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Cornmeal and blackspot

Holly Webster(7bNC)
July 11, 2016

I decided to post this here and not risk setting off a powder keg over in the general rose forum. Recently, a newbie posted asking about bs management and a terrible battle broke out. I felt so bad for her, plus I don't think she ever had her concerns addressed.

Anyway, I have had a tough couple of years due to medical issues, and last yeat my roses were without much care. We have a lot of bs pressure here, even in resistant varieties. Every rose in my garden has at least a few bs leaves; some had naked canes until 3' up. This year, I was able to work more in them and was aghast to see new spring growth quickly show bs & infect the new roses. I did not know bs could winter over. We had a really mild winter, too, and I am thinking the fungus wasn't effected at all.

I will try almost any non big chem remedy that seems safe and doable. I began spraying with a buttermilk solution, and it did help. I put fresh mulch, 3"-4"in the entire bed. I did some more research and decided to try cornmeal. I sprinkled a handful all around the splash area of each rose and then dusted each rose with another handful of cornmeal. That was only 2 days ago, and I really believe the bs situation is improved! I have not used a good empirical method, so I can't report the results in a meaningful way, but just looking at my roses I see less bs and shinier leaves. Sprinkling the cornmeal took very little time and was quite inexpensive. NC grows a lot of corn for cornmeal, so I was able to support local producers. For those who don't use cornmeal in the kitchen, this is plain finely ground corn. Not cornmeal baking mix and not grits. In my area, superfine grind is what people like, so my cornmeal is almost as fine as white flour. It was super easy to sprinkle over the bush and its fineness helped it to cling to the leaves. This cornmeal did not make a mess when rain hit it.

This might just be anecdotal info, but I encourage others to give cornmeal a try. There are different strains of bs fungus and maybe the bs in your area will respond the way mine did.

Happy rose gardening!

Comments (10)

  • strawchicago

    Hi Holly: thank you for the confirmation that dusting with corn-meal works. The pH of corn meal is very acidic, below 4. Pathogenic fungi like blackspots don't like it too acidic like corn meal, or too alkaline like baking soda (pH 8.3 as in sprayed solution).

    I tested dusting with corn meal on the lower leaves of my 10 Austin roses, and posted pictures in Rose forum in 2011 ... yes, I got nit-picked by some although I posted tons of clean & healthy pics. as proof.

    I used WHOLE-GRAIN cornmeal in 2011, which encourages Trichoderma, the fungus that competes with blackspots. Refined corn-meal flour (stripped off the fatty germ) didn't work as well as WHOLE-GRAIN corn-meal.

    As of now, I no longer use corn-meal since I find that DIGGING DEEP FOR DRAINAGE solve the blackspots problem permanently (for clay soil). From my experiments with cut-blooms in the vase, Pink Peace' leaves break out in BS if soaked in the vase (with acidic rain-water) for more than 1 day. If drainage isn't fast enough, roots don't like standing acidic-rain-water, and leaves break out in BS.

    Also throwing a few red-lava-rocks for constant potassium, plus pea-gravel for calcium really helped. We just had 2 days & 2 nights of constant rain, it will shoot up to 87 deg. tomorrow, and more rain to come: Base of Comte de Chambord, July 10 pic.

    Base of Evelyn, July 10 pic. in humid, after 2 days & 2 nights of rain. See the rain-drops on the leaves which didn't dry out.

    Holly Webster(7bNC) thanked strawchicago
  • Holly Webster(7bNC)

    Nice to see lovely, healthy foliage. We tend to focus on the blooms, but I also love to see lush, glossy, healthy and abundant foliage. I want my roses to be pleasing to the eye in AND out of bloom.

    I am awaiting the results of my soil analysis. If my potassium is in the lower end, I will probably add some lava rock. It's inexpensive. I also find it ugly : ( However, if it helps my roses, I will get over it.

    Straw, in an older post, did you not remove your lava rock? What happened to lead you to put it back? Or am I misremembering?

  • strawchicago

    Holly: Excellent question, Thanks.

    When I first tested red-lava-rock in 2014, I put WAY-TOO MUCH, which blocked out rain, plus release too much iron, which stained leaves .. leaves got this pinkish/brownish stain.

    In 2015, I removed the pile-up, leaving only 4 pieces per large bush, plus 1/2 cup pea-gravel (for calcium). Then I topped with alfalfa hay for nitrogen & lock in moisture. This approach works only if there's EXCELLENT DRAINAGE below. If the planting hole doesn't drain fast enough, roses will blackspots NO MATTER HOW MUCH NUTRIENTS it gets.

    I spent at least 1 hour digging hole deep and wide, like 2.5 x 2.5 feet deep. The hole should drain a 5 gallon bucket in less than 10 min. I prefer draining in less than 5 min. for grafted-on-Dr.Huey, which likes very well-drained & alkaline.

    Very healthy roses as the result. Posted lots of pics. in my thread, "Roses & plants in heavy-clay". I always date all my pics. Folks only post their best pics. in spring, but it's late-fall that show proof of healthy roses.

    Holly Webster(7bNC) thanked strawchicago
  • Samuel Adirondack NY 4b5a


    I really like the ingredients that you used as mulch( the alfalfa hay the pea gravel and the lava rocks and the corn meal. Those are really good mulch ingredients . For mulch I use compost made from yard waste and shredded leaves and compost and also the alfalfa meal and hopefully I get trichoderma from the compost. I use my compost as mulch that is fully composted in my compost pile so that when I mulch with my compost there is no uncomposted organic material in my mulch. My mulch has a little uncomposted material left like alfalfa and a little bit of shredded leaves that are mostly composted. And the alfalfa meal is mostly composted when I make the compost alfalfa tea in a barrel for two weeks before I put it on.

