Irises forum→rhizome rot

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Nov 2, 2017 5:37 PM CST
I planted a trial garden of irises to see how they would do. Got a good raised bed and made sure tubers were really shallow, even showing above the surface and used no mulch. They were looking great even through all the rains of Harvey but now I have lost two very health plants. Based on what I read it is a classic case of Pectobacterium attack. I understand all the pro active steps but I find it hard to believe that there is not some systemic fungicide that can help. Yet there seems to be no mention of same on line. Question, anything at all I can treat with?? I have dug up and disposed of the damaged rhizomes sifting through the dirt to get as much root matter out as I can.


Nov 2, 2017 5:57 PM CST now not sure what I got. the Fans look health, just fell over and the rhizome is mushy abut there is no particular smell.

Probable spoke too soon with my novice diagnosis.

Name: daphne
san diego county, ca (Zone 10a)
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Nov 2, 2017 7:19 PM CST
welcome to the iris forum, ed. sorry i cannot help you. i've only had 2 irises that completely rotted over the 45 years that i've grown them. once was in northern california, and the second time was last year in southern california. both times, the irises were stricken with the smelly rot.

the first two times, i didn't know the irises could be saved with surgery.......i just threw them away, and didn't even treat the soil or surrounding area. however, the third time i discovered a potential rot, it was before the rhizome was rotted. a bug ( i think) had chewed a hole on the top of the rhizome, and it had started to turn black inside, like a tunnel where the bug had been chewing. i took a sharp, sterile knife, cut away the black parts, dusted all the healthy bits with comet (chlorine cleanser) sprinkled some of the cleanser, on the ground replanted the iris and cut off bits. all the pieces grew back. that was jan. 2017, and it was in the middle of blooming. the same plant just bloomed a few weeks ago.

you might want to look at some of the old posts in "goners 2017" you might get some clues from others, plus they have also posted how they treat for rot. i don't use too many chemicals, but will use common household stuff such as chlorine, garden sulfur, lime, organic diatomaceous earth, and hydrogen peroxide. as far as insecticides, pesticides, and herbicides i use eucalyptus soap, alcohol, 'sluggo', neem oil, pheromonal traps, and only occasionally a product called "safer spray"and 'preen'.

it would help to know what growing zone you're in, it might make a difference with the advice others might give you. different areas of the country have specific/different growing conditions....such as hot and humid, cold and wet, etc.
Name: Tom
Southern Wisconsin (Zone 5b)
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Nov 3, 2017 3:42 AM CST
Rot can be caused by fungus, or bacteria. I "think" the rot from fungus is the smelly one, and the one from bacteria isn't. Sometimes like Daphne mentioned, you can cut out the soft parts of the rhizome with a spoon and treat it with something like diluted bleach water, or Comet cleanser with bleach if most of the rhizome is firm and healthy it may send out new increases and stay alive. I also think that some cultivars are more prone to rot then others. Welcome to the forum by the way, I'm sure some others will have additional information for you. Smiling
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Name: Bonnie Sojourner
Harris Brake Lake, Arkansas (Zone 7a)
Magnolia zone
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Nov 3, 2017 5:00 AM CST
If you do surgery on a rhizome allow it to callous over before replanting it. If you do surgery without digging it up do not put soil over the freshly cut part until it calluses over. And welcome to the iris forum. Big Grin You will get more 'rot' info, or will be directed to the relevant threads to read, if you stick around a bit.
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Name: Arlyn
Whiteside County, Illinois (Zone 5a)
Irises Beekeeper Region: Illinois Celebrating Gardening: 2015
Nov 3, 2017 6:49 AM CST

You may get some diagnostic info , here.

My "guess" is that you have the bacterial form of rot ("soft rot"), as the fungal types are usually "cold weather" rots. You have had some good advice . already, on treatments, so I won't add anything, other than to make sure your soil fertility, and ph are as close to "correct" as possible.
South central PA (Zone 6a)
Irises Region: Pennsylvania
Nov 5, 2017 12:30 PM CST
Bonnie is right, as other some others as to the repair and salvage of rhizomes. I cut mine while in the ground, let the sun callous it over, spraying it sometimes with a bleach cleanser like Lysol or some other brand. After calloused you can add soil back over it, but sometimes I let it be. As to the soil where I have had a bad problem with a clump I leave the hole open to the sun over the winter before replanting in that area.
Welcome Ed; hope you enjoy the company and advice by the many who have the iris virus themselves.
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Nov 5, 2017 6:33 PM CST
TomballEd now not sure what I got. the Fans look health, just fell over and the rhizome is mushy abut there is no particular smell.

Probable spoke too soon with my novice diagnosis.


I have had bacterial soft rot in hyacinth bulbs and in iris rhizomes so I know the smell well. It is a very distinct smell, but at least in some plants there can be watery rot that initially doesn't smell. This is well documented in potatoes.

It can be a bit hard to find good info about bacterial soft rots specific to iris, but we need to look no further than Wikipedia to find a pretty good general description about how the infection develops.

"Soft rots are characterized by their distinct maceration of hosts' cell walls with pectolytic enzymes, and subsequent digestion of the intracellular fluid as the bacteria grows. But little is known about the pathogen's interaction with its host at earlier stages when it is still attaching to, and growing within the host with no symptoms present. In fact, the bacteria may develop large populations within a plant before any symptoms can be seen. No one knows exactly why the bacteria have this dormant stage, or what factors influence the bacteria's virulence, but the research is being done."

If there is a symptomless stage where the bacteria spreads and at least in potatoes a stage where the rot doesn't smell even though it is watery, then I do not think it unrealistic that you could end up with a bacterial rot in iris that doesn't exhibit the typical smell until it is quite advanced.

I had quite a lot of rot damage last winter and I noticed soft rot both with horrible odor and some with virtually none. Symptoms looked the same to me. That said I can't be sure that they actually were the same, this is just an assumption that I'm willing to change if I find good info that contradicts it.

Nov 6, 2017 3:28 PM CST
Thanks for all the input,
Name: Ian McBeth
Lincoln, NE (Zone 5b)
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Nov 6, 2017 3:51 PM CST
shizen said: "a bug ( i think) had chewed a hole on the top of the rhizome, and it had started to turn black inside, like a tunnel where the bug had been chewing."

Yeah I had that happened to me, but it was with ants. Ants started to crawl out of some holes of a rhizome of my favorite white iris clump. I eventually forgot about it during the summer. In August of this year, I checked on the rhizome and the iris plant itself were doing just fine. Thank goodness the ants moved out. Big Grin
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