Dahlia disease

Asked September 12, 2017, 11:49 AM EDT

A number of my dahlias have some sickness or disease this year. This has not afflicted my garden ever before. What is it and how can I get rid of it? Thank you.

Lane County Oregon diagnosis of plant problems horticulture flowers: annuals and herbaceous perennials

3 Responses

Thanks for your question about these very peculiar-looking Dahlia leaves! Unfortunately the resolution of the photos is not high enough for me to see what is actually going on, so I will ask a few questions. If you have a hand lens, please use it to examine the leaves closely as you answer these questions.
Are the dark spots on the leaves removable, or are they discoloration of the leaf tissue itself?
Is there any fuzzy or cottony texture in the white parts?
Is there any fine webbing visible, especially on the underside of the leaves?
And most important, do you see anything moving when you examine the leaves closely - tiny black, white, or red insects?
Also, how are the plants blooming? I see that the new growth appears unaffected - is that the case on all of them, or is it spreading to the new growth too?

My phone timed out as I was posting a reply, so please excuse me if this is a duplicate response.

The brown spots are on the underside of the leaves and on the stems. The spots on the leaves can be rubbed off (much to my surprise). The leaves feel brittle. I thought when this first started that it was smoke and heat related. I began watering more often during the hottest period.

I don't see any white spots, mildew, webbing, or insects.

None of my plants are doing well this year. They are usually prolific bloomers, but blooming has come late this year with fewer blooms altogether.

The plants affected still have green, living new growth at the top, but the brown spots seem to be creeping upwards and will take over the whole plant.

Thanks for the additional information. I think there may be several things going on. First, this has been a tough year for many plants - first we had an exceptionally wet, chilly spring, which is hard on Dahlias. The rain also may have leached more nutrients than usual out of the soils, leaving them depleted. Then we segued right into a very hot, dry summer. So your plants may well have been stressed to begin with, making them more susceptible to whatever has attacked them.
I will make some suggestions, but I think you need to take some leaves into your local extension office so they can be examined in person, with a strong lens. It is located at 996 Jefferson St,, Eugene.
The little black spots, coupled with the pale color of the leaves, suggest two very different possibilities. Thrips are very tiny insects, which you might not see, and an extremely bad infestation could do the sort of damage you have. In this case the black spots are their droppings. Since you say there is no mildew, this is my first guess - the second would be a very bad case of powdery mildew, in which case the spots would be the fruiting bodies of the fungus. A definite diagnosis will require the in-person examination.
However, regardless of what has caused the damage, you have few courses of action at this point. I recommend you remove and dispose of all dead, damaged, or infested foliage, and also all flower buds. Leave only foliage that is healthy and green, and keep a close eye on it for any spread of the pest. Once you know what it is, you may be able to treat it if it recurs, but at this point there is nothing you can do for the damaged parts. What you want to do is leave some healthy foliage for the plants to regrow a bit, and nourish the tubers for next year. (That is why I suggest removing the flower buds, so the plant can put its energy into recovering strength).
Or, if the Dahlias are not particularly valuable to you, cut everything back, get rid of them, refresh the area with some compost, and plant some nice Chrysanthemums (Dendranthemums) or Asters for the fall... sometimes its better to just cut your losses rather that try to fight a pest!