Expression of resistance in primary leaves of Avena sativa inoculated with an incompatible race of Puccinia coronata avenae was decreased by subsequent inoculation with a compatible race.  This decrease was diminished as the time interval between inoculations was prolonged, and the reinoculation with a compatible race at 12 or more hours after the initial inoculation did not appreciably affect the expression of resistance.  Heat-treated leaves inoculated with a uredospore mixture of incompatible and compatible races (95 : 5, w/w) and subsequently inoculated with a compatible race expressed a significant increase of hypersensitive necrosis and a marked decrease of uredosorus formation compared with leaves inoculated with only the uredospore mixture.  It therefore appeared that, in this case, the compatible race was recognized as incompatible and the resistance reaction was amplified.  The host cellular conditioning towards the resistance response seemed to be completed sometime between 8 and 12 hours after inoculation with an incompatible race.

Additional key words: cross-protection.