Thank you for the acorn!
Here's a translation of the leaf-spot article (same as before, apologies for inelegance of translation)
Leaf spot in irises
Leaf spot (French technical term is hétérosporiose; sorry, I don’t know the English equivalent)
This iris disease is in a sense the most spectacular because it is visible - it makes the flower bed look unsightly and gives an impression of a gardener who does not take care of his garden. However, it is a long way short of being the most serious disease. Nevertheless, if not treated it can spread and ultimately hinder the proper growth of the plants.
- The different stages of the disease
The disease first shows itself, often after rain, in the appearance on the foliage of small, dark green spots that quickly turn brown, followed by discoloration of the surrounding area.
This then spreads to the whole leaf. The stains become dark brown and the leaf dries out.
- What is the agent provocateur of this disease?
The cause is a fungus (called ‘lower’ because not bearing any carpophore): hétérosporium iridis. The pustular spots are the fruiting bodies of the fungus, which is preserved in the soil when the leaves fall to the ground and is thrown up on to other plants when it rains or the bed is watered.
This disease, even if not fatal, is nonetheless damaging (in addition to its unaesthetic characteristic), because by destroying the chlorophyll, it reduces the surface of leaves involved in photosynthesis. In short, it can interfere with proper plant growth and reproduction.
- What are the methods of control:
a) They are first, preventative
Plant in clean soil in which no irises have been grown for at least two years.
Avoid watering the leaves. In most areas irises do not need to be watered (except for on planting and in extreme dryness). Collect up and destroy (by burning) all affected leaves.
Apply a fungicide solution in spring and then 3 to 4 times up until autumn (Dithane, Maneve, Zineb) or Bordeaux mixture. Organic gardeners will use Bordeaux mixture.
Important comment: water has a tendency to slide off iris leaves, making the treatment less effective. So add to the solution a ‘wetting agent’. A coffee spoon full of commercial washing-up liquid does the job perfectly.
b) And finally, the methods of control are curative
As soon as the spots appear, you should cut off the leaves below the affected area and burn them. Above all do not leave them on the soil, or the fungus will be preserved.
Some gardeners decide, in order to avoid the displeasing sight of brown leaves, to remove the leaves as a preventative measure after the irises have bloomed. This may not endanger the life of the plant, but it is a method that is to be discouraged because the leaves are indispensible to photosynthesis, which contributes to feeding the rhizome. It is better therefore to wait until the end of the growing season (October, November) before cutting back iris leaves.
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Notes
Carpophore = The stem of the fruiting body in higher fungi (I looked it up on Wiki!)
Bordeaux mixture – used to spray grape vines as fungicide - you probably know it.
I liked the bit about washing up liquid as a wetting agent.
