Excessive sunlight can cause discoloration and sun-scald to leaves. General bronzing or an orange coloration of the foliage is also due to environmental stress usually following hot, dry spells in summer, although a contributing factor may be root damage due to winterwaterlogging.
Over winter, leaf tips and margins commonly yellow due to low temperatures. Leaf discoloration will tend to be more common in exposed sites. Young growth in spring can also get caught by frost, turning them pale brown and often papery-looking. An application of fertiliser in spring to encourage fresh new growth, along with clipping, usually rectifies these problems.
General yellowing of foliage may be symptomatic of waterlogging, especially on heavy clay soils. Roots subjected to waterlogging are usually a blue-black colour in cross section and fall apart when teased out. Damaged roots should be removed, top growth trimmed back and the box replanted.
Box may sometimes suffer from the following problems: box blight, box sucker, box tree caterpillar, plant establishment problems, red spider mite and scale insects. If in doubt, our page on box problems is a good place to start.
( From the RHS site)