The Effect of Temperature, Light and Leaf Age on the Frequency of Appressoria Formation and Infection with Uromyces phaseoli (Pers.) Wint.

The frequency of appressoria formation is reduced by changed conditions of infection such as adult leaves, higher temperature, or continuous light during penetration. In correlation with the reduced number of appressoria, the infection frequency decreases too. The behaviour of growing germ tubes on the plant surfaces did not show any indication of the fungus being led to the stomates by leaf surface structures. Obviously a recognition signal proceeds from the stomates sometimes helping the fungus to get into the leaf. Probably the gas exchange through the stomates is involved. Carbon dioxide, produced by the leaves in the dark, can build up pH-gradients towards the stomates as the inoculated leaves are covered with a water film.