Dahlia Pests and Diseases

Stanford, CA(Zone 9b)

Do we have ways to solve these pesky problems? You be we do. And we'll have lots of opinions. Leaf miners, cut worms, mildew......

We can use this sticky to help each other with them.

Thumbnail by doss
Mount Angel, OR(Zone 8a)

My biggest problems are slugs, slugs, and slugs when they are first coming up. Then later Japanese cucumber beetles. Occasionally there can be a problem with mites, these little buggers are microscopic.

Stanford, CA(Zone 9b)

I agree. Slugs and snails make me a huge investor in Sluggo. What do you do about your beetles and mites?

I get powdery mildew at the end of the season on early cultivars. I was using Neem oil but now I find out you can't apply it in hot weather. It did burn the leaves pretty badly. I'd rather use something else. Any ideas?

I do have leaf miners. Another fun thing.

Seattle, WA(Zone 8b)

Diluted milk and water works GREAT for powdery mildew, and doesn;t burn the plants the way some other treatments can.


Stanford, CA(Zone 9b)

Thanks RikerBear. I'll try it.

Seattle, WA(Zone 8b)

Your welcome. :-)

Oklahoma City, OK(Zone 7a)

For spider mites, aphids and leafhoppers, Malathion works very well. For earwigs and cucumber beetles, Sevin is great. Actually, Malathion would probably work there just fine, too.

If you have a problem with Mildew, try any product that contains Daconil.

If you have any kind of wilting diseases, the plant needs to be removed (including the tuber) and thrown away. If you put it in the compost pile, you'll likely just spread the problem as the pathogens are in the soil.

Stanford, CA(Zone 9b)

If you live in an area with rain or you use sprinklers, a stickative is usually very helpful - you'd have to check if you can use it with Daconil. I'm not sure.

Smiln, do you ever use systemics?

Oklahoma City, OK(Zone 7a)

I do not use systemic insecticides. They can be poisonous to people and plants as the chemical causes the plant (or parts of the plant) to become poisonous. Here is some information about them, though.


Merit and Orthene are two examples of systemic insecticides.

The insecticides I use are applied to the exterior of the plant.

I rarely use fungicides, but when I do, I use Immunox. Part of that is because I used to work for the company, though. :)

Stanford, CA(Zone 9b)

Thanks! I meant systemic fungicides. The milk does help and I've used other things but I was hoping to extend my Dahlia season by keeping the powdery mildew at bay ahead of time.

Oklahoma City, OK(Zone 7a)

From what I have read, it's actually best to use a combination of both systemic and contact fungicides. Each fungicide works on a different "class" of fungi, so one type typically won't erradicate all fungus problems. There are over 10 types of fungi that can affect dahlias, so correctly identifying the problem is step one. Then, figuring out which fungicide works best is step two.

Here's a link that has tons of information:


Stanford, CA(Zone 9b)

Thanks, it's a great resource.

Willamette Valley, OR(Zone 8a)


When you say wilting diseases, what do you mean? What kinds of things cause wilting diseases?

I have a real problem with cucumber beetles. Some of my plant's leaves are looking like the photo I attached. Doss suggested it might be Dahlia Mosaic Virus, but I am also thinking Cucumber Mosaic Virus.

What do you all think?

Thumbnail by mgh
Willamette Valley, OR(Zone 8a)

Hmmm....does anyone even look at these threads anymore? ;-)

I found out today that all my dahlias need is some iron. The owner of my favorite garden store said that my dahlias have an iron deficiancy so.....he sold me some food for them.

Just thought I would post this in case someone out there in dahlia land is still looking at these threads! LOL

Everson, WA(Zone 8a)

I am glad you did post it, mgh. It is good information.

I keep threads I am watching marked "watch thread" indefinitely. You would be surprised how many times somebody responds to a thread 6 months or a year later. Usually they haven't looked at the date to see how old the thread is but sometimes they have additional info or a question on the topic.

If no one responds to your post try 'bumping' it up again. Sometimes people miss it or forget to get back to it or the person who could provide you with some answers was away for a few days and never saw it.

I am glad you found out that you could easily fix it and it wasn't a virus that could have spread to your other plants. :>}

Stanford, CA(Zone 9b)

I'm glad that you posted it too. And I'm sure that people read these things. Do you know how many times I've answered the same questions about culture - in addition to many other people - in the Iris forum? This was a very important piece of information. We are just beginning this forum and these stickies will help everyone.


Willamette Valley, OR(Zone 8a)

Thanks, guys. I'm sure people read these too. :-) I know they are a great idea to have instead of answering the same questions over and over again. I think we should try to implement them on every forum page.

