Iris diseases can be caused by fungi, bacteria or viruses. Some affect the foliage, causing symptoms such as leaf spotting, mottling or decay. Others attack the parts of the plant below the soil surface (either the bulb or the rhizome, depending on the type of iris), causing decay.

Depending on which disease is to blame, symptoms may vary as follows:

Leaf spot (Cladosporium iridis)

• Seen most commonly on Iris germanica (bearded iris), but other rhizomatous and bulbous types can be affected
• Watersoaked leaf spots develop, which turn brown and may have a yellow margin
• The spots are lens-shaped in the early stages but usually enlarge and merge together
• Heavily infected leaves may have much of their leaf area affected, and will eventually shrivel and die
• A ‘sooty’ growth, consisting of large numbers of fungal spores, may develop on the surface of the spot in wet weather

Ink disease (Bipolaris iridis)

• Seen most commonly on the bulbous Iris reticulata and Dutch irises
• Leaves develop black blotches and, if severely affected, turn yellow or reddish-brown and wither
• Ink-like black blotches may be present on the exterior of bulbs
• Severely affected bulbs rot to leave an outer ‘shell’ containing a mass of black fungal spores

Rust (Puccinia iridis)

• Both rhizomatous and bulbous iris can be affected
• Numerous small, pale green or yellow leaf spots develop
• Close examination will reveal that each spot contains an orange-brown ‘pustule’ of spores
• As summer progresses, these orange-brown pustules are replaced by black ones
• Severely affected leaves may turn a more uniform yellow and wither from the tip

Rhizome rot (Pectobacterium carotovorum)

• This primarily affects bearded irises
• The rhizome develops a slimy, often foul-smelling rot
• Decay usually starts in the youngest part of the rhizome, but may spread to affect a large proportion of it
• Leaves develop a soft rot at the base, turn yellow and wither. Eventually the whole ‘fan’ of leaves may collapse onto the ground

Bulb rots (Fusarium and Penicillium species)

• Bulbs may rot in the ground or during storage
• White, orange or blue-green fungal growth may be visible on or within the bulb
• Plants grown from affected bulbs may be stunted with leaf yellowing, and may die

Viruses (various)

• Leaves develop light green or yellow flecks and mottles
• Flowers may develop patches of irregular colouration (‘flower breaking’)
• Plants may lack vigour

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Non-chemical control

• Remove and dispose of all leaf debris at the end of the year. This is particularly important where iris leaf spot is a problem
• Picking off leaves lightly infected with leaf spot or rust can help to slow the spread of these diseases, but take care not to remove too much foliage as this may do more harm than good
• Plants affected by ink disease are best disposed of. The soil in which they have grown should be replaced or rested for a few years from bulbous irises
• In order to flower well, bearded iris rhizomes should be planted so that they are partially exposed, on well-drained soils. These conditions will also help to prevent bacterial rhizome rot
• Where only a small part of the rhizome is affected by bacterial decay, this can be cut away using a clean, sharp knife that is disinfected regularly. Allow the cut surfaces to dry before replanting
• Check iris bulbs before planting and discard any with softening, decay or black inky patches on the scales.  If bulb rots develop in the garden, dispose of the plants and rest the soil from bulbous irises for a few years
• Dispose of virus-affected plants if performance is impaired, or if there is a risk of transmission to neighbouring healthy plants

Fungicides

The RHS recommends that you don't use fungicides. Fungicides (including organic types) may reduce biodiversity, impact soil health and have wider adverse environmental effects. If you do intend to use a fungicide, please read the information given in the link and download below to ensure that use, storage and disposal of the product is done in a responsible and legally compliant manner.
  
The products listed in the ‘Fungicides for gardeners’ document below are legally available for use by home gardeners in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally. Homemade products are not recommended as they are unregulated and usually untested.

Because of the vertical growth habit of iris leaves it is very easy for the fungicide spray to run off onto the soil. Use a fine spray to get good coverage, stopping just before the spray starts to run down the leaves – do not spray in breezy or windy conditions as a fine spray can drift over a considerable distance. Do not apply fungicide sprays to irises growing in close association with ponds or other water features, as they are likely to be harmful to aquatic animals.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Link

Chemicals: storing and disposing safely

Leaf spot is a fungal disease favoured by wet weather. The microscopic spores produced on the surface of the spots are dispersed by rain-splash and wind. The fungus overwinters at the base of the leaves and on any other leaf debris left on the soil surface. Repeated infection over several years can greatly reduce the vigour and flowering potential of the plants.

Ink disease is also spread by wind-blown or rain-splashed spores. The fungus is thought to persist on affected bulbs and crop debris, and as spores in contaminated soil. This disease is most common in south-west England, and can occasionally affect Lachenalia and Crocosmia.

Rust is another wet weather, fungal disease. The orange-brown pustules contain huge numbers of ‘summer’ spores that spread the disease during the growing season. The black pustules contain overwintering spores. Like many rusts, Puccinia iridis affects two host plants in its life-cycle, the alternate hosts in this case being species of nettle (Urtica spp.).

Rhizome rot is a bacterial disease, favoured by wet, poorly-drained soils. The bacterium usually requires damage of some kind to the leaf bases or rhizomes in order to colonise the plant. The disease progresses most rapidly under warm, wet conditions. Pectobacterium carotovorum also causes soft rots on a range of vegetable crops (e.g. potatoes, carrots) and on some other ornamentals.

The fungi causing bulb rots are generally found in the soil. Infection can occur through the roots or bulb scales. In some cases wounds predispose the bulbs to infection.

The viruses of significance on irises are all spread by aphids. These include Bearded iris mosaic virus, Iris mild mosaic virus and Iris severe mosaic virus.