Dahlia foliage problem: whitefly, mites, underwatering or virus?
I have 20 dahlias in my Oakland backyard, and more or less all of them are exhibiting issues with their leaves. First pale, grayish, translucent spots form, and then they yellow and brown and eventually the leaf whithers and dies. This tends to start with the bottom/older leaves.
It's incredibly disheartening, as I'm sure you can imagine; my dahlias all grew beautiful foliage and now it seems there's nothing I can do to stop this.
I don't really see any evidence of spider mites. I do have whiteflies in the garden; I spray once a week for them. I haven't seen a great abundance nor any whitefly eggs on the underside of leaves.
Is it possible I'm underwatering? I have a soaker how run the length of my dahlia bed and typically leave it on for 2 hours 1-2 times a week (the flow is very low).
Could it be fungus or virus? I don't do any overhead watering at all, but I do spray neem oil once a week in the early evening.
The stems look very healthy; this issue seems strictly limited to the leaves.
I'm attaching pictures of the leaves. Any tips or suggestions would be helpful!