Wow, thanks for the info. Any idea how it could've gotten the mold or mildew? I live in such a dry area, with nearly no humidity or rain and I always try to water at the base. It also seems to have appeared almost over night looking like this!

I do have neem oil on hand so I'm going to aplly that right now but will look into other solutions tomorrow. So sad, as I just adored that plant.

Thanks again for your help.

I suspect that the spores can become wind-borne and can travel via insects, often spread from nearby infected plants of different species. The local University extension office would most likely be able to identify your specific type of fungus in your region, and suggest the best treatment now that it is full-blown for you.

The disease spread is so advanced that I will suggest that you either deploy a chemical control such as a product containing myclobutanil [controls leaf blights & powdery mildew] or rouge the plant. The longer it remains in this state, the greater the spread and even greater the likelihood of it infesting your soil to come back later to infect other susceptible dahlias. Do not compost diseased plant parts. Instead, bag the infected material and dispose of it in a plastic of paper bag. A preventative biological fungicide such as Serenade Garden Fungicide would serve you well, but, because it's a preventative it must be applied prior to an outbreak.

spider mites are another possibility. Looks like webbing between the leaves on the first photo. They like hot and dry conditions.

I see the webbing too. It could be a two fold attack by grey mold and the mites. Any rate, I would yank that plant and dispose like Gary had suggested. Dahlias are too cheap to let something like that continue in your garden. The chance for contamination of surrounding plants is great with either ailment.

One thing about mites that I have discovered over the years is that if I fertilize using a complete fertilizer [meaning, one that contains both the macro and micro nutrients] the issue with mites can be mitigated. So, it is my practice to take the time to amend the soil, add the proper nutrients and keep up on the maintenance through sanitation [litter removal] and watering [moist on the dry-side.] As I pass various landscapes in my area and see first-hand the distinction between the well maintained planting versus the neglected, its obvious to me that a passive approach to one's gardening is tantamount to all manner of problems as the season progresses.

The spider mites make sense! I should have mentioned in my original post that when I first noticed a problem with this plant, I also noticed tons of webbing. Considering we don't have very many spiders near our house, I was surprised and creeped out by all the webbing. The webbing seemed to happen overnight, which I thought was even more bizarre.

Spider mites are pure evil. Just saying.

Are you seeing any positive effect after spraying the Neem oil?

This is a serious infection! I can see the webbing on the first image, now that it was pointed out. I've never seen it this bad.

Here's a post from a dude on a 'roll Em up' site about growing pot, (amazing what dahlia growing tips you can get on those sites!) which has very interesting commentary on fighting mites with Neem Oil:

"so here are a couple things about neem oil... i use it and swear by it so ill give you my personal experiences with it...first i've used neem oil under HO satellite IV floros on a 24/0 crop with NO problems.. i didnt have any burn from the water or the oil magnifying light... no ill side affects at all.. so if you are using floros i would venture to say you'll be fine spraying during a 24/0 light schedule... just move the lights when you spray so you dont spray the lights themselves... make sure you get under the leaves and the "inside" of the plant just as well as the topside of the leafs and "outside" of the plant... like was said those little bastards live under the leaves so take time to spray well...

secondly neem oil can be root fed to your plants as well.. during the entire time i have an infestation i feed neem to my plants... ill explain what neem oil does and why root feeding is good in the next paragraph.. but i feed each plant about 6 ml per gallon of water every other feed.. you can just incorporate the neem into your regular feeding schedule... only do this if its soil though... i wouldnt recommend it for hydro...

here is how neem oil works.. neem oil is NOT a spot killer.. you wont spray your plants today and be gone of mite tomorrow... expect about a 2 week battle... neem oil works a couple ways.. first if you can spray the eggs well it covers them and suffocates them... no more eggs.. when the adults eat the neem oil it messes with their hormones... it makes it so they cant reproduce and lay eggs.. immature pests that eat the neem oil dont reach adult hood and are never able to reproduce... thats why spraying the plant well is so important.. you want to make sure the mites have no choice BUT to eat the neem oil... root feeding work very similar in that the pests hormones get ***** up when they suck the juices from your plant, but it also deters them from sucking on your plant because the neem oil tastes horrible... it also contains some trace minerals so its actually good to feed your plants,, i personally will root feed my plants every other time during and infestation, and spray them every 2-3 days.. the spray i use is 3ml neem oil per 1L of water with a few drops of biodegradable liquid dish soap... usually following this over a 2 week period.. then just go down to spraying once a week as a precaution... ill do this up to the 4th weeks of flowering.. havent ran into mold problems yet, or had neem flavored buds, or any other side affects due to the neem oil... its always worked for me.. like i said.. it just takes about a week before you see progress, 2 weeks before the infestation is gone... but its a natural way to get rid of them... if you want an immediate solution get some NO PEST STRIPS from walmart, lowes, home depot, ect.. they will work in a few hours and kill both mites and eggs... but it is a chemical strip (not a spray)... personally i dont want to think that what im inhaling was treated with a chemical... NO PEST strips work tho.. just make sure you arent in the area because they arent anything you want to breath in... also AVID and FLORAMITE are instant working sprays, but again they are chemical... chemical = instant, but it is a chemical you are using on your plant... natural = longer time fighting the pests, but its not chemical.. just depends on you...

also.. a couple smaller ways of helping slow down their life cycle and battle them is;
1) keep humidity up... they hate humidity... spraying with cold water will help by raising the humidity, spraying them off the leaf surface, and they dont like cold.. which brings me to number two..
2) drop the temps to between 65 and 70 if you can... it can take 10 days for mites to reach maturity in cooler humid temps, but as little at 2 days in warmer less humid places... each female can lay 100 eggs a day... they are 75% females anyways... in the right climate they can very very very quickly run your garden over... so until you decide how you are gonna deal with them and they are dealt with making the enviroment as unhospitible to them as you can will help slow their growth..."

simpsonsampson420, May 13, 2009