Saturday, June 15, 2013

Natural Treatments for Black Spot (on Roses)

“It was June, and the world smelled of roses. The sunshine was like powdered gold over the grassy hillside.”
Maud Hart Lovelace, Betsy-Tacy and Tib

One of the first things I did when I moved into this home was purchase several rose bushes to plant in the garden. Now, at the time, I was extremely inexperienced with gardening and had no experience with roses, but just decided to dive right in. The house already had several climbing rose bushes and a few other plants and I figured, what the heck, how hard can it be??

Ha ha ha ha... well, several years later I am still inexperienced but I know enough to know that growing roses can be a bit challenging. In my experience, they prefer morning sun, thrive with plenty of water (soak the ground, not the plant), and do their best in well-drained soil. I formerly selected my roses based on how pretty the blooms looked. But, that transitioned to include fragrance and then finally disease resistance.

Very fragrant, small purple roses after pruning away diseased leaves

Actually, this is a very good point to keep in mind - if you purchase only disease-resistant varieties, you will be able to enjoy your roses more and worry about them less. One solution for black spot that I read about was a "shovel" and the remedy was to get rid of the plant completely and replace with a disease-resistant variety!

If you have had to battle black spot on your rose bushes, you can understand this suggestion. However, if you are like me and grow attached to these little plants, I hope the following information can help keep you from this drastic measure. Also, the Nashville Rose Society offers a great list of disease-resistant roses, if you are interested. And more here at Heirloom

I have listed five natural treatments below and a few other preventative tips. Hope they are helpful to you and your garden.

 The dreaded black spot, out of control on my climbing roses

  • When plants are dormant in spring, spray with sulphur (dry or wet) and a fungicidal soap, which are both available at garden centers or make your own. In order to be effective, the sulphur should be applied before spores land on the leaves and it must be reapplied after it rains. Do not apply when temperatures are above 80-85 degrees and be sure to use a face mask when applying the sulphur in dry form as it can irritate the eyes and lungs. Finally, sulphur will lower the soil pH, so keep in mind that you may want to test your soil after this treatment.
  • Ensure plants are properly spaced (3-4 feet apart) and water only in the mornings.
  • Water around the base of the plants and not the leaves. The fungus prefers a wet, humid environment. Don't water from above.
I love the intricate design of these blooms. After pruning this plant, I had very little left.
It is my most susceptible to black spot, but so pretty that I cannot bring myself to remove it.

Once you are infected
The very first thing you will need to do to treat your roses will be to get out there and prune your infected plants (cut well past the obvious signs of infection and don't let your prunings touch the ground). This is no fun if you have a lot of plants, but a very necessary step.

Black spot is a fungus and you will want to remove it from your garden. Do not compost the diseased leaves as that will simply keep the black spot hanging around your yard - throw them in the trash or burn them.

Another close-up of the dreaded fungi.

For all remedies, if you are using city water, let the water sit for a few hours first to de-chlorinate. Before spraying, remove any discolored leaves and never use a sprayer that has held herbicides, pesiticides, or other chemicals. When spraying be sure to also spray the undersides of leaves as well as the tops.

Remedy #1 - Baking Soda and Water
This is suggested in many various forms on the internet and is a variation, I believe, of the Cornell Formula. It is purported to be effective against powdery mildew as well, although I have not used it for that function myself.

1 quart water
1 tsp baking soda
2 drops liquid soap

Combine everything in a spray bottle. Shake well and spray onto plants in the morning, every three to four days. Before spraying, shake the spray bottle well.

The official Cornell Formula adds a horticultural oil but I have read that it will kill insects too (good and bad insects). I do not have have an insect problem and do not want to kill any beneficial insects in my garden, so I do not add the horticultural oil. Also, I read that sulphur and horticultural oil combined is toxic so if you have applied sulphur to your garden as a preventative, you will need to wait 30 days before using a horticultural oil there.

Finally, if you decide to use a horticultural oil, Mike McGrath recommends a newer, light-weight summer oil. Vegetable oil will work in a pinch, but official horticultural oils work better.

Very fragrant, large lavender blooms. This is a late bloomer, but gets the least morning
sun so that may be why. It was not affected too seriously by black spot this year. This is a Neptune Tea
Rose - I remember because of the unusual name.

Remedy #2 - Manure Tea (Compost Teas)
The great thing about a manure tea is that it fights fungal diseases (blackspot, mildew, and rust) and provides nutrition for the plant.

Basic Manure Tea (Rose Magazine)
Place 1 part well-composted manure in five parts filtered water. Stir the mixture well and let sit in a warm place for three days. Then strain through a cheesecloth or mesh and use the resulting "tea" to spray the affected plants. The solids left behind can be applied around the base of the plants as an added fertilizer.

According to Mike McGrath, the Cornell Formula and various compost teas should keep black spot and other diseases at bay unless you've got over-crowded, wet roses. He suggests rotating his"teas" weekly and always treat in the mornings. You can read the full article here. I highly recommend that you check out Mike's facebook page, which has a lot of really valuable and interesting information related to all sorts of gardening and lawn topics.

Basic Compost Tea (Mike McGrath)
In the morning, add about 1 cup of compost to an old sock or tie in a cheesecloth, let steep in a gallon of filtered water for 24 hours, strain and then spray immediately (a morning spray). Return the contents of your "tea bag" to your compost pile. 

