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Response of Pythium aphanidermatum zoospores to ultraviolet radiation in a laboratory collimated beam assay

Ultraviolet radiation has been used for disinfection of greenhouse hydroponic solutions for nearly two decades, but information on the dose response of specific pathogens is still lacking. To determine the sensitivity of Pythium zoospores to different UV doses, a collimated beam assay was developed with a controlled UV-light intensity. An applied UV dose (D) was calculated according to D = ωl x T, where ω = [1-e -( x L) l/(A x L). The survival of Pythium zoospores was reduced to 34.0, 1.2, and 0.02%, respectively, at doses of 10, 20, 40 mJ cm -2 in a suspension of 2.8 x 10 5 zoospores ml -1 . A small portion of the damaged cells was recovered from UV irradiation by exposure to visible light before plating. The percentage survival decreased when the transmittance of the suspension was improved by filtration. The germination of Pythium zoospores was inhibited by bacterial isolates of Variovorax paradoxus CE26, Xanthomonas maltophilia CE27, Alcaligenes faecalis SS faecalis CE24, and Comamonas acidovorans CE22, but not by isolates of Xanthomonas maltophilia CE17, Alcaligenes faecalis SS faecalis CE8, Comamonas acidovorans CE21, and Alcaligenes eutrophus CE19. The bacterial effect on the germination of zoospores was diminished at high doses of UV irradiation. The mechanisms of bacterial inhibition on the germination of zoospores needs further research and elucidation.