My googling established that quince fleck is Fabraea maculata.

The effect of boron deficiency on the fruit of apples/pears is described as "corky pit" - corky patches on the skin and inside the fruit. Also new shoots suffer die-back. Boron deficiency is mainly a problem in high rainfall areas with sandy soils, where it gets leached out of the soil, and above granite bedrock which contains little boron. It can also be a temporary problem in very dry periods, when there isn't enough water to deliver it to the plant. Also some types of soils are said to fail to mobilise boron to plants, though the sources I have seen disagree what these a compa
http://www.garden-care.org.uk/files/Fertilisers.pdf
http://en.wikipedia.org/wiki/Micronutrient

Organic fertiliser, ie compost, is rich in boron. So if you are already putting organic fertilisers on, there is no need to add boron, and adding boron won't work unless you are adequately watered.