The alteration of sugar level responsible for the development of a poplar leaf rust, caused by Melampsora larici-populina Kleb., was investigated.
In diseased leaves, before visible symptoms appeared, sugar alterations occurred rapidly upon infection. Total content of sugar and sugar alcohol in the diseased leaves increased after inoculation. The amount of arabitol in rusted leaves differed remarkably and this may be attributable to the presence of mycelium in the tissue, though the possibility that arabitol occurs in the host cannot be excluded.
The most interesting result was observed in the correlation between the content of sucrose and an unknown substance. The lower level of sucrose was reached at an earlier stage of infection, when the maximum concentration of the unknown substance was detected.