Although you would think such elegant-looking flowers would be difficult to grow, the iris family requires very little maintenance. These long-lived, hardy perennials can grow in a variety of soils providing it drains well and they can flourish in full sun to partial shade. However, just because they’re easy to grow does not mean that they’re without their share of problems.
Winter Rot (Botrytis)
The sooner you identify the signs of winter rot, the faster you can remove the affected plants from your garden so others aren’t compromised. There is not a lot known about this fungus other than its destructive and is typically seen during the freezing and thawing periods associated with spring.
This corky, dry rot will often leave the rhizome practically weightless. You can identify it by black sclerotia that resemble a brain, measuring about half an inch wide.
Generally removing the affected rhizomes is enough to remedy the problem, side shoots and new growth will often be free of infection. The easiest way to avoid winter rot is to mulch heavily in cold climates.
Mustard Seed Fungus
Sclerotium rolfsii is the pathogen that is commonly referred to as mustard seed fungus. It receives this name because it presents itself as spherical and brown, similar to the color and size of mustard seeds. Depending on geography, some also know this iris problem as crown rot or southern blight. It is soil-borne and just waits patiently for you to plant an iris so it can attack.
Mustard seed fungus doesn’t usually show up until later in the spring or the beginning of summer when the fans are starting to expire. When you inspect the plant, you will notice that the rhizomes appear to be scale-like, crusty and the roots will look like a mass of threads. Around the plant, in the ground is where you’ll see these seeds scattered.
This problem can be controlled by offering sanitary growing conditions, exposing the rhizomes to direct sunlight, cleaning the infected tissue and dipping in a 10 percent bleach solution before transplanting.
This is a fungus that overwinters on dead remains of the iris such as leaves that have not been cleaned away. The affected leaves will first look like they have wet spots which later turn brown and often multiply, killing the infected leaves.
Humid, foggy or wet conditions are often to blame for leaf spot but it is an iris problem that can be addressed. Remove the leaves immediately and provide the plant with good air circulation.
Always avoid transplanting irises in wet weather because you can spread the disease. It is also important to note that leaf spot does not permanently affect the plant’s rhizome. It does generally offer a two-year cycle and will have an adverse effect on blooming the following year but the plant should be fine the year after.
Bacterial Soft Rot
Of all iris problems, this one is most common and easy to recognize due to the foul odor that it offers. Bacterial soft rot can spread to and destroy nearby plants so it is imperative that you catch the problem in its early stages. Overwatering and too much fertilizer are common causes which is why it usually makes it presence known in the spring.
To treat an infected plant, all you need to do is use a sharp knife to scrape the mushy part off the rhizome. Once this is done, clean the area with a 9:1 ratio of water to bleach and then dust with sulfur. Avoid watering the plant until symptoms disappear. Making sure that you plant your iris in well-drained soil will reduce the risk of bacterial soft rot.
Most commonly seen in the southern states, the exact cause of scorch is undetermined and prevention is up for debate. The center of the leaves get very dry and begin dying at the tips. Leaves fade to brown; the roots die after rotting yet the rhizome often remain firm.
Some plants can be saved by digging them up, drying them in the sun and replanting but you won’t really know until the following season if your efforts have been successful.