    I really like your pictures of healthy plants. What you use as mulch is so good for your healthy plants. Straw your mulch is so good. Your mulch is the key for growing disease free plants.

    It is really good of Straw to take the time and show pictures and tell what she used for mulch. This forum is a good forum where people share pictures and tell what they used for mulch.


    I am so thankful for this forum where we learned what to mulch with.

    Straw I have enjoyed having healthy roses and learning about what ingredients to use as mulch in this forum. Thank you Straw for sharing what ingredients that you use for mulch. The mulch is the key.

  • Valrose FL Zone 8b

    Hi Straw, first time I heard of Trichoderma, a fungus that competes with blackspot. Yet another reason not to use fungicides. I am going to be doing some googling tonight to learn more.

    Thanks for posting that info

  • strawchicago

    Hi Val: Trichoderma works against mildew too. 1st year Wise Portia was in 2 hours of sun in late fall (tall house shaded it), plus we get zero sun in Nov. .. I dusted the lower leaves with cornmeal, and that stopped mildew. In second & latter years, no blackspot nor mildew on Wise Portia .. also from the horse manure that raised the pH (stable here put lime in horse manure to deodorize).

    Sam: Gorgeous & very healthy pic. of Mayflower .. the best that I had ever seen of Mayflower. Agree with you on the type of mulch can influence the health. Wood-chips rob soil of nitrogen, plus harden & dry out the soil below. I would rather spend the money on cracked-corn or alfalfa hay, than zero nutrients woodchips.

    I re-post the info. from the other thread when I answered Khalid's question:

    The easiest way to solve BOTH copper and iron deficiencies is to put cracked corn (pH 4) to neutralize the high pH of wood-ash (pH over 11). Cracked corn is dry corn-kernels that's partially broken, so less corn will sprout. I get less than 10 baby-corn sprout, but I kill that easily.


    NPK of corn meal is 1.6 / 0.65 / 0.4 .... that's better than horse manure NPK of 0.44 / 0.17 / 0.35. Whole-grain corn's minerals profile is impressive, with 53% magnesium, 25% iron, 35% phosphorus, 14% potassium, 40% manganese, 37% selenium, 24% copper, and 26% zinc.

    Dusting the lower half of the bush with cracked corn works against mildew/blackspot, plus giving anti-fungal agents of zinc and copper for roots.

    Cracked corn has 25% iron, 24% zinc, and 26% copper. Zinc, copper, and calcium are anti-fungal. Zinc kills fungus best, next is copper (folks use copper-spray as fungicide against rust), and last is calcium. Corn has 37% selenium, that's another anti-fungal agent against pathogenic fungi like blackspot and mildew.

    I always get perfect large & dark-green & shiny leaves with DECAYED cracked corn. It takes at least 1 month for that to break down so it's less acidic, but in hot climate and mixed with wood-ash it will decompose fast. Fresh cracked corn has pH 4, too acidic .... but a month later I dug that up, it's decomposed, and found tons of earthworms. Earthworms love cracked corn buried in the soil, more so than chicken manure.

    I'm going to bury some cracked corn tomorrow in my pH 8 hard-clay, to get the soil ready so when I plant in late September, that will be filled with earthworms.

    Below is Internet-pic. of cracked corn, it's small so it breaks down fast ... quite acidic at pH 4 .. best with alkalinity like wood-ash at pH 11 or my pH 8 clay. Great for dusting on leaves against fungus, and the chunky chips is good as fertilizer for alkaline soil. Birds love to eat the chips.

  • strawchicago

    Re-post the info. I posted in August 31, 2013 with cracked corn on the base of Frederic Mistral hybrid tea:


    Picture taken August 30, 2013, hot temp with 70% humidity.

  • strawchicago

    Re-post the info. I posted in July of 2014 to show how acidic cracked corn is, it's pinkish at pH 4 just like pine-bark. People use pine-bark as mulch with zero nutrients, well, cracked corn is just as acidic, but it has nutrients, plus feed the birds.

    See below from top to bottom, left to right: Blood meal (medium blue), pH around 7.5, compost (just a touch of green, slightly alkaline), corn (very pink) pH around 4, and red-cabbage boiled in distilled water (purple, neutral pH):

    This post was edited by Strawberryhill on Fri, Jul 25, 14 at 8:48

  • strawchicago

    Here's the acid-group. I don't have vinegar sample there, but it's fuchsia pink (gaudy-reddish pink). From Answers.com: "commercial distilled white vinegars contain 5-10% acetic acid and have a pH roughly around 2.40 - 3.40."

    Left column, from top to bottom: Composted willow branches is same color as rain water, pH 6.5. Paper-birch is acidic, pH 4.5. Pine bark is VERY ACIDIC, pH 4. Left of pine bark is rain water (pH 6.5 to 6.8).

    Next column from top to bottom: whole-wheat flour is acidic, pH 5. Organic popcorn pH 4, and Brewer's yeast pH 5.

    This post was edited by Strawberryhill on Fri, Jul 18, 14 at 21:52

  • strawchicago

    Re-post the info. I posted on Sept. 15, 2015, of Tchaikosky which I put tons of cracked corn (pH 4) in the planting hole months in advance, so it's decomposed by the time I planted (that's to neutralize my alkaline clay at pH near 8). The below is 2 1/2 months growth from a gallon-own-root:


    Here's bush-shot of Tchaikosky, some brown-leaves underneath, but zero blackspots. I count 12 buds as of today, Sept. 15, bought from RU end of June. This rose is worth it: nice scent, disease-resistant, and lasts long in the vase. It's down to 4 hours of sun since my house shades it in the fall.

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