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spotting on zinnia leaves

9 years ago

Hi all,

I've done a much better job this year thinning out my volunteer zinnias. All thriving, but have noticed a rusty blotchiness on some of the lower sections that has me concerned. Not all areas are thoroughly mulched yet to prevent water splashing. Want to know now if I should remove all the affected leaves and dispose of them.

Your advice will be much appreciated!
Rosie, Sugar Hill, GA

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Comments (3)

zen_man
9 years ago
Hi Rosie,

Without pictures, it is difficult to know the appearance of the "rusty blotchiness on some of the lower sections". If it is a disease, just handling the plants enough to remove the affected leaves could tend to spread it, but removing the affected leaves could help.

I gave up mulching my zinnias several years ago, because the mulch was serving as a breeding ground for several pests, including earwigs, slugs, and pill bugs. When I feel the need to mulch now, I use sand, which does not serve as cover for pests.

Pill bugs can build up to destructive numbers in the right organic mulch environment. Pill bugs can't live in sand because in it they die of dehydration. Their rolling-up behavior is more to prevent dehydration than as a defense against a predator.

Pictures could help. Are you an organic gardener? I'm not, but knowing whether you are or not could help me skip recommending measures that would go against your principles.

ZM
rosiew
Original Author
9 years ago
zen, thanks. I missed seeing this mail - my mailbox is in a ridiculous state.

I'm basically organic, not completely. Use Miracle Gro occasionally.

Was reading about pill bugs the other day - consensus was that while they can harm seedlings, not many problems after. They feed on detritus.

I must mulch. It cuts down on my water usage (and bill) significantly. Largest garden is on a slope, close to 2000 sq. ft., and the mulch helps holding moisture. Use shredded cypress, which doesn't slide down the slope. I'm just north of Atlanta and have super hot summers.

Will keep an eye on the plants, ask for help in posting a pic.

Would like to know what you and others use on powdery mildew also. Noticing some already.

Thanks!!
Rosie
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Hello Pv and also Dee, Actually, purplish-colored spots, such as appear in the big photo above, are fairly common in the various spot diseases that afflict zinnia leaves. Since the spots here do not have white or yellow centers, fungal diseases are eliminated and bacterial leaf spot (Xanthomonas campestris pv. zinniae) is strongly suggested as the causal agent of leaf spot on these zinnias. The symptoms of bacterial leaf spot infection on zinnias are purplish spots that usually remain small but can become angular in shape but do not develop the white or yellow centers typical of fungal infections. The purplish color of the spots in the photo is probably a realistic depiction. Spray zinnia plants weekly with a copper fungicide following periods of leaf wetness to reduce bacterial disease incidence. Copper fungicides are effective against many bacterial plant diseases. ZM
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zen_man
9 years ago
Hi Rosie,

"Would like to know what you and others use on powdery mildew also. Noticing some already."

I suspect your mulch may have something to do with you seeing Powdery Mildew already. But that's just a suspicion of mine. I grow a lot of zinnias and don't consider Powdery Mildew to be a problem. If I see it at all, I don't see Powdery Mildew on my zinnias until late Fall, when they are already rather far along in setting a seed crop. However, in the past I have used a product called Green Cure and found that it is quite effective, provided that you spray it on frequently. Green Cure is primarily just potassium bicarbonate with a proprietary spreader sticker wetting agent to cause it to completely wet the foliage you are spraying it on. Potassium bicarbonate is quite safe for humans -- I have even seen it listed as one of the ingredients in a baking powder.

Its drawback is that it is completely water soluble, and remains so on the plant. So any rain, or even a heavy dew, can and will wash it off. Where heavy dews are common you probably should spray it on early in the morning so it can do some good before it gets washed off during the night by a heavy dew.

Green Cure is supposedly effective against a host of foliage diseases, although my experience has been only with its prevention of Powdery Mildew. Those claims are presumably true, provided that you spray it on frequently enough. I rather suspect that Green Cure also acts in a minor role as a foliar feeding of potassium, which plants need. The sodium in sodium bicarbonate (which organic gardeners frequently recommend) can actually be harmful to a plant, though it is relatively safe for humans, provided they are not on a low-sodium diet.

In my experience organic mulches can promote or even carry foliage diseases. I never add my Fall cleanup dead zinnias to my compost pile. Instead I put them in the trash pickup to be sent to the landfill. And I use compost as a soil amendment, and not as a mulch.

ZM
(not associated with any product or vendor mentioned or linked)

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