@UndertheSun and anyone else who might be interested - here's a translation (by me, so apologies for any lack of elegance) of an article about preventing and/or treating iris rot. It may not say anything you don't know, but I thought it worth a try.
Bacterial rot in irises
Caused by the bacteria Erwinia carotovora
Symptoms: a plant that is magnificent one day presents a blackening of the leaves (in 24 or 48 hours in the worst case), the leaves come away on pulling them, revealing rot on the rhizome accompanied by a foul odour. If you don’t intervene, in a few days the plant is dead and the disease risks to spread.
Spread of the disease seems to be by 2 methods:
- starting with an attack on the leaves. The rot descends to the rhizome, which is soon transformed into a gluey puree.
- Starting with the rhizome. The disease is harder to identify, because by the time you realise it’s there the future flower bud has been destroyed.
Origin and propagation
The origin of the disease is well known - Erwinia carotovora, a bacteria of the genus enterobacteria, which causes serious damage to crops of carrots (hence the name), potatoes, turnips, beetroot, etc. This bacteria acts by secretion of enzymes, notably lyase pectates, which hydrolyze and degrade
the cellulose tissue causing rhizome cell death.
The bacteria can be present in the soil and enter the rhizome through a wound, via a hole made by larvae (wireworms) etc.
It can also be transmitted by insects at foliage level. Lawrence Ransom considers that fruit flies (which proliferate near fig trees, eating from figs that have dropped to the ground) may be a vector.
That said, there must be conditions favourable to its proliferation: a raised temperature and a humid atmosphere. It has been shown (confirmed by research on cole crops) that the richness of the soil in nitrogen is a contributory factor.
Perhaps there are other factors that affect propagation. No doubt we don’t know everything, because this disease is relatively rare and those affected are not necessarily keen to communicate, especially if they are professionals. We would welcome your experience and you can leave a commentary which we will use to benefit everyone.
Precautions and treatments
Because there is no general and specific treatment, it is important to emphasise prevention:
- choice of soil: old gardens and ancient meadows are full of wireworms. So we should avoid planting iris in these areas, unless the earth has been worked for one or two years beforehand, letting it rest. One can also spread a soil insecticide, but being cautious about the products used in order not to sterilise the soil. Steam treatment of the soil is not within the reach of most amateur gardeners.
- Avoid the use of nitrogen fertilisers. It is better to use slow release organic fertilisers (for example horn fertiliser) or fertilisers rich in phosphate and potassium.
- Do not plant too closely to limit the spread of disease
- Do not bury the rhizome too deeply. Its base should be flush with the ground.
- Avoid unnecessary watering
- Finally, unless harvesting seed after hybridation, cut close to the rhizome stalks that have bloomed and remove weeds that restrict sunlight
- As a preventative treatment, we would recommend Bordeaux mixture, because bacteria do not like copper.
If after all these precautions the disease strikes, as is unfortunately sometimes the case, it is important to act quickly.
Treat to save
It is possible to save affected irises, contrary to what one sometimes reads, at least as regards future growth, if not always the next bloom.
There are different levels of treatment depending on the scale of the problem and the value attached to the plant.
1. Elimination
One plant in a clump is affected. Cut out with a knife the affected rhizome, without qualms, and burn it. Disinfect the soil with a bleach solution (1 coffee spoon per litre) and do not touch it. Inspect the rest of the clump from time to time.
2. Treat the affected iris
You have an iris you have recently bought, it has not had the time to make increases and you really value it (especially when you paid a lot for it). Several methods are possible, from the lightest to the most extreme:
a) leave the iris in the ground, but remove the affected part of the rhizome in such a way as to not leave any trace of rot. Sometimes what is left is very little, but if there are increases, the iris can regrow. This solution is appropriate only where the disease is slight. This ‘cleaning’ is accompanied by a series of interventions more or less ‘heavy’.
o Do nothing and leave the sun (a powerful bactericide) to take charge of eliminating the bacteria.
o Disinfect the wound in the rhizome with a permanganate solution or bleach solution or Dakin’s solution (if like me you don’t know what it is, see
https://global.britannica.com/...). You can also use antiseptic solutions (povidone iodine of the Betadine type, or chlorhexidine of the Hibiscrub type), to which Gram-negative bacteria such as Erwinia are sensitive.
www.betadine.com (link if you haven't come across it before), I think Hibiscrub is quite well known?
b) lift the iris, with the aim of isolating it in a pot in order to watch it more closely and proceed as above.
One can then, after drying, coat the wound with an antibiotic ointment if you have one.
In all cases, it is important to continue to inspect the plant carefully and do not hesitate to repeat the treatment if the disease persists.
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Apologies if the words 'granny' and 'eggs' come to mind... There's also an article on leaf spot prevention/treatment - also in French, but I'll give it a shot if anyone's interested.
