In my experience thymes are disease free.  As far as pests are concerned slugs and snails have a liking for Thymus pulegioides and its cultivars.  Generally they just tend to 'graze' the plants and with most varieties do very little damage.  However if I am preparing thymes for a show they can be a problem as I end up with a plant covered in half eaten leaves!  Slugs and snails are particularly fond of Thymus 'Ruby Glow' and to a lesser extent Thymus 'Pink Ripple', both of which have Thymus pulegioides in their parentage and they can do a lot of damage.  It is therefore advisable to grow Thymus 'Ruby Glow' in pots so that the plants can be moved around the garden if necessary or given barrier protection such as a copper ring around the pot.  Nightly slug and snail patrols and the encouragement of predatory wildlife such as frogs, hedgehogs, blackbirds and thrushes is recommended.

The main pest which affects thyme is Pyrausta aurata, a tortrix moth which feeds on labiates.  This moth can wreck havoc in a large collection of thymes and it is essential to check plants regularly, especially when thymes are grown in pots and are under cover during the winter months.  The moth is approximately half an inch (12 mm) in wing span, brown with orangey yellow markings.  The olive green coloured larvae feed on the young leaves at the ends of the stems.  They are easily detected because they spin a protective web within the leaves at the end of the stem.  They can be squashed between finger and thumb or cut in two when detected.  All moths should be also be squashed when seen.  Vigilance and mechanical means are all that is needed to protect thymes from this pest.  There is absolutely no need to resort to the use of chemicals.

Pyrausta aurata (sitting on a kitchen cupboard door) and squashed after picture was taken!

Advice on buying thyme plants

Personally my advice would be to avoid the large pots of thyme available from garden centres and some nurseries.  Although they look good at first they will have been planted in non-soil compost which is not suitable and will invariably lead to problems.  Non-soil composts tend to dry out easily and then become difficult to re-wet.  As thyme needs to be grown in well drained soil, inevitably plants grown like this will become too dry and eventually will die.  Although thyme cuttings need to be started in non-soil compost, ideally they should only be potted on into a maximum 6 cm square pot.  This technique enables a good root system to develop prior to being planted in soil.  This size is also suitable for planting between paving slabs once the soil and roots have been gently flattened sufficiently to fit the space!  All my thymes are sold in this size of pot, ready to be planted.  For thyme to grow really well it needs to be grown 'hard' and these smaller plants will soon become established in their permanent growing positions.