I usually control blackspot for the entire season with about 3 or 4 applications of Bayer's tebuconazole. This Spring, I decided to try Bayer's new Natria product and I began spraying it on a weekly basis on May 1. I saw the first signs of blackspot on May 22 and by May 27, blackspot was bad enough that I went back to the tebuconazole. If Natria has any effect at all on rose blackspot, I couldn't detect it. I am quite certain that it doesn't control rose blackspot to an extent that satisfies me.

The OP knows this, but for kibitzers, Serenade and Bayer Natria are similar products based on bacteria with antifungal properties.

You Do NOT,Can NOT simply spritz and spray with the natural Serenade type products the way one would with the toxic synthetic products.
It fails for some because they do not change their application mode.

I have used Serenade for years with total success but I do also remove dead leaves and suspect debris under the rose bush.
I literally liberally soak the ground during application and the rose bush also.
One can use a (two gallon minimum preferably) watering can with a diffuser nozzle although sometime I make a five gallon pail and use one gallon ice cream pails to douse the bush and ground.
I have two new roses this year, one of which appears to have come with black spot.
It did spread to a bush close to it but is now gone for a month and did not spread to any other bushes.

One good extra with Serenade of the Bayer version of said same, is the water used helps water the rose.

Buy it in the bulk 2.5 gallon jugs and you will save tens of dollars.

I've tried both the Green Cure and Serenade. They are only minimally effective and like RpR said, you have to use a lot on each rose and use it frequently for it to be of any use. And don't bother with the preventive ratio, go right to the cure ratio.

RpR, I am interested in your application method. How much blackspot pressure do you have in your area? What's the thought behind drenching the ground as well, instead of just spraying on foliage? How often do you spray? Where do you buy?

I'm wondering if I can just apply Serenade using my ortho dial-n-sprayer. This hose-end sprayer will siphon up the solution, mix the solution with water, and allow me to quickly drench the plants and ground. I grow 100+ roses, have high blackspot pressure, and need something faster than mixing with a watering can.

I noticed that Serenade is made by AgraQuest, and the Bayer Natria label says that it is "powered by AgraQuest". So I compared the ingredients, and they are the same! I guess Bayer OEMed Serenade as Natria. (OEM = original equipment manufacturer)

ACTIVE INGREDIENT:
QST 713 strain of Bacillus subtilis*..............1.34%
OTHER INGREDIENTS................................98.66%
TOTAL...........................................100.00%
*Contains a minimum of 1 x 109 cfu/g

What's the latest thinking on these blackspot control methods: spreading cornmeal around the base of the plant and/or spraying with 1 part milk to 2 parts water.

Do they work?

They do not work.

Kate

What's the latest thinking on these blackspot control methods: spreading cornmeal around the base of the plant and/or spraying with 1 part milk to 2 parts water.
Do they work?

Didn't work 10 years ago, still doesn't work today.

Milk spray is effective against powdery mildew but not against blackspot.

Cornmeal on the soil is helpful against some soil fungi such as crown rot, but that's not a rose problem.

"Everyone loves the idea of using milk to control blackspot on roses, and it's no wonder! There are some nasty chemicals out there. When we tested milk against other home remedies it beat everything else easily. In fact, it was just about as good as the synthetic fungicide Chlorotalonil and the organic fungicide Bordeaux mix. The recipe for using milk is very simple. Just mix one part milk (any type of milk will do -- except maybe chocolate) with two parts water and spray that mixture on your roses once a week. Besides working on blackspot, other researchers have shown that there's a good chance this spray may work on other diseases too."

The author is an Associate Professor of Horticultural Science at the University of Minnesota.

http://www.horticulture.umn.edu/Who_sWho/Faculty/JeffGillman/

Here is a link that might be useful: link for above quote

Gardenerzone4:
As we can, and are having, sub-tropical weather, I would imagine in the worst summers and this is only for a few months, as bad as any area that has high heat and high humidity.
A large public rose garden in my town is often heavily infested with black spot after a few years as most of the help is volunteer and it appears no one wants to go through the bushes removing the black spot, and I have never seen them spray anything of any type probably due to lack of funds( It is a very large garden and has more than just roses but it was created for the roses initially); therefore this year, a LARGE number of roses, at least fifty percent, are new roses.

In my newest garden, it seems to only show up when I put in a new rose.
In the old garden, which is over fifty years old, there seems to be one area that will have minor infestation that once it is killed seems to be gone for the year.
That area is close to grape vines and areas where black spot spore could blow to or splash to and not get killed as I do not flood the entire area though I wish I could.

