Induced Resistance to Apple Scab: Microscopic Studies on the Infection Cycle of Venturia inaequalis (Cke.) Wint.

Application of Methyl 2,6-Dichlorisonicotinate (DCINA) induced resistance in apple against Venturia inaequalis. The effects of induced resistance on the infection cycle of the fungus were investigated by fluorescence microscopy. Induced resistance interfered with all stages of infection except conidial germination and appressorium formation. The establishment of primary stromata after penetration of the cuticle was affected by induced resistance as indicated by a delay in the visualisation of these structures. Only 10% of the primary stromata further expanded and differentiated runner hyphae in induced resistant plants. Contrarily in unprotected plants, runner hyphae were formed at 100% of the successful penetration sites. Characteristically, circular fluorescing structures appeared associated with stunted penetration sites. The epidermal cells underneath these infection sites showed no hypersensitive response. On induced resistant plants the number and length of runner hyphae was reduced. In most cases neither colonisation nor sporulation occurred due to the inhibition of secondary stroma formation. Possible mechanisms of induced resistance in apple plants are discussed