Irreversibility of Primary Host Cell Recognition of Pathogen as Demonstrated by Yellowing Response Barley Leaves to Races of Erysiphe graminis

Although barley leaves inoculated singly with races of Erysiphe graminis hordei exhibited no or slight yellowing, they responded with marked yellowing on double inoculation, especially when an incompatible race was used as inducer (conditioner) of reactions. Leaves conditioned by an incompatible race responded much extensively to a compatible challenger than to an incompatible challenger race. No visible colonies were formed on leaves hypersensitively responding with yellowing. More colonies were formed on leaves which had been both induced and challenged by a compatible race than on those singly inoculated with the compatible race. These results were discussed in relation to the primary recognition and subsequent cellular conditioning in the host-parasite interaction.