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Blackspot on Roses
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Posted by
jessaka 7b (
My Page) on
Fri, Apr 6, 12 at 11:25
| Hi, I live in east Oklahoma where it is really humid, and my B.R. Cant roses have developed blackspot. So far I have picked off the bad leaves, even off the ground as much as possible. I have sprayed them with 1 T. distilled vinegar, 1/4 plus 1/8 t. baking soda, and i think 1/4 t. veg. oil in 32 oz container. I have done this twice now and the leaves are still getting it. If there is something better please let me know. Nothing that will kill praying manthis' though. And if this will work, how often do I need to use it? |
Follow-Up Postings:
RE: Blackspot on Roses
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| What you are applying is known as Cornell Mixture. The original formula, which has been tested and found somewhat effective, is 1 gal. water
2 TB horticultural spray oil
1 rounded TB baking soda So your recipe is too weak. For a quart you would use 1-1/2 tsp oil and 1 tsp baking soda. Hort oil would distribute better than veg. oil, but some people use a bit of soap and warm water along with frequent agitation. If you use the correct mixture, you would need to apply it thoroughly once a week except in very hot weather, when it might burn foliage (and when blackspot is less active). Like other organic remedies, it does not provide perfect control, but maybe good enough. See the FAQ of the Organic Roses Forum for more information. Years ago, I wrote the entry there on sulfur as an organic fungicide. Today I would recommend 2 TB of micronized sulfur rather than 3 TB of "wettable" sulfur. But I confess I've backslid into using synthetic fungicide because it doesn't need to be applied so often. |
RE: Blackspot on Roses
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Keep in mind, if the organic method worked, everyone would be using it.
My advice is to purchased only disease resistant varieties. Although few hybrid teas fall in that catagory, The shrub rose varieties that do, repeat well. I grow 400 roses, mostly disease resistant shrubs. I spray neither fungicides nor insecticides and haven't for 10 years.
I'm not a bit disappointed in my garden.
As a bonus, I have butterflies and birds galore, with lots of predatory insects that keep the bad guys at bay.
I must admit a severe stroke in 2002 forced my hand and made spraying extremely difficult but I'm glad now that I don't spray. I can truly say growing roses can be easy, especially if one chooses disease resistant hardy varieties. |
RE: Blackspot on Roses
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| Thanks so much. So it is only somewhat effective. Do I need to go by something else that is effective? What would work if this doesn't? Just that I have a praying mantis egg sack in my roses and don't want to poison it. I could move it. |
RE: Blackspot on Roses
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| If you want to use a fungicide, I recommend Bayer Advanced Garden Disease Control for Roses, Flowers, and Shrubs --I'm never sure if I have that name right or not, but it is close. Should be available at Lowe's or online. And don't confuse the fungicide with one of Bayer's "combo" products--avoid the combos. Apply every 2 weeks during BS season. Kate |
RE: Blackspot on Roses
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| I think Michaelg missed the fact that you are also adding vinegar to your mix of baking soda and oil. Don't add the vinegar. It isn't part of what is usually called the Cornell Formula and it's addition makes no sense (vinegar reacts with baking soda, eliminating any positive effect that either vinegar or baking soda might have, and forming sodium acetate, which does nothing at all for blackspot). |
RE: Blackspot on Roses
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| I agree with mike rivers. One has to wonder how this how this Cornell "modification" got started and how it keeps coming back. If your child is interested in having a volcano science demonstration mix baking soda with vinegar (behind a safetly shield and with safety glasses). http://www.ehow.com/way_5393104_vinegar-baking-soda-science-experiment
s.html |
Here is a link that might be useful: what happens when baking soda and vinegar mix
RE: Blackspot on Roses
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| I use the Bayer product recommended by Kate, and it is very effective at 2-week intervals. However, for growing tea roses in East Texas, the Cornell mixture or micronized sulfur may well be satisfactory. These are preventative only and will not disable infections that have already started. You would continue to notice new spots for a few weeks. Field Roebuck, a garden writer of some distinction, used Cornell mixture for decades in the DFW area. (Admittedly, your disease pressure in E. TX would be somewhat higher.) I don't think the Bayer product would affect beneficial insects, and it will clear up your problem quicker. However, there are possible health and environmental concerns with synthetic fungicides, particularly for women who might become pregnant. Right, no vinegar in the Cornell mixture. |
RE: Blackspot on Roses
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| No vinegar? Where did I come up with the idea of using it? I read it somewhere. Nothing happened when I mixed vinegar/baking soda. I am lucky. Thanks for all the dded info. |
RE: Blackspot on Roses
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| I have been using Serenade and have had great results. Buy the two and one-half gallon size and beside last longer you will save a lot of money. Serenade ASO is its commercial name. You have to do a net search--and it is hard to find the large size--- but DO NOT--- buy it on Amazon you will pay many dollars more than you have to as the shipping is more than twice what other sellers charge. I ususally type in Serenade 2.5 gallons, or Serenade ASO. Windcrest Farms is the cheapest place I have found recently but you have to call them, they will not do a electronic sale for something that large. Pick the leaves absolutely as if it rains, the splash from the rain hitting the fallen leaves will spread it farther. I vacuum most of the dead leaves and make sure I soak the ground when I apply it. |
RE: Blackspot on Roses
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- Posted by seil z6b MI (My Page) on
Sat, Apr 7, 12 at 8:42
| You have to remember that even though you have sprayed them other leaves were already infected but not showing symptoms yet so there will continue to be some leaves spotting and yellowing for some time afterwards. It's probably working to protect any leaves that were not already infected but the ones that already were it can't "cure". |
RE: Blackspot on Roses
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| Sorry for mixing up E. Texas and E. Oklahoma. I should have reread the original post. But the point stands: disease pressure in these areas is bad, but not the worst in the country. |
RE: Blackspot on Roses
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| Title: "Resistance of Shrub and Groundcover Roses to Black Spot and Cercospora Leaf Spot, and Impact of Fungicide Inputs on the Severity of both Diseases" Authors: A.K. Hagan , M.E. Rivas-Davila , J.R. Akridge , and J.W. Olive Authors affiliation: Department of Entomology and Plant Pathology Alabama Agricultural Experiment Station, Auburn University, AL |
Here is a link that might be useful: blackspot research report
RE: Blackspot on Roses
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| I've read lately that you don't need to pick up the infected leaves from the ground because they can no longer transmit the disease. I have blackspot for the first time this spring but I don't spray and think it will go away when the weather is warmer and drier. I try to take the leaves off the plant because they're unsightly but just leave them under the plant as mulch. Getting rid of varieties that are blackspot magnets would be my first choice. I don't know if you can grow the old tea roses (not the hybrid teas), they have the least blackspot of all. I wouldn't consider using anything toxic since I have so much wildlife in my garden. Having the right roses is to me the first line of defense. Good luck! |
RE: Blackspot on Roses
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| I think it's just unreasonable to suggest that people should gather all the fallen rose leaves. Those who have enough available time to do so are likely to be too old and stiff to be willing to do it. Picking spotted leaves off the plant is probably worth doing, if you feel like doing it. Fallen leaves are known to play a role in transmitting cercospora spot. It's not clear whether they play a significant role in black spot. Adding fresh mulch would help to suppress any contribution by fallen leaves. |
RE: Blackspot on Roses
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| It was once thought that BS in the ground was further spread by water splashing up on the leaves. I can't remember the study that disproved that - it was 5 years ago, at least. Maybe it was even something that Henry referenced. Whichever, it seemed absolutely conclusive that BS was not transmitted from ground contact - even from splashing leaves. Airborne spores seems to be the only method. (That's certainly bad enough!) |
RE: Blackspot on Roses
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| These posts are so interesting. Thanks. If blackspot is airborn couldn't the dropped leaves have spores that are airborn? |
RE: Blackspot on Roses
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| "These posts are so interesting. Thanks. If blackspot is airborn couldn't the dropped leaves have spores that are airborn?" Excellent question - the kind that many studies seem to ignore in their conclusions. Perhaps keeping the beds as clean as you can might be the prudent thing to do, anyway? |
RE: Blackspot on Roses
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| Blackspot, like many fungi, has alternate life cycles. The minor or rare one involves airborne spores released by fallen leaves. The main one involves short-range transmission of spores by splashing water (or the gardener's hands). If there are no infected leaves on the plant, these water-borne spores are still produced by lesions on the canes, which is the main way that blackspot carries over the winter. (Source: Horst, Compendium of Rose Diseases) |
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