Does neem oil control black spot?

Bob_B(Sunset 14, Ca.)May 28, 2009

I know Banner Max and Indicator (something or other) does. But I have a friend who wants to stay organic.


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michaelg(7a NC Mts)

No. Oils have a slight inhibiting effect at best.

See the FAQ of the Organic Rose Forum for the use of sulfur. It works better than the Cornell mixture also described there.

    Bookmark   May 28, 2009 at 12:12PM
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Title: Effect of inorganic salts and vegetable oils on black spot of roses.

Authors: Osnaya-Gonzalez, M.; Schlosser, E.

Authors affiliation: Institut fur Phytopathologie und Angewandte Zoologie, Justus-Liebig-Universitat, Giessen, Germany.

Published in: Mededelingen - Faculteit Landbouwkundige en Toegepaste Biologische Wetenschappen (Universiteit Gent), volumn 65, pages 725-729, (2000).

Abstract: "Five inorg. salts, Na2CO3, CaCO3, NaCl, K2HPO4 and (NH4)2SO2 were tested in combination with rapeseed oil or neem oil in a detached leaflet assay for their efficacy in the control of Diplocarpon rosae, the causal agent of black spot on roses. All inorg. salts in combination with vegetable oils reduced the development of Diplocarpon rosae on artificially inoculated leaflets of Rosa canina pollmeriana, when applied as protective treatments. With the exception of NaCl in combination with Telmion (rapeseed oil), the other combinations were as effective as the synthetic fungicide Saprol, commonly used in the control of black spot on roses. It is possible to substitute NaHCO3 for other salt in combination with Telmion or neem oil, although it provides better control. In plants, all the treatments reduced the percentage of leaflet damaged area with necrotic lesions, fallen leaflets and conidial germination. With exception of K2HPO4 and (NH4)2SO4, in combination with Telmion, the other treatments were as effective as the fungicide. Disease control could be due the redn. in the conidial germination."

    Bookmark   May 28, 2009 at 1:36PM
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helmut_ii(z9 Houston, Tx)


I've tried Neem Oil in the past with very poor results. I've used Banner Max and several other products, but I got tired of arriving at my garden at 6:00 A.M. in the morning with all my spray garb on just so I could get it done before it became to hot.
I did not spray last year at all determined to cull out the roses that could not hold up to a no-spray garden.
That said I have about a dozen roses that I can not part with so I went on a search to find something easy on me and the garden. My local nursery recommended Serenade Garden. It is organic and I've found it easy to use. I've been using it for about six weeks and so far so good.
I would recommend you google it if you are interested.


    Bookmark   May 28, 2009 at 11:11PM
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rochesterroseman(zone5 NY)

I bought Serenade a few years ago, but it got such bad reviews for black spot from rose growers, that I was afraid to use it. Let me know if it works for you. 6 weeks is not long enough to know anything for sure.
I find that chemical fungicides are very low toxicity. I just wear a mask so I don't breath it in, but I have a fish pond, and my fish are fine. I also tried Neem a long time ago and it was a total disaster. Good luck.

    Bookmark   May 29, 2009 at 7:51AM
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It probably would be useful if those who used Neem would tell whether their "Neem product" contained Azadirachtin or not. Apparently most so called "Neem products" have had the Azadirachtin removed.

    Bookmark   May 29, 2009 at 10:51AM
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This research paper identifies the compound in Neem that is an anti-fungal agent:

"The compound was suggested to be trihydroxy-tetra acetyl-azadirachtin"

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

    Bookmark   May 29, 2009 at 2:42PM
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roseman(Z 8A GA)

Re: The question: Does Neem oil control black spot? Not around here it doesn't. I guess that's why we stick to Banner Maxx - we know it works, and very well too. I realize some people want to stay organic, but sometimes, and black is the exception perhaps, you have to use a chemical product to be rid of a disease.

    Bookmark   May 29, 2009 at 4:31PM
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roseman, is your experience with a "Neem product" that contained a significant amount of Azadirachtin or one that did not?

    Bookmark   May 29, 2009 at 5:02PM
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funny you should bring this up-i just sprayed all of my roses a few days ago with 100% neem oil. my viking queen then developed what looked like a dead ringer for downy mildew the next few days. this rose has never had anything, so i was puzzled. i took a leaf in to my rosarian mentor and the same thing happened to one of his roses after spraying as well. turns out that some roses do not like it, however, the rest of my roses look fine.
if i am going to curtail disease, i spray occasionally with copper (bordo) but rarely at best. the neem makes the leaves look shinier and healthier and it smells good too, but i couldn't say if it worked for BS.

    Bookmark   May 29, 2009 at 5:12PM
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michaelg(7a NC Mts)

Henry, the product I used did not contain azadirachtin. There doesn't seem to be any research on azadirachtin and blackspot. The effect of the az-less neem oil would likely be similar to that of the canola oil in the abstract you linked. I have read that oils somewhat reduce sporulation by blackspot, presumably by coating the acervuli. Since the az-less neem oils make label claims, the manufacturer must have demonstrated some degree of control in their trials, i. e., it's at least a little better than nothing..

    Bookmark   May 29, 2009 at 5:33PM
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I hope I don't cause a war with this... but
I use neem to control fungal diseases. It's important to note though, I use pure cold pressed neem and not a distillate product or the laughable serenade. I use it blended with a sticker/carrier (fish hydrolysate) and an emulsifier (a dash of biodegradable dish soap - otherwise the oil is just going to sit on top like un-whisked salad dressing) and I apply it weekly to bi-weekly.

Now, I certainly don't have the same disease pressures as you, but I can say after our ridiculously wet and cold spring last year here in Iowa - I was the only one in town without a spot of scab, black spot, or anything else of fungal nature on my plants.

    Bookmark   March 25, 2014 at 3:58PM
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What is your source for cold-pressed neem?

A few years back, a British poster on another garden forum claimed success in deterring fungal diseases by adding neem cake (that is the residue from neem seeds after the oil has been pressed out) to his fertilizers.

    Bookmark   March 25, 2014 at 10:19PM
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I buy from ahimsa alternative

They also carry neem cake, but personally I think the oil is easier to work with. I highly recommend using with fish hydrolysate (i buy neptune's harvest - you don't want emulsion/hot processed).

Here is a link that might be useful: ahimsa

    Bookmark   March 26, 2014 at 9:31AM
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Corrected for a few errors in original text.

This post was edited by jjstatz29 on Wed, Mar 26, 14 at 14:51

    Bookmark   March 26, 2014 at 10:11AM
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Let me expand further on the topic...

I use neem oil from Ahimsa alternative -
I buy my fish/seaweed hydrolysate from neptune's harvest

or I buy from brown's for just straight fish hydrolysate

Some notes about using...

Neem - you have to keep it cold, I split the gallon container into smaller bottles (it hardens when cold, so you'll have to either dunk the gallon container in a sink of hot water or leave it sitting out on a warm day) - Shake it thoroughly and divide into very tightly closing smaller bottles.

When you plan on spraying - take the bottle out, put it in a sink of hot water until it returns to liquid state. I usually pour the portion I need (.5 fl oz per gallon - a tablespoon) into a bucket and then return the bottle to the fridge, add my 1 ounce (1/8th cup per gallon of water) fish hydrolysate (be sure to thoroughly shake the container before measuring it out - be warned - this stuff is smelly), and scant tablespoon of biodegradable soap - I get 7th generation - it's cheap & readily available.

Then I add about half the water I need and stir it several times in one direction and then switch directions with a dedicated whisk. Pour into sprayer add remaining amount of water needed, close your sprayer, shake thoroughly and start spraying (make sure their are no oil droplets at the top of the solution - neem has to be thoroughly mixed to be effective - think like salad dressing - who likes separated salad dressing? yuck.

I prefer spraying right before sundown or even at night (with a flashlight... my neighbors naturally think I'm nuts) and to the point of runoff. Be sure to get undersides of leaves.

Why near or at night? This is when the stomata are opening up on the plant - in other words, the pores of the plant are open. You'll get the maximum uptake and benefit from spraying at this time and it gives the maximum time for the plant to avoid sunburn. I do this about once a week during times of high disease pressure and then back off to bi-weekly when disease pressure goes down - if it rains, I spray the next day or as soon as possible (assuming it isn't just going to rain the next day... then I wait)

As always - be sure to test an inconspicuous area of the plant and allow a week to make sure their aren't burn issues.

If you, after a few weeks, aren't seeing an improvement in plant health - up your spray concentration by half a percent. Per gallon, for example, if you started at .5% dilution - go up to 1% (1 fl ounces per gallon, 2 tablespoons).

Few other pieces of advice - this stuff is smelly. Neem has a very strong garlic-nutty smell and fish hydrolysate... well it smells like rotten fish (trust me the cold process hydrolysate smells A WHOLE lot better than the cheaper, less effective hot processed stuff... ugh). Be assured it dissipates quickly - if the scent really is bothersome you can add a drop or two of a fragrance oil - peppermint is particularly effective.

Neem and fish hydrolysate once mixed - have to be used immediately upon mixing - they break down rapidly in solution - if you sprayed everything and have some left, use it as a soil drench or spray it on something you don't normally spray. It'll be glad for it. Thoroughly rinse your sprayer after every use - I fill mine with water and just use it to water plants, silly to waste the good stuff. I try to think of it as being like licking the brownie batter off the spoon.

This should go without saying, but don't spray it on the blooms. Buds are absolutely fine - but blooms and less so buds are much more prone to phytotoxicity (burn). Do not spray during foraging times for beneficials - (shouldn't be too much of a problem at night).

DO NOT spray on host plants for butterflies/native endagered insects - Azadirachtin (the main active compound in Neem) interrupts the molting process of insects that consume it. Butterfly caterpillars, that eat and live on their respective host plants that are treated with neem (or any other spray for that matter) will die. If you don't know what that includes, I'd contact your local extension office.

You should see improvement in just about any plant you use this on. I pretty much use it on everything that isn't a host plant in my garden and this is also the same basic formulation I spray on my orchard. Also, the fish hydrolysate is more than just a sticker/carrier - it's also a fantastic fertilizer - I use the same spray formulation (1/2 fl oz neem, 1 fl oz fish per gallon) as a soil drench aka fertilizer.

Hope that answers some questions

Jordan - Organic/Holistic Orchardist and Gardener

This post was edited by jjstatz29 on Wed, Mar 26, 14 at 10:18

    Bookmark   March 26, 2014 at 10:13AM
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Michaelg - There doesn't seem to be any research on azadirachtin and blackspot.

Well, that's because azadirachtin (the primary active compound in neem oil) isn't the anti-fungal agent working within neem oil.

Neem works as a fungicide

by 1. Coating as an oil like any oil coating spray and boosting cuticle defense on the leaf surface - this can also be boosted by applying fermented compost tea of horsetail/nettles which boost silica content in leaves.

2. Secondary plant metabolites stimulate an immune response