I have never had case where I could not make it go away or the whole garden was infested.
I have removed all bush type roses from the Hybrid Tea area to an area on the other side of a building and those roses get minimal care and yet have never had a serious infestation.
They will get doused once in the spring and once in the fall and do not get any real special treatment for winter other than a few leaves heaped on top as they are covered with snow from the drive-way so the chance of deep freeze hurting them is small.
They do get thinned either spring or fall, and any branches that show black spot coming , or are one too many, is removed.

I try to check the roses all summer removing any branches and leaves that are not necessary or do the rose no good.
I.e. for air flow and maximizing strong blooms. I will cut off buds that will be weak.
I use a shop vacuum to suck up debris under the roses and now that I have switched to mostly cocoa bean hulls for mulch it is easier to see what should be there and what should not.
When in doubt the mulch gets sucked out and replaced. I do not want any mulch that has black spot, or any other nasty, down there to make life miserable.
This is also why I douse the entire bush and ground.
Those on heavier soil, it will take seconds before the solution soaks away.
I try to keep a bowl, for lack of a better word under each rose, so the solution, whether Serenade of fertilizer, goes where it belongs.
Over the past eight years I have gone from approx. four dozen roses, plus or minus, to less than half that due to mostly fighting miserable warm winters and springs that wreak havoc on covered roses.
Although a winter with days of twenty plus below, or below zero highs, can be deadly if one gets careless.

Spring is worse- like this year.
I buried almost all last winter and they came out of the ground looking great but I lost four due to warm-cold-warm--cold and I could not let them in the ground any longer as they were already sending shoots up out of the ground under the cover.

Even when I had the largest number of roses, I would pick a day, often more than once a summer and spend the literal whole day going through the roses removing leaves and branches. I always try to keep the bottom six to eight inches of the rose clear of leaves although in a summer like this that is easier said than done.

If I failed to do this black spot was more of a problem but still controlled --although the amount of Serenade I was going through is what made me look for the bulk jugs.
I look at the recommended amount and, at least, double it--always.

To apply the way you want to, can be done, but you are going to need a sprayer than can handle a base solution that is as thick as some of the heavy water soluble fish fertilizers.
Anything that can spray those thick fish emulsions can handle Serenade.
I have considered getting a barrel, wood, and using that as that way I can mix up forty to fifty gallons at shot.
I did that this year with a plastic forty gallon barrel for fertilzing. I have had enough trouble with the hose end sprayers that while I have a half-dozen they have not been used for years.
I am not saying they do not work but when I mix it up in five gallon pails, my normal way, I know exactly what I am doing and any snafu is my fault without exception.
Depending on how much time I have or why I am doing it, I most often apply it with an old ice cream pail, or a metal watering can with a diffuser nozzle.
I have several types of watering cans including some smaller ones that I can maneuver with one hand while using the other to maneuver the rose.
Yes I do get stuck and as I am slightly allergic, within five minutes I will find any deeps ones I did not see.

Where do I get it?
Go to the organic rose site and look for a thread about Serenade.
I listed for a women there several places I found on the net.
I cannot find the receipt I bought my last one at so I can not give their name.
DO NOT go through Amazon, their shipping is asininely high.

How often?
Heavily spring and fall, with one in July (I did it just this weak for all roses, usually a weak after I fertilize.
I soak the ground as I want any spores down there dead.

When I spend the time to keep the bottom of the rose clear, any infestations are usually very small and easily seen.
Failure to keep the bottom clear has on occasion been been a nasty experience where I will simply remove all except the three or four strongest and douse those after picking off leaves up to a foot off of the ground.

Beyond that only roses that get it, get doused more often, although if my gut instinct says so, I will do all roses any time I get a bad feeling.
If my tomatoes or squash show signs of a disease, I will do the roses also.

Now what I have listed as being done here is a compilation of decades, including before Serenade was around.
I do not go through the same routine every year.

I apologize for the overly long response, but this is an abstract of all I have learned in the past few decades and why I do it the way I do.

Henry posted a link saying that a trial in MN found milk spray to be effective. I found a fuller account of (what I think is) the same trial. See link. There is also a research article about this trial that is under a paywall.

These treatments were judged effective:

30% milk solution
a copper product

These were judged not effective at all:

Serenade
sulfur
Cornell mix (soda and oil)
horticultural oil
neem oil
aspirin

These results are somewhat puzzling, since sulfur and Cornell mix have been found somewhat effective in other trials. But the positive results on milk are encouraging, so I retract my dismissive statement above.

Linda Chalker-Scott's summary of research on milk as fungicide is at

http://www.puyallup.wsu.edu/~linda%20chalker-scott/horticultural%20myths_files/myths/milk%20and%20mildew.pdf

In studies of milk vs. powdery mildew, skim milk was effective; it would be cheaper and less stinky than whole milk.

Here is a link that might be useful: MN horticulture research summary

Michael, I actually tried it in my yard on couple roses two years ago. It did help to postpone a little the start of desease. In comparison with other roses that were not sprayed, milk treated roses gor their first spots and defoliated approx. 2 weeks later. So probably in the areas with moderate BS pressure this could be a somewhat valuable option. In climates like yours or mine, I don't think this is sufficient. Definitely sulfur was more effective for me. You know I used it for several years. Not sure about above trial, why sulfur was not working for them.
Milk makes leaves look shiny and there is no bad smell :) I used 2% and whole milk.
Olga

Michael, I actually tried it in my yard on couple roses two years ago. It did help to postpone a little the start of desease. In comparison with other roses that were not sprayed, milk treated roses got their first spots and defoliated approx. 2 weeks later. So probably in the areas with moderate BS pressure this could be a somewhat valuable option. In climates like yours or mine, I don't think this is sufficient. Definitely sulfur was more effective for me. You know I used it for several years. Not sure about above trial, why sulfur was not working for them.
Milk makes leaves look shiny and there is no bad smell :) I used 2% and whole milk.
Olga

Thanks, Olga. I know from my own experience that sulfur (considered "organic") will substantially slow the blackspot down if you start early and apply at least once a week.

Thanks, RpR, Henry, and everyone who posted. I've tried milk solution before, but it was later in the season, after BS had already taken hold. I'm running an experiment with a combined solution of milk and Greencure. If there are any chemists out there, I'd value your thoughts on what combining Greencure with milk does. I don't know if it'll mess up the pH (if pH is integral to the effectiveness of either against BS) or somehow neutralize each other.

Anyway, without better chemistry knowledge, I test sprayed Day Breaker yesterday with the combined solution to see if it would burn the leaves. If not, I might start spraying all roses with the combo just to see what would happen.

I too wish that there was a truly effective organic solution against BS. I wish that I was a chemist b/c searching for such a solution is something I would love to work on. What a great job that would be! And think of the money to be made too, and the boon to the environment if such a solution could be found.

Gardener, if you like to experiment, Jeff Gillman suggested that the active black spot fungicide in milk might be a protein called lactoferrin. The concentration of lactoferrin in cow's milk is about 150 mg/liter while the concetration in human milk is 1400 mg/liter. I'll leave it up to you as to what experiments that might suggest. Also, it turns out that you can buy lactoferrin capsules, see the link below.

Here is a link that might be useful: one source of lactoferrin capsules

oho - my breastfeeding daughter has kindly obliged - we LOVE these sort of recipes - I will not go into what I use for plant feeding as I will be kicked off the web for poor taste - suffice to say, it is a monthly sort of thing.

Milk contains lactic acid - a weak acid. Greencure is a weak base. This mixing would be similar to those who mix baking soda with vinegar.
-----------------------------------
"Antibacterial activity of lactoferrin is best studied; it originates from the iron-binding properties of lactoferrin, which deprive the bacterial flora of an element necessary for its growth.[22] Antibacterial action of lactoferrin is also explained by the presence of specific receptors on the cell surface of microorganisms. Lactoferrin binds to lipopolysaccharide of bacterial walls, and the oxidized iron part of the lactoferrin oxidizes bacteria via formation of peroxides. This affects the membrane permeability and results in the cell breakdown (lysis).[22]

Although lactoferrin also has other antibacterial mechanisms not related to iron, such as stimulation of phagocytosis,[23] the interaction with the outer bacterial membrane described above is the most dominant and most studied.[24] Lactoferrin not only disrupts the membrane, but even penetrates into the cell. Its binding to the bacteria wall is associated with the specific peptide lactoferricin, which is located at the N-lobe of lactoferrin and is produced by in vitro cleavage of lactoferrin with another protein, trypsin.[25][26] A mechanism of the antimicrobial action of lactoferrin has been reported as lactoferrin targets H(+)-ATPase and interferes with proton translocation in the cell membrane, resulting in a lethal effect in vitro.[27]"

I would be hesitant to add anything that would change the hydrogen ion (H+) concentration/equilibrium as often biological mechanisms are very pH sensitive.

Here is a link that might be useful: Link for quote

Henry says don't mix milk and Greencure.

I agree the Greencure plus milk experiment might not work but I think it's definetly worth trying.