spp. include spindle-shaped vegetative cells that move in an ectoplasmic network. The biology and pathology of L. terrestris is not clearly understood; therefore, the ultrastructure, life cycle, and histopathology of L. terrestris were investigated. In addition, improvements in axenic culture and long-term storage methods were made to better culture L. terrestris. Ultrastructure studies showed L. terrestris cells contain cytoplasmic contents consistent with relatives; bothrosomes, cell surface organelles that secrete the ectoplasmic network, were also elucidated. Examinations of cultures revealed that cells initially divided repeatedly with little motility, later cells were highly motile in ectoplasmic networks, and finally cells moved in a whirling motion and formed round aggregates. Aggregate formation was also observed on the host. Aggregates gave rise to new colonies and appear to play a role in survival under stress. Asexual reproduction was through mitotic cell division and sexual reproduction or production of zoospores was not observed. Histopathology investigations showed that L. terrestris infected the host through stomata, wounds, and trichome bases. L. terrestris occupied all foliar cells in the host, multiplied inside the host, and moved across host cell walls. The basic culture medium called SSA (serum saline water agar), typically used for L. terrestris, resulted in poor growth and viability. SSA was modified with additional nutritive sources; this medium was designated as SSA+A (plus additives). Another medium, GESSA (grass extract SSA) was developed by adding grass extract to SSA. Results showed that the growth rate was significantly higher on GESSA followed by SSA+A and SSA. Cultures were successfully stored for up to two years with the modified long-term storage method