The most widespread disease of irises is the mosaic virus, both mild and severe types, mostly affecting bulbous irises such as Dutch, Spanish, and Morocco types. Spread by aphids, the best deterrent is controlling aphids in the yard and the weeds that may harbor them.
Iris Mosaic Symptoms
Iris Mild Mosaic Virus exhibits symptoms such as light-green mosaic-like streaks on new leaves which become more apparent as the plant matures. The flower stalk and bud sheath may show more mottling. Many irises can withstand the disease and may not even show symptoms. Other infected irises may show symptoms one season, but not the next. Iris Severe Mosaic Virus may cause mild to severe stunting of iris stems; wide, pale green stripes; or dark teardrop markings in the flowers of white, lavender, and blue cultivars. Yellow flowers may exhibit feather-like markings. Flower quality is reduced bearing smaller flowers which often are twisted to one side.
Iris Mosaic Control
Iris mosaic virus is transmitted by aphids, a sucking insect, as they move from plant to plant ingesting juices. The best control of the virus is vigilance for aphids and taking measures to reduce or eliminate them from the garden.
How to Treat Iris Mosaic Disease
Examine irises for mosaic virus in early spring, mid-spring, while flowering, and end of season. Dig and dispose of severely affected iris.
Spray aphids with insecticidal soap as soon as they are noticed. Repeat regularly.
Purchase large, healthy bulbs and rhizomes from reputable growers.
Reduce weeds in and around iris beds. Weeds can provide a home for aphids and viruses.
While mosaic virus infects bulbous irises predominantly, rhizomatous irises such as tall bearded irises are occasionally affected, and the disease has also presented in crocus.
