APPLE scab is among the deadliest of all apple diseases. It is caused by a parasitic fungus, the spores of which arise from partly- decomposed debris from last summer’s diseased fruit and fallen leaves.
The fungal spores settle on the leaves and then quickly move on to the fruit. We can see it developing at first with slightly- shrivelled, dirtylooking leaf- clusters.
Later, as it moves on to apples, we can see lop- sided, distorted young apples forming with dark, sometimes cracked, scabs on one side.
I’ve long- since learned to control apple scab with rigorous hygiene around vulnerable varieties – mainly Granny Smith, Pink lady and Mutsu.
It is essential to spring- prune all susceptible apple trees to allow a free flow of drying air through canopies
This involved raking up and carting away every fallen fruit and all fallen leaves.
I also closely mow, right to the ground, any grass or weeds with a grass- catcher attached, especially in early winter and spring.
In May, this is followed by a wide sprinkling of builders’ lime beneath and around the trees.
Lime helps to speed up decomposition and encourages earthworm activity during winter to get rid of any remaining diseased material.
In late winter, a good scattering of blood and bone further accelerates decomposition while helping to fuel growth.
It is essential to spring- prune all susceptible apple trees to allow a free flow of drying air through canopies.
This should be followed by spraying all leaf surfaces with a mix of builders’ lime and water when the first blotchy leaves indicate disease is attempting to move in.
Scab organisms prefer acidic conditions, so the lime covers the leaves with a protective, alkaline coat. It works brilliantly to keep this disease at bay. Luckily builders’ lime is an extremely cheap form of protection so the overall cost is low.
If the lime spray goes on too early, scab organisms will attack all new, unprotected leaves which emerge afterwards, so it may be necessary to spray again. This is what I’ve been doing.
Apple scab can only penetrate leaves that remain wet for several hours under relatively cool conditions. Unfortunately, these conditions have happened many times in Tasmania, especially at night- time during spring and early December. This is why this disease has become particularly prevalent this year.
However, the lime coating is not easily washed off and usually remains for several weeks, despite persistent rain.
In most home gardens we can give vulnerable apple trees this type of close attention. It also means going over the trees regularly to pick off any diseased apples and to prune out all dirty- looking leaf- clusters.
If we can control apple scab during early summer there is little chance of re- infection happening once temperatures rise above 25C.
Once the disease is eliminated, the trees will not be re- infected, unless neglected apple trees are growing nearby.
I should add that lime sprays also prevent calcium deficiency, the main cause of ‘ bitter pit’ disorder in apples, while spring pruning and fruit thinning also helps control brown- rot disease.
Lime sprays applied during December and early January are also an effective means of controlling pear and cherry slug.
This is a slug- like grub that attacks and skeletonises the leaves of cherry, pear, Japanese plum, quince, hawthorn and related plants.