Mike's Fermented Compost Tea
Place your brewed compost tea in a bucket with a mosquito screen covering the top. Set the bucket in a shady spot for two weeks. Scrape off the scum, avoiding the sediment at the bottom, strain it, and spray. Return the scum and the sediment to the compost pile.

Mike's Aerated Compost Tea
Make the basic compost tea, but drop in a few aquarium bubblers to add air during the brewing process. Mike indicates you can purchase a commercial device to add air, like The Soil Soup Machine or Gardens Alive's Compost Tea Kit.

A pretty pink rose. I still need to prune away the damaged parts of this plant.

Remedy #3 - Milk and Water
I love this idea and have read various testamonies regarding its effectiveness. Some stated that using this once a week was all that was needed to to keep black spot from their roses.

2 parts water
1 part milk

Combine in a spray bottle and spray once per week. According to this article, the lactoferrin in milk may be what works against the blackspot. Wikipedia states that lactoferrin is a bacteriocide and fungicide, which would explain why this method works. Should you use pasteurized or raw milk for this? I could find no indication either way and I tried both on my plants. Just considering the nature of raw milk, it is reasonable to expect it to provide additional benefit.

A dark red bloom. This bush did very well this year, despite having major issues with black spot.
Hopefully it will bounce back now that it is pruned and being treated regularly.

Remedy #4 - Essential Oils (Scotch Spearmint & English Thyme)
Probably as a result of my mathematics background, I just love it when I can incorporate a scientific study directly into my gardening hobby. This remedy is based on a study published in the Journal of Medicinally Active Plants dated 1-26-2012 entitled Evaluating Natural Products for Control of Black Spot Disease on Roses. The study tested the effects of several essential oils and hydrolats on black spot.

The results demonstrate that essential oils of English Thyme (Thymus vulgaris L.) and Scotch Spearment (Mentha x gracilis Sole) were promising in the control of black spot on roses and, in fact, may have an ability similar to a fungicide treatment to suppress black spot disease. In The Complete Book of Essential Oils and Aromatherapy by Valerie Ann Worwood, it is suggested to use 4-8 drops of essential oil per gallon of water for use as a garden spray. Shake well in a spray bottle, then, as discussed above, spray the leaves in the morning once per week.

I had a hard time finding these oils online, but did eventually find English Thyme on the Young Living website here - thyme and the Scotch Spearmint here on a chemical website (BSI). Weird! Now Mountain Rose Herbs has only Red Thyme (thymus zygis) and Spearmint (mentha spicata), and it is sort of annoying to me that the experiment uses oils that are difficult to obtain. Ha!

Caught someone having lunch!

Remedy #5 - Baking Soda and Vinegar
The recipe below is based on one found at the Organic ROSE Care website. As stated on the Good Earth Organic ROSE Care website,

"If you remember your high school chemistry, when you mix an acid (vinegar) and a base (baking soda) they cancel each other out and you get water that has a neutral pH. So the question is, how does this formula work to kill mildew?

When baking soda and vinegar are mixed they produce water, carbon dioxide (which bubbles away), and sodium acetate (which stays in solution). I found many references to sodium diacetate as a mold and mildew inhibitor in baked goods and some references to sodium acetate as a fungicide. Apparently, the active ingredient of the baking soda-vinegar solution is the sodium acetate."

I have altered the recipe to exclude the insectisidal soap (or liquid soap), which you could include in the amount of 1 1/2 tbsp. Also, you could include 2 tbsp vegetable oil or horticultural oil, but please read my comments related to these oils in Remedy #1 above.

Mix together 1 1/2 tbsp baking soda (and any of the optional oils) in one gallon of cold water. Gently stir in the vinegar at the end but DO NOT SHAKE, unless you intend to make a volcano. Pour into a spray bottle and spray the entire plant weekly, in the mornings before it has reached 80 degrees F.

There is a remedy using only vinegar that is floating around online as well. However, I have to caution you that vinegar is typically used to kill weeds and/ or grass and this remedy may (probably will) kill your roses, perhaps depending on the concentration used. I do not recommend using a "vinegar-only" solution as a fungicide, but if you need spot weed control in your yard, a spray bottle filled with vinegar applied directly to the offending weed will work very well.

This bush needs major pruning to get it under control and remove all traces of the disease. It is a fairly
good sport, though, and manages to produce lots of pretty pale yellow blooms all summer despite my neglect.

Advice for the Ground/ Base of Plants
The last thing I can recommend based on my research is to sprinkle about 1/4 cup corn meal around the base of each plant. The corn meal is a natural mild fungicide. It is different from corn gluten meal, which can be used to deter weed growth in your lawn, so make sure you are using just a regular corn meal, something like this. Also, horticultural corn meal is the same thing as corn meal, so just purchase whichever is least expensive and easiest to find.

You will also want to switch out your mulch each year to ensure that the new season does not see any carry-over fungus from last year, and with this same message in mind, be sure to pick up and throw away (or burn) any damaged leaves you might find on the ground.

Happy Gardening to you and may your yard be filled with disease-free and happy roses!  

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  1. Thanks for the great ideas to protect my roses. I don't have too many problems but I do get some pests. I am going to try some of these and see what happens.
    I am a new GFC.
    Have a wonderful weekend,
    @ Eclectic Red Barn

    1. I hope some of these ideas will work for you. I think part of my problem is that I was doing nothing because I didn't want to spray chemicals all over my yard. New follower as well!

      Thanks for the comment!

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