EVIDENCE FOR LOSS OF ONTOGENIC RESISTANCE OF APPLE LEAVES AGAINST VENTURIA-INAEQUALIS

The susceptibility of apple leaves to Venturia inaequalis was investig ated by assessing disease on individual leaves of seedlings and labell ed shoots of orchard trees. Four sets (A-D) of potted seedlings of cv. 'Golden Delicious', which had been grown in a glasshouse, each with a pproximately 30 mature leaves, were exposed to a high-inoculum orchard . Sets A and B were exposed after each other for 47 and 42 days, respe ctively. As a result of the six and three infection periods during exp osure, 94% and 81%, respectively of the seedling shoot tips in set A a nd set B became infected. However, due to ontogenetic resistance, dise ase incidence was low in both sets on leaves which were fully expanded at the beginning of exposure. Set C was exposed during both periods ( 89 days) in which sets A and B were exposed. Not only were all the see dling shoot tips in set C infected, but also - due to the loss of onto genetic resistance - nearly all of the mature leaves. Ontogenetic resi stance was also lost in set D, which was exposed for 57 days at the en d of the growing season. The symptoms on fully expanded leaves on plan ts in sets C and D were typical and similar to those on young leaves. A time course symptom assessment was performed on leaves which had dev eloped early in the season on labelled, field-grown shoots of cv. 'Gol den Delicious' trees. A steady increase of disease incidence was detec ted, which could not only have resulted from infections followed by ex tended incubation periods. In addition to the increase of typical lesi ons on both leaf surfaces, there was also an abundance of diffuse myce lial development on the lower surfaces of the leaves of the held-grown trees.