Defence reaction of apple against fire blight: Histological and biochemical studies

Systemic movement of Erwinia amylovora in apple leaves was investigated by histological and biochemical methods with respect to defence reactions of the plant. Presence of the bacteria was noticed in xylem vessels and intercellular spaces of cortex and parenchyma tissue. Infected xylem vessels were frequently filled with a mucilaginous substance. Occasionally, these occluding masses were found in xylem vessels of inoculated leaves even though no bacteria were visible. Based on this phenomenon, it can be discussed whether accumulation of this substance represents a reaction of the plant that is comparable to gummosis in cherry. Accumulation of phenolic compounds was observed, which is interpreted as a chemical barrier inhibiting the spread of Erwinia amylovora throughout the cortex tissue of petiole and stem. High contents of phenolic acids in apple leaves were correlated with a restricted disease development. In samples with slight symptoms the content of these phenolic acids were up to ten times higher as in samples showing pronounced symptoms.