Yikes, she doesn't look happy! I posted a thread almost identical to yours a few days ago and have been doing some research since. I hope this site works for you:

Here is a link that might be useful: WSU.edu on dahlia diseases

Those are exactly the same effects mine are having due to spider mites. Eventually the entire leaf will go brown and dry as if its been unwatered.

There's a very good article on spider mites on the American Dahlia Society's site:

Coping with Spider Mites

I couldn't manage the regiment described there. I have received a "Bug Blaster" from Lee Valley which I'm going to try. It is simply a high pressure spray attachment for your hose that gives off a 360 degree spray at right angles to the ground, so you can get under and over leaves.

I have already tried spraying with Green Earth's Bio-Mist but it has had very little effect.

The ADS article talks about simply pulling up affected plants and destroying them. I am hoping I can avoid that, but it is looking like it may very well be the most effective means of dealing with them.

FWIW, I am also planning on increasing the water on my Dahlias since they aren't tubers (I don't have to worry as much about rotting tubers) and it may very well alter the spider mites cycles enough to throw them off.

Good luck.

Cheers,
Russ

I had the same thing destroy one of two miniature dahlias. ferocious little beasts. I'm seeing a small amount of damage on a few other plants in an entirely different bed too. I think I am going to try the bug blaster - thanks for the link. So disappointing-this is my first year with dahlias, and they have been looking so pretty until now (no blooms, but otherwise healthy and green).

It doesn't really look like spider mite damage to me, but I've been having more trouble with the little buggers than ever before in the history of my gardening. I've had several roses with heavy infestations. Controlling them is as simple as spraying the tops and bottoms of leaves on affected plants on a daily basis with a forceful spray of water from your hose. Just an ordinary nozzle with provide enough pressure to blow them off the plant. Just spray them very thoroughly - I do a couple of minutes on mine. Since this can in no way harm your plant, you can do it to them just in case it is spider mites.

Excerpt from the Dahlia Society bulletin linked by Sturgeonguy:

"Dahlia growers from 50 or 60 years ago would be astonished to hear our concerns with mites, because these tiny creatures were not a serious problem until after World War II. Interestingly, DDT created the mite problem two ways. First, DDT killed many mite predators. Moreover, DDT vastly increased the fertility of mites -- by greating increasing the number of eggs each female mite would lay and by substantially enhancing the percentage of mite eggs that hatched.

A widely used miticide in the United States today is Sevin, a carbamate (same chemical class as DDT). I have always heard warnings not to use Seven on dahlias because it worsens mite problems. Dr. Ronald Ochoa's story about DDT explains why carbamates cause more problems than they cure for dahlia growers."

An excellent and shocking article on how our use of one pesticide has had the unintended effect of being a fertility drug for at least one insect beastie if not many others...

Thank you all for your input. I'm inclined to agree that it is indeed spider mites, as there is webbing on some of the most damaged areas. Egad. One of my dahlias, a Mrs. Black, is pretty bad so I'll probably yank her. But one of my favorites, Midnight Moon, is also showing early signs. I'll give the water spray a try on this one. But won't spraying the little buggers off the dahlias just transport them to another area of the garden?

-Ray

Ray,

According to the writings I've read, a good forceful water spray should actually kill them, not simply dislodge them. They're ultra-tiny soft-bodied insects...as you spray, keeping imagining the sounds of *MUSH* while you chant; "Take that you little beggers!"...;-]

Cheers,
Russ

LOL!

Thanks for the chuckles, Russ. :)

-Ray