Issaquah, WA(Zone 7a)

I'm reading this despite the age of the original thread.

Why is it called a "sticky" as opposed to any other thread here?

Riker Bear! Good to see you again. Hope your gardens are flourishing.

Lenjo, when you mention Japanese cucumber beetle, is that the same thing as the dreaded Japanese Beetle I hear so much about as it ravages the Midwest and Eastern gardens? I hope not!

My biggest early dahlia pests are far and away slugs, then later the occasional earwig or aphid. Crossing my fingers- they have not been too bad this year.

And while I'm packing multiple questions into one post LOL: have any of you had a leaf like this, and what did you do about it, if so? Turns paper thin, brown, no green veining seen. The plant is short and should be tall ( stunted) and otherwise looks normal.

Willamette Valley, OR(Zone 8a)

Poochella, Wecome to Dave's Garden! It's good to have you here!

The thread is marked "sticky" so it will always stay at the top of the forum. That way if anyone has questions about this subject they can look here instead of starting a new thread. If their question isn't answered already, they can add to the thread and keep all the information in one place.

I have had some leaves like the one you posted. I figured it was just sunburning. My plants were not stunted by it though.


Seattle, WA(Zone 8b)

Hey poochella! Nice to see you here at Dave's :-)

Stanford, CA(Zone 9b)

Sometimes first year daylilies don't meet their maximum potential. This could just be a watering problem. Root systems can be very shallow. Do you have your dahlias mulched? If you don't, do it now. Just make sure that the mulch doesn't cover the actual tuber. It looks as if the newer leaves are fine so you should be just OK.

Issaquah, WA(Zone 7a)

Thanks MGH on the "sticky" info- makes good sense to me.

Hi back at yah Riker Bear.
Doss- yikes! yes I mulch with shredded leaves or shredded hay, or compost if I have enough. But I mulch all around the stake for about 18 inches, including right over the tuber/s. Would you recommend putting it further out just to cover the root ends? And why?

I don't know how I'd ever remember where I planted the dang tuber in the first place!

Albany, OR(Zone 8a)

I mulch right over the tuber with mint compost in the fall and the spring. The stems come up right thru the mulch. Of course, this is on top of the soil.
Oh Poo, you can become an addict to dahlias. Just get some more and you will see!

Stanford, CA(Zone 9b)

Daisyruffles is the expert. I'd follow her recommendations. I haven't ever lost a dahlia to mulching either and I've taken to mulching with alfalfa and it works very well. I also plant dahlias in an organic planting mix that everything (except my iris) really love. It's fairly retentive and very rich. What's your soil like?

Albany, OR(Zone 8a)

Oh my goodness! Doss!!!
Don't say I am an expert, lol!
Edited to say, Doss grows the most wonderful dahlias too! Her pics are so beautiful!

It is just how I do my stuff here, hehe! And all my 2 cents advice is worth only that. I am just a mulch fanatic because I need to save money on water and to keep the tubers from freezing in the raised beds. So far, so good.

It sounds like Poo already has a lot of dahlias from the other posts on the Dahlia Forum. That bouquet she posted shows that she has a great start already!

This message was edited Aug 26, 2005 2:02 PM

Malibu, CA

Hydrogen Peroxide diluted with water in a spray bottle really helps with mildew...I use it on my roses as well. As far as beetles...there's a really great thread that talks about organic gardening and purchasing good predators for all the insects discussed in this thread. e.g. Nematodes are great for beetles. You can purchase them at www.bugilogical.com. It beats using pesticides, which ultimately makes your garden more susceptible to pests, because it kills all the good predators along with the bad ones.

Aptos, CA

For powdery mildew I use a new product with is OMRI ( Organic Materials Review Insititute ) certified. It is called "Serenade". Owrks amazingly well! For bugs I use various OMRI products...they haved worked really well at keeping bugs at bay. I used predatory mites to kill harmful mites. These little guys have voracious appetites. You can get them from insectories or look online to order them. Worm compost repells aphids as well...there is also a product you can use called "Aphid Chaser", it utulizes pheremones to repell the little critters!

Perham, MN(Zone 3b)

Zippity, what proportion of hydrogen peroxide to water?

And regarding slugs -- up here in the north land, if you disturb the places where they tend to lay eggs late in the fall, you can sometimes reduce their numbers because the eggs get turfed out and frozen.

Anyone have any comments on Escar-Go? (Might not be the right spelling.) I tend to use what I can get at the local hardware store -- Ortho, generally -- but I've heard that this product can be more effective.


Issaquah, WA(Zone 7a)

Wow, Joanic, I worked in Wadena Co. for a year way, way back when- what a small world.

Escar Go is an iron product just like Sluggo. It constipates the little monsters so they rapidly lose their appetites, curl up and die. I like these products as opposed to the toxics as we have tons of animals here, so they are safe to use liberally. And needed here where the slugs roam in packs.

The only minor drawback is that they may still munch on a leaf or two until enough appetite is gone to make them knock it off for good. Our first dog was poisoned by the neighbors' metaldehyde slug bait, luckily we got her to the vet in time.

Stanford, CA(Zone 9b)

Deadline works better than Sluggo for sure, but it is deadly. Whatever you do don't use a poisonous snail bait in areas where you are going to use bone meal. That will certainly lead to catastrophe.

Most of the discount stores have a generic product that's much cheaper. Also, Garden's Alive has deals all during the year where EscarGo is on sale. I buy it there pretty often. You can join a club and it's 10% off. I don't pay sales tax but do have to pay shipping which is reasonable.

I have been using it religiously for two years now - spent a fortune on it, but my dogs are healthy and the snails are pretty gone. But you do have to keep up with it every two weeks for the first year or so or your hostas will be lace. Now I use it monthly in the summer and more often in the spring when the clematis and dahlias are coming up.

(Zone 7b)

I have a deep dark purple dahlia with white tips, that I just love, but every year, the earwigs munch the white tips off the petals!!! this is so unsightly that I'm thinking of getting rid of it, even though I love the colour so much - help!

Eastlake, OH(Zone 5a)

For powdery mildew, I use Soap Shield by Gardens Alive. For insects, I use Pyola Insectide. Both are organice products, and can be bought at: www.gardensalive.com. Good lucK!! Remember, use the soap shield about three times a week.

Eastlake, OH(Zone 5a)

For powdery mildew, I use a product called Soap Shield. You spray it on the leaves of the plant and the stems also. I start using it when my seedlings get their first set of true leaves. They are sprayed twice a week. It is organic and safe to use. Best time to spray is in the evening. Will not burn, as long as you spray when the sun goes down. You can buy it at www.gardensalive.com Good luck and Keep Gardening!

Hanover, VA

I planted 2 dahlias this year for the first time. I just saw the first of them peeking through the ground, and it looks like something has chewed them off. There are ants everywhere in the area, but I also have a bad earwig problem. Are they destroyed, or will they continue to come up? Also are ants causing the problem or maybe something else?

Willamette Valley, OR(Zone 8a)

It's most likely slugs. The dahlias should come back, but you really should start fighting the slugs.

Hanover, VA

I know what slugs look like, but have never seen one here. Is there a particular place or time to look. Also, is there anything I can do in case there are slugs? What kind of treatment do I need?

Olympia, WA(Zone 8b)

I usually go out at about midnight, or in the EARLY AM, like just before sunrise with my trusty salt shaker, to sprinkle ON the slug, in hand. Slugs are a snail without the shell.
Here is a pic, but he has been salted, but will give you an idea. I will go out tonight and get you a better pic. Your slugs color may vary. They come in all colors from black, to camo color, orange, yellow, etc. There are any number of slug "baits" on the market that are usually pretty effective.

Thumbnail by bluelytes
Willamette Valley, OR(Zone 8a)

As bluelytes stated, the best time to see them doing the damage is at night or early morning. I have heard of people putting a shallow dish of beer around their plants. The slugs are attracted to the beer and crawl in and die. I like to use sluggo or something similar that is safe to use around pets. I have also used Deadline and gotten GREAT results, but it washes away in the rain.....and we get LOTS of rain. ;o)

Issaquah, WA(Zone 7a)

Deadline, Corry's or anything with metaldehyde should not be used if you have pets or neighboring pets who visit your yard. Stick with Sluggo- a safe iron compound that constipates the little @#%$*s to death in a day or two.

We have rampant slug populations out here (although kind of a mysterious drop in population noticed around the NW this season) and the best advice is to use bait early and often to reduce their population. I squish them with a nail, pruning shears or stick as I work in the garden. You can often find them huddled under low dense foliage until dusk or shortly after dawn, under rocks or logs adjacent to their feeding grounds.

A squirt of ammonia water will kill them too, like salt. Don't know if it's as messy though! Slug eggs are pearly off-white to golden round beads ~1/8 inch wide, kill those too if you find them. The less the merrier.

Albany, OR(Zone 8a)

Sluggo is the best slug bait, I think. It works great even in the rain and is safe for the pets.

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