“Soft rot” attacks many plants and tall bearded iris are no exception! It can attack anytime when days are warm and it is much more prevalent in humid climates in the east and Midwest. Soft rot is a disease caused by Erwinia carotovora, a bacterial phytopathogen. Erwinia carotovora is widespread in occurrence and there are many strains.
Iris growers in the west and southwest have to cover their rhizomes to prevent sun scorch. In the snow belt they cover their rhizomes for protection from the cold! In the temperature extremes and moisture in the southeast, this would never work unless you are planting in sand. Being a native and life-long resident of Tennessee, we do things a bit differently.
When is it most likely to strike?
1.) Be alert when the daytime temperature is (say … 80 degrees) and the
relative humidity is (say .. 70%). I have coined a red-neck term I refer to as “humiditure”. When the humiditure exceeds 150 (70% + 80 degrees), be on the alert! Monitor the combination of heat and moisture.
2.) Tall Bearded iris need lots of sunlight to bake the top of the rhizomes and the foliage. Clumps growing too close to each to each other greatly compromises this serious need of the bearded iris plant. Best spring bloom typically follows a miserable, scorching hot summer. Bearded iris are original natives of the scorching Middle East! Planting your irises in shade will reduce their vigor and bloom production.
3.) Air circulation is another requirement for your iris to remain free of this pesky bacterial infection. Poor spacing between iris clumps is an open invitation for big trouble in an iris planting.
4.) Iris rhizomes planted too deeply are the #1 cause of this infection here. In the southeast, the upper half of the rhizome needs to grow exposed to the elements. (One of the best iris growers I ever knew in this area said to plant them to look like “crawdads”!)
5.) Planting on slopes can be deadly. When soil washes up against the back of the rhizome where the foliage attaches OR covers the rhizome, you can expect problems. A rain shower followed by blazing sun with soil smothering the rhizome will cook the leaf bases and rhizomes. You may be clueless until the leaves start collapsing and the stench of soft rot is evident. and encourage soft rot. Excellent drainage is essential!
All soils contain this bacteria. You can sterilize your soil but it is a matter of months before it re-establishes itself in your soil! Be ever watchful as immediate treatment will be the most successful approach.
Diluting the bacteria will also play a giant role in eradicating the infection in your garden. If soft rot is detected, it is imperative to physically remove all infected leaf and root tissues—scraping away the decayed portion of the rhizome and leaving it fully exposed.
Treatment for bacterial infections of patients in medical facilities typically includes immediate fluid infusions. That being said, you can use a very narrow stream using a high-pressure nozzle on a garden hose and “power wash” away the rotten tissue, being careful to not splatter and spread this infection to neighboring plants. Diluting this bacteria sufficiently will result in its instant death.
Whether you scrape or wash away the infected tissue, be certain that only VERY solid tissue remains. Drench with a bacterial fungicide labeled for the bacteria (see paragraph #1) . (I define “drenching” as having coated every leaf and rhizome in the clump. )
I use Consan Triple Action that is now available in most garden centers—2 Tablespoon in a one gallon sprayer. I have used it successfully for the past 15 years. It is a detergent developed for the restaurant industry to wash cutting tables, walls and floors of restaurant kitchens. It smells wonderful (there are no chemicals) and it will not harm you in any way. If you use more than the prescribed amount, you will find your iris plants covered in suds. Why waste this wonderful compound when just a little will do the trick??!!
Too many beautiful new plantings completed in soft, silky soil will often produce beautiful, lush growth. These same iris plants are often decimated by soft rot come spring when the unsettled, fluffy soil has settled around and over your rhizomes, burying your iris plants. These same well grown and gorgeous plants will produce magnificently IF ONLY the gardener will wait until the soil was well settled before planting.
Prepare your planting areas in advance. Have your soil tested!
The late Alan Ensminger of Lincoln, Nebraska advised that you cultivate your new soil in early spring, plow in a generous amount of balanced fertilizer and sow rye or another grain crop. Do NOT let it go to seed, but plow it under as a soil conditioner. You are ready to plan for planting the following spring and summer.
I would suggest that you build your elevated bed at least 12” high because the soil will settle to at least half that height by spring. Extreme fertility can cause lush growth that is an invitation for soft rot. By following Mr. Ensminger's advice, your soil will be well regulated for planting when your new irises arrive.