Pests and diseases
In the main, iris are not prone to major pests and diseases in the UK. Newly imported bearded varieties being overwintered in a cold greenhouse can be attacked by grey aphids, but these can be killed by spraying with a suitable systemic insecticide, or, if you prefer, systematically squashed! (Running a finger and thumb up the fan squeezing at the same time, is effective, albeit slightly messy).
Green aphids can attack in the garden during hot but damp weather, and the same solution can be applied as above.
The worst pests are slugs and snails, which can decimate newly planted iris and seedlings overnight. Preventative measures range from a scattering of slug pellets (or better still the lesser-known liquid alternative), to 'green' remedies of nematode control, beer traps, or even the simple, but time-consuming process of collecting them by hand in the mornings and evenings. The job needs to be done using whichever method you prefer, otherwise your iris will open with damaged blooms.
Slugs and snails
There are three main diseases to look out for: rot, scorch and rust.
Rust manifests itself as brown spots on the leaves, usually appearing after bloom, but can happen at any time. While it is disfiguring to the plant, it is not usually harmful, but when the leaves die down they should be collected and burnt to prevent spreading the disease. A fungicidal spray will control the rust, although it will not remove existing spots.
Scorch is a condition where the leaf starts dying back from the tip, first turning orange/red then brown. It usually affects only one plant in a block, leaving all its neighbours untouched. There is no known remedy for scorch, so all affected plants should be lifted and burnt to prevent spreading the disease.
Rot attacks the rhizome turning it buff/yellow in colour and becoming soft. Quite frequently the first sign of rot is when a healthy looking fan keels over, and examination shows that the cells at the base have turned translucent. The rhizome will yield to pressure, and if the skin is punctured, a noxious smell is released.
Lift the affected piece, and cut back to white flesh, discarding and burning the bad part to prevent spreading the disease. Dust the cut end with sulphur powder, and replant 24 hours later in a different place. If possible, remove the soil from the original position, and replace with new before planting anew.