Induced resistance of sweetpotato to Fusarium root rot by UV-C hormesis

'Jewel' sweetpotato storage roots previously treated with a low hermetic dose of ultraviolet light-C (UV-C) were stored for 30 days before inoculated artificially with Fusarium solani. Storage roots showed an increase in resistance to Fusarium root rot, as indicated by a reduction in lesion diameter, depth and weight of rotted tissue following UV-C treatment. The rate of decay development around F. solani inoculum plugs on UV-C treated sweetpotato storage roots progressed slowly, and about 55% failed to develop lesions on wounded UV-C treated, compared to 11% of untreated sweetpotato storage roots, 10 days after inoculation. There was a polynomial curvilinear regression relationship between percent incidence of Fusarium root rot and hermetic UV-C doses. The hermetic dose of UV-C which suppressed decay to the greatest degree was 3.6 kJ m(-2). Exposure of sweetpotato storage roots to doses of UV-C promoted phenylalanine ammonia-lyase (PAL) production with the maximum PAL activity occurred to the greatest degree at a UV-C dose of 3.6 kJ m(-2). Similarly, a relationship between PAL activity and the incidence of Fusarium root rot was established. Crude extracts from UV-C treated sweetpotato storage roots reduced germination, germ tube elongation, and growth off. solani compared to extracts obtained from untreated storage roots. (C) 1999 Elsevier Science Ltd. All rights reserved.