EFFECT OF SELECTED COMBINATIONS OF MICROWAVE TREATMENT FACTORS ON INACTIVATION OF USTILAGO-NUDA FROM BARLEY SEED

The effects of combinations of absorbed microwave power (AMP) and puls ing (PUL) on barley seeds highly infected (68.9%) with the loose smut pathogen Ustilago nuda (Jens.) Rostr. were investigated. On the basis of a previous study the moisture content of the seeds was fixed at 21% on a dry weight basis and the microwave treatment duration at 75 min. Barley seed germination and seedling vigour, the latter measured as t he mean plumule length (MPL), were not significantly affected by most of the microwave treatments compared to a non-treated control, except at high levels of AMP and PUL. The percentage of seed germination was highly correlated with the MPL (r = 0.86). A multiple regression model explained 82% of the variation in the percentage seed germination and 74% of the MLP as functions of AMP and PUL. The effectiveness of micr owave treatments to inactivate U. nuda was evaluated based on seedling infection (SI) at the second true leaf stage and plant infection or s mutted plants al maturity (PI). There was a poor correlation between % SI and %PI (r = 0.46). Although most of the microwave treatments had a significantly lower %SI than the untreated control, there was no sign ificant effect of AMP or PUL within the range of levels studied. In co ntrast, the %PI data showed that there was a significant interaction o f AMPPUL on %PI. The SI and PI were reduced to 26.7 and 11.6%, respec tively, for the best microwave treatments, as compared to 50.1 and 45. 3%, for the untreated control. Therefore, up to 74.3% reduction of the effective inoculum was achieved by using 0.5 W/g AMP and 50/10 s of P UL. At 0.5 W/g AMP, when PUL was reduced to 40/20 s, the effective ino culum was reduced by more than 56.0% without significantly affecting s eed germination and vigour.