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

Conidia of Fusarium oxysporum f. sp. cyclaminis and zoospores of Pythium ap hanidermatum were suspended in water and hydroponic nutrient solutions and treated with various doses of UV radiation (253.7 nm) by means of a collima ted-beam apparatus and a flow-through apparatus. Germination and colony-for ming ability of spores from treated suspensions were estimated on agar medi a. Colony-forming ability of F. oxysporum f. sp. cyclaminis decreased logar ithmically, with increase in applied dose of UV radiation, and in curves th at were 3 to 10 times steeper for conidia in water compared to crop nutrien t 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 25 4 mW-s.cm(-2), respectively. In the collimated-beam apparatus, applied dose s to inactivate 99.90% of conidia were two to three times higher. Colony-fo rming ability of P. aphanidermatum zoospores decreased logarithmically, wit h UV radiation dose, in curves that ranged from similar to twice as steep f or zoospores in water compared to plant nutrient solutions. Inactivation of 99.99% of zoospores in water. fresh plant nutrient solution, and pepper cr op nutrient solution, respectively, required doses of 12, 17. and 38 mW.s.c m(-2) as estimated in the flow-through apparatus, and 34. 46, and 213 mW.s. cm(-2) as estimated in the collimated-beam apparatus. Colony-producing abil ity of zoospores of P. aphanidermatum and an unidentified species of Pythiu m from hydroponic lettuce declined more rapidly than did germination incide nce as UV dose was increased. In conclusion, UV radiation doses of 30-40 an d 20-30 mW.s.cm(-2) inactivated most conidia of F. oxysporum f. sp. cyclami nis and zoospores of P. aphanidermatum, respectively, when applied to suspe nsions of the sports in water. For spores in plant nutrient solutions, the applied dose must be increased to compensate for absorbance of UV radiation in the solution.