There are a few leaf spot-type diseases that can affect snapdragon, with sometimes very similar-looking symptoms. With this pattern of pale centers and purplish borders, the culprit might be fungal like anthracnose (Colletotrichum), blight (Phyllosticta) or leaf spot (Cercospora), or bacterial (Pseudomonas). Our recent spell of recurring wet weather and occasionally-cooler conditions earlier this month likely aided in the spread of infection. As the weather turns more consistently warm and drier, the rate of spread will hopefully decrease. You can pluck off heavily-infected leaves if you prefer, though that might not be necessary.

If you opt to try using a fungicide, look for a product listing as many of the above pathogens as possible (the scientific name more so than the common name, so Pseudomonas, etc.). Broad-spectrum active ingredients often involve copper, at least when the product can also suppress bacteria, since few other fungicides can. Follow all product directions about application dosage (if buying a concentrate) and when to repeat applications, since multiple treatments would be needed to continue to suppress spore infection throughout the season. Copper sprays, for example, tend to have limits on how many applications can be made per growing season. Bear in mind, though, that fungicides could risk harm to pollinators or other organisms, so do not get the spray on open flowers and avoid making applications during periods of high pollinator activity (morning versus evening, usually).

The damage will probably wane and only be cosmetic more than threatening to the plant's overall health, but that can depend on the pathogen responsible. If lesions of infection wind-up affecting stems and girdling them (killing tissue around enough of the stem circumference), then entire branches may die back. If that happens, removal and replacement of the plant would be more practical than treatment. (Fungicides are preventative measures only to protect healthy growth; they cannot cure existing disease, especially once symptoms manifest. You're right -- rain will definitely interfere with a re-treatment schedule if we wind-up having a rainy summer, potentially rendering a treatment regime not super effective.)

Miri