ROT AND REMEDIES
by JoAnn Mukherjee
Ah spring! The sun is shining, the birds are chirping, and the minis are blooming their hearts out. All is right with the world.
You see the welcome sight of new lush green leaves in your tall bearded iris beds. So you decide to check up on things and do a little post-winter maintenance. As you draw close, you see some of the leaves are turning yellow and brown and some fans are falling over. You think to yourself, “It’s too early for borers…isn’t it???” You start to remove the hapless leaves that are turning to mush. Oh no! The rhizome is mushy too…and what is that horrible smell! You, my friend, have soft rot.
Soft rot is a disease caused by Erwinia carotovora, a bacterial phytopathogen. Ervinia carotovara (Ec) is widespread in occurrence and different strains affect many vegetable and horticultural crops.
It typically enters the iris rhizome via an injury to the plant. Wounds that provide access for the bacteria can be created by borers, slugs, snails, beetle larvae, wind damage, cultivation, animals or even careless leaf or stalk remove.
Ec secretes multiple enzymes that degrade the cell walls of the iris. This allows the bacteria to spread into the rhizome and leaf bases. In experiments with calla lilies, researchers found that bacterial production peaks at 82.5º F, while enzyme production peaks at 57.2º F. Therefore, the bacteria get a foothold in the plant during warmer, drier parts of the year, but soft rot problems don’t appear until the cooler weather in the spring or fall, when the enzyme production is highest.
Typically, the problems start towards the center of a fan and progress to the outer edges. You may observe the center leaves tuning yellow, then brown, and eventually the whole fan collapses. Often part of the clump will appear healthy and only one or two fans will be affected. In cases where the decay is progressing more slowly and hasn’t infected the lower leaf sheaths, you may initially see wilting of the leaves. The result of the infection is a mushy or slim, foul-smelling rhizome, and usually, rot and decay in the lower leaves.
The Ec bacteria can be spread from plant to plant via water runoff contaminated garden tools, and quite possibly insects. The bacteria can overwinter in soil and on infected plant debris.
There is a lot of inaccurate information on treatment of this problem, even from normally credible sources. I have read that you should remove yellowing leaves promptly to prevent spread of the disease. Wrong. The problem starts in the rhizome, so you have to address that source if you want to stop the spread of the soft rot. I have also read that you should remove and destroy infected plants, that there is no cure. Again, wrong. In most cases, at least a portion of the plant can be saved and the planting area corrected to minimize chances of re-infection.
If the rot is extensive, you should dig up the plant; but if only a portion of a plant is infected, the plant can be treated in place. Since the soil immediately surrounding the infected plant has very high Ec levels, it is best to scrape away and dispose of that as well and replace it with uncontaminated soil. With a sharp knife, Cut off the rotted portion of rhizome and leaves, and put into the trash. Do not add diseased leaves and rhizomes to your compost pile! Make sure you cut back the rhizome to all white healthy tissue.
Do not follow guidance that recommends just scooping out the mushy part with a spoon. Any brownish rotted material remaining has Ec bacteria in it and you really increase our chance of re-infection.
At this point you have several options for sanitizing the rhizome. You can dip the cut rhizome in a 10% or stronger bleach solution or apply the solution to the rhizome with a sponge if you are treating the plant in place. In both cases, leave the rhizome exposed and let it air dry for a couple days before putting soil around it again.
You can also use either a chlorine-based powdered cleanser like Comet or garden sulfur, a.k.a. elemental sulfur, to coat the cut ends and sprinkle around the affected area.
Both the chlorine and the sulfur create an inhospitable environment for Ec but in different ways. The chlorine bleach is a strong base with a pH of 12.6, which makes the treated area alkaline. Sulfur on the other hand lowers the pH and creates an acidic environment; a pH of 5.2 has been shown to be protective against the pathogen. Sulfur should only be used when temperatures are below 80º F. However, don’t use sulfate forms of sulfur, since they will not lower the pH.
Since bearded irises prefer neutral pH soils for growth, don’t over apply whichever treatment you choose. Usually, the replanted rhizome will survive, thought it may not bloom that year.
The best treatment, however, is prevention, and proper cultural practices can dramatically reduce the occurrence of soft rot in your garden.
Plant your rhizomes in sunny areas with good drainage. Make sure the plants are not overcrowded since the bacteria are susceptible to drying and sunlight. Ensure the rhizomes are not planted too deeply and that the bases of the fans have not sunk in the soil. Check your plants in the spring; the freezing and thawing may have pushed dirt up around the fans – if so, just pull it away from the leaves.
The use of fresh manure or excess nitrogen coupled with poor drainage has been shown to contribute to soft rot development. Experiments have also shown increased incidence and virulence of Ec with the application of superphosphate fertilizers. It is best to use composted manure and/or gentler slow feeding fertilizers like Osmocote on your irises.
If your drainage is poor, raise your beds, or at least plant your irises on elevated hills and add one-third sharp sand to your soil. Control iris borer populations since their damage to the rhizomes creates opportunities for secondary infections like soft rot.
When you remove spent bloom stalks from your plants, cut them at an angle close to the ground, don’t break them off at ground level. If they don’t break perfectly you risk creating wounds Ec can use to enter the rhizome. Use scissors for removal of diseased or injured leaves; do not tear them near the soil. Lastly, remove and dispose of your dear iris leaves and plant debris in the fall and spring to reduce pest and disease populations and improve air circulation around the rhizomes.
Careful planting and good garden hygiene should create many enjoyable years of healthy trouble-free irises.