For your description what comes to my mind are .....Spider mites ! I hate those, they suck the life out of the plant and if not treated it just withers and withers.

Check closely the webs during the hottest hours of the day under direct sunlight, you can see the little critters moving around the web.

Raid and most commercial insecticides don´t work on spider mites, you need an insecticide that contains permetrin and treat accordingly ( spraying once or twice a week for three or four weeks ) to get rid of them.

Hot weather and dry conditions ( I don´t mean not enough watering ) are what those critters love. Those and slugs ( when it begins to rain and rain and rain for days ) are the ones I have to deal the most during dahlia season.

Russ,

Do you have any pics of the "moss"? Webbing on the underside of leaves from spider mites I've seen more than I'd like to but not moss. It sounds like you suspect something may be growing underneath the patches of moss, ready to break out when it matures!

Here are three pictures of the affected plant. It probably is spider mites, but I'm posting just to be sure. I cannot get a picture of the webs themselves, they're too fine to show up.

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The picture links to a set of three pictures: the entire plant, the front and back of the leaf.

Cheers,
Russ

Russ,
That looks all too familiar like spider mites. I hope you've founds something to spray with by now but just in case you haven't...

I've used everything from acephate (ortho product) to pyrethrins (Garden Safe product). I found that it's not what you use it's how effectively you apply it. I've only been able to eradicate the problem in the plants that I successfully sprayed the UNDERSIDE of EVERY single leaf on the plant. Not kidding you. EVERY SINGLE leaf. They are supposedly blown around by the wind so spread pretty quickly if you have windy days. I'd definitely move it away from your other plants. Good luck and I hope for the best for your plant! They are a nightmare!

I would pick off and throw away (in the trash - not on the ground near your plants) those leaves most severely affected. Then you can use a watering wand and a good hard stream of water daily (hit all the leaves thoroughly, both tops and bottoms) for a few days to wash them off and give the plant a new start. Most plants will recover fairly quickly. I have the most trouble with these stupid little things on my potted roses that I bring inside over the winter. They love the dryness, as someone else said.

Oh No, I have it too. Have been reading about it all morning. They say that some insceticides even spread it. Has anyone found something for the spider mites that works?

Oh yes ! that´s how spider mite damage looks like. Like explained, permetrin or pirethrin based insecticide gets rid of the problem if applied correctly and frequently, once or twice a week for the first two weeks and then weekly for the next three or four weeks.

Insecticides do not spread it Pdshop, they simply don´t kill them, they seem impervious to anything you throw at them.

Down here it´s the never ending story with dahlias, since they begin to grow as early as mid march ( when it´s hot and dry ) then your plants get spider mites, then it begins to rain, it gets warm and moist so slugs and snail come out and if it´s not slugs and snails then it´s powdery mildew ( too much moisture ), oh well, if it wasn´t that way then it wouldn´t be fun to grow them.