Iris Root Rot: How To Control Or Prevent Root Rot On Iris Plants

While they tend to be quite resilient, some diseases can still affect irises.

It can be frustrating for iris lovers, as some diseases attack mainly bearded or non-bearded varieties while others attack mostly bulbed or rhizomatous species.

Nasty Iris Root RotPin

One notable example of this is in the form of root rot.

Some irises like the Blue Flag (Iris Versicolor) are well-adapted to wet or marsh-like environments, while others prefer drier soils.

It is particularly true of irises with a rhizome instead of a bulb.

These irises are susceptible to a particularly nasty form of root rot, commonly known as soft rot.

What Is Iris Root Rot (AKA Soft Rot)?

Caused by the bacteria Erwinia carotovora, soft rot is a form of root rot usually caused by damaged rhizome instead of waterlogging.

You often won’t notice this disease right away because the bacteria are most active at 82.5° degrees Fahrenheit, but the enzymes it produces are most active at a cooler 57.2° degrees Fahrenheit.

Thus, the plant will often become infected during the summer but might not show symptoms until the fall or spring, when temperatures are ideal for the enzyme to do its job.

Unlike more common types of root rot, this form can be challenging to treat and not always so easy to prevent.

What Damage Does Soft Rot Cause?

The first signs you’ll see of soft rot are in the leaves, which will begin to yellow in the middle of the fan.

This yellowing will slowly spread up the leaves, with the center turning brown.

Eventually, the browning fans will collapse.

While the disease is commonly seen on multiple fans, it may affect only a few fans, with others looking perfectly healthy.

There won’t be any yellowing in a few cases, and the first signs will be leaf wilting.

The real damage occurs underground in the rhizome.

As enzymes break down the cell walls to spread the bacterium, the rhizome will become mushy and give off a rotting smell.

The infected rhizomes may ooze slime or even collapse under little pressure if excavated.

As the disease can overwinter, symptoms may appear long after the initial infection.

How To Control Soft Rot?

While we understand a lot about this disease, part of the problem is that there’s a lot of misinformation out there, mainly from sources that confuse this form of root rot with fungal root rot or other types of bacterial rot.

Two Common Myths Regarding Soft Rot

Soft rot begins in the rhizome, so some sources claim it’s incurable, and the plant is automatically lost.

However, if caught in time, at least part of the plant can often be saved and the soil treated.

Another common myth is that you can treat the disease by pruning away yellowing leaves.

While this does indeed work for some types of rot, it won’t affect soft rot because the disease isn’t in the leaves you’re removing.

Treating A Diseased Plant

Soft Rot is caused primarily by the rhizome being damaged, either grazed by tools while gardening or by a garden pest.

Examples of garden pests are:

Learn more about Iris Pests and Diseases here.

  • The first and most crucial step is determining the extent of symptoms and finding the likely source of the infection.

The first and most crucial step is determining the extent of symptoms and finding the likely source of the infection.

Here are the tips to follow for inspection:

  • Dig up the rhizome and inspect it somewhere safe, carefully for severely infected plants.
  • For plants showing symptoms in only a few fans or otherwise appearing somewhat healthy, you can uproot them and work on them right in the garden, but make sure you put something down (such as a plastic bag) under the plant.

Performing Iris Rhizome Surgery

Here are the steps to follow in performing surgery:

  • Use a sharp, sterile knife and carefully cut away the diseased parts of the rhizome.
  • Dip the knife in rubbing alcohol or some other sterilizing agent between cuts.
  • Cutaway until there’s no trace of brown infection on the rhizome, or the disease will just spread again.

In some more advanced cases, the damage may be so severe the entire plant is a loss (which is why you want to work on it somewhere you can then sterilize after disposing of the plant).

  • Make sure you dispose of the infected tissues safely.
  • Once this is done, remove or sterilize the soil where the iris was planted.
  • It’s usually easiest to scoop out the soil a couple of inches around the excavation site and dispose of it, adding healthy soil in its place.

However, you may also use various sterilization methods on the soil if you’re feeling bold.

Post-Surgery Care

You will want to ensure no traces of bacteria are left after cutting away any visible signs.

This can easily be achieved by dipping the plant in a solution of 1 part bleach to 9 parts water (you can give the rhizome a sponge bath using this solution if working on-site).

You can further treat the soil area by sprinkling a chlorine-based cleaning powder (Comet works well) to raise the soil pH to a strong alkaline 12.5 or if the temperature is cooler than 80° degrees Fahrenheit, you can use garden sulphur to drop the soil pH to a strong acidic 5.2.

To help avoid harming your iris, it’s best to do this at the same time you’re giving it its bleach bath.

Allow the rhizome a couple of days to air dry before replanting and, if you treated the soil area, consider flushing the soil to help bring the pH back to a more hospitable level right before replanting.

You can also choose to temporarily pot your iris so the soil treatment has more time to do its job.

Note that your iris may not bloom until the following year due to the pH changes if you plant it in freshly treated soil.

Prevention

Ensure no traces of bacteria are left after cutting away any visible signs.

This can easily be achieved by doing the following:

  • Dip the plant in a solution of 1 part bleach to 9 parts water (you can give the rhizome a sponge bath using this solution if working on-site).
  • Treat the soil area by sprinkling a chlorine-based cleaning powder (Comet works well) to raise the soil pH to a strong alkaline 12.5.
  • If the temperature is cooler than 80° degrees Fahrenheit, you can use garden sulphur to drop the soil pH to an acidic solid 5.2.

To help avoid harming your iris, it’s best to do this at the same time you’re giving it its bleach bath.

  • Allow the rhizome a couple of days to air dry before replanting.
  • If you treated the soil area, consider flushing the soil to help restore the pH to a more hospitable level right before replanting.

You can also choose to pot your iris temporarily, so the soil treatment has more time to do its job.

Note that your iris may not bloom until the following year due to the pH changes if you plant it in freshly treated soil.

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