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.