Dahlia foliage problem: whitefly, mites, underwatering or virus?

katrosa731(9b)June 19, 2013

I have 20 dahlias in my Oakland backyard, and more or less all of them are exhibiting issues with their leaves. First pale, grayish, translucent spots form, and then they yellow and brown and eventually the leaf whithers and dies. This tends to start with the bottom/older leaves.

It's incredibly disheartening, as I'm sure you can imagine; my dahlias all grew beautiful foliage and now it seems there's nothing I can do to stop this.

I don't really see any evidence of spider mites. I do have whiteflies in the garden; I spray once a week for them. I haven't seen a great abundance nor any whitefly eggs on the underside of leaves.

Is it possible I'm underwatering? I have a soaker how run the length of my dahlia bed and typically leave it on for 2 hours 1-2 times a week (the flow is very low).

Could it be fungus or virus? I don't do any overhead watering at all, but I do spray neem oil once a week in the early evening.

The stems look very healthy; this issue seems strictly limited to the leaves.

I'm attaching pictures of the leaves. Any tips or suggestions would be helpful!

Many thanks!!

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Here's a photo showing the beginning stages of this.

    Bookmark   June 19, 2013 at 4:18PM
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Hopefully someone more experienced will respond. It certainly doesnt look like insect damage to me. Is it only happening to the bottom leaves, or is it continuing all the way up the plant? My experience is that the bottom leaves pretty much always die off. I just snip them and dont worry about it. However if it is moving up the stem quickly, thats another story.

    Bookmark   June 21, 2013 at 7:26AM
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I agree with Mandolls concerning the lower branches... they tend to deteriorate, and become bug food if you don't snip it off when they are on the way out. That helps airflow, avoiding leaf mold and makes it harder for bugs to 'climb aboard.'

With that said, it is possible there is something else going on in your dahlia bed, but its hard to tell without seeing the plants and what percentage of each plant is effected at any given time. I, too, would defer that kind of diagnosis to a long time grower.

It is certainly likely that you're under watering, especially if you're getting lots of sun. If the plant isn't hydrated well, it is more likely to be effected by virus and other ailments. Personally, I'd soak the soil all the way around the plant at least each week with a hose if you don't get a soaking rain, to ensure even moisture and tuber growth. i have single soakers down each double row, but have occasionally seen dehydration signs in the leaves, so I've been supplementing when needed. I really need to rework my watering system next year, as I don't want to invest more right now.

Through the grapevine, I heard that a long-time grower runs four slow drip lines per double row to ensure enough water, and in the peak summer temps of 95 degrees, supplies two gallons of water per plant per day by running his system 24 hours a day.

    Bookmark   June 22, 2013 at 12:24PM
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Thank you for the feedback!

The damage seems to be on about 25% of the leaves, or less. It does not move up the plant quickly, but there are a few of the middle leaves that are affected.

Based on CCvacation's feedback, it does sound like I'm underwatering. The bed they're planted in gets about 8 hours of sun and is right up against the side of a house which radiates the heat. I've seen the leaves look a little limpy during the peak hours of a hot day, but they perk back up with a bit of supplemental water. But I guess I need to rig up a second soaker hose for them.

Many thanks!

    Bookmark   June 24, 2013 at 2:03PM
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Hi Katrosa

Assuming you mean Oakland CA - my experience may help you.

As you can see, I live in San Francisco (I guess people could think I meant Santa Fe or something!) and this spring/summer has been very sunny - especially for me in the Sunset. I have sandy soil and I know it drains well but I am watering (not heavily but certainly giving each plant a good soaking) at least every other day unless it is foggy. Then I back off to every 3rd day unless the dahlia leaves and buds are drooping.

If I went to twice a week, most of mine would fall over. And I know Oakland is much sunnier than western S.F. So I cast my vote with the underwatering analysis also. Underwatering stresses any plant, which then leaves it vulnerable to any virus in the area. Which is what I think you are getting.

But if your buds are not drooping (which is the first indication of lack of water) then perhaps you aren't underwatering. But I sure know I could NEVER water mine once a week. Even last weekend when it rained I had to water mine on Wed and it actually RAINED Tues night! But again, my soil is amended sand and it drains REAL FAST so it needs more moisture than most gardens I guess.

    Bookmark   June 30, 2013 at 12:58AM
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