Hello Terri,

This appears to be a common leaf infection of Iris, simply referred to as Iris Leaf Spot. Susceptibility to this pathogen can vary from cultivar to cultivar, but likely none are immune. As with many plant diseases, severity could vary from year to year based on weather, since often spores spread with rain or more easily infect leaves that remain wet for long periods. You can learn more about this disease in this Virginia Tech publication (PDF document).

There are two actions you can take to reduce disease severity in future years, because once the symptoms appear any measures will have limited impact for the rest of that growing season. First and foremost (and it sounds like you are already doing this), remove all dead iris foliage at the end of the growing season and dispose of it so spores don't overwinter immediately beneath the plants.

Second, you can apply a copper-based fungicide to the foliage, according to label directions for iris (or more broadly for flowering plants or perennials, depending on how specific the label gets). You will probably need to add a "spreader-sticker" type product to the mix so the spray sticks to the leaf better; iris leaves are otherwise fairly water-repellent and would shed most of the spray droplets applied otherwise, rendering a treatment not very effective. Another term for these products are adjuvants, and the fungicide label may mention that an adjuvant is recommended or that a particular product shouldn't have any added because it's already included in the formulation. Spreader-sticker products are sold by garden centers in the same area they stock the pesticides because they are often used together to improve the performance of the pesticide.

Fungicides help to protect uninfected growth from pathogens; they cannot cure existing infections, and leaf lesions already present cannot heal, even if the fungicide is working. This is why they're best applied before a disease manifests, assuming one knows what the disease in question is at that point. Even so, since this fungus produces more than one generation of spores during the growing season, you still might gain a bit of disease suppression this year if you happen to start treating after the spots begin appearing. In future years, now that you know what the ailment is, you can begin treatments when the leaves are still emerging in spring (if that's what the product label instructs). The fungicide label should give more detailed instructions about when to start spraying and how often re-application is recommended.

Some formulations of copper are organic and others are not. We do not know if any of the adjuvant/spreader-sticker options on the market are organic, but for either case you can see if the label says "for organic gardening" or has an OMRI certification. The OMRI organization certifies ingredients suitable for organic production. If you wish to avoid pesticide use entirely, then the annual clean-up of old leaves along with possibly transplanting some of these iris further apart to give clumps more breathing-room should be enough to noticeably reduce infections.

Miri

Thank you so much, this is enormously helpful!

You're welcome!