Relationships of ultraviolet radiation dose and inactivation of pathogen propagules in water and hydroponic nutrient solutions

Abstract

Conidia of Fusarium oxysporum f. sp. cyclaminis and zoospores of Pythium aphanidermatum were suspended in water and hydroponic nutrient solutions and treated with various doses of UV radiation (253.7 nm) by means of a collimated-beam apparatus and a flow-through apparatus. Germination and colony-forming ability of spores from treated suspensions were estimated on agar media. Colony-forming ability of F. oxysporum f. sp. cyclaminis decreased logarithmically, with increase in applied dose of UV radiation, and in curves that were 3 to 10 times steeper for conidia in water compared to crop nutrient solutions. In tests in the flow-through apparatus, applied doses required to inactivate 99.90% of conidia in water, in fresh nutrient solution, and in nutrient solutions from pepper and tomato crops were 20, 93, 104, and 254 mW·s·cm−2, respectively. In the collimated-beam apparatus, applied doses to inactivate 99.90% of conidia were two to three times higher. Colony-forming ability of P. aphanidermatum zoospores decreased logarithmically, with UV radiation dose, in curves that ranged from similar to twice as steep for zoospores in water compared to plant nutrient solutions. Inactivation of 99.99% of zoospores in water, fresh plant nutrient solution, and pepper crop nutrient solution, respectively, required doses of 12, 17, and 38 mW·s·cm−2 as estimated in the flowthrough apparatus, and 34, 46, and 213 mW·s·cm−2 as estimated in the collimated-beam apparatus. Colony-producing ability of zoospores of P. aphanidermatum and an unidentified species of Pythium from hydroponic lettuce declined more rapidly than did germination incidence as UV dose was increased. In conclusion, UV radiation doses of 30-40 and 20-30 mW·s·cm−2 inactivated most conidia of F. oxysporum f. sp. cyclaminis and zoospores of P. aphanidermatum, respectively, when applied to suspensions of the spores in water. For spores in plant nutrient solutions, the applied dose must be increased to compensate for absorbance of UV radiation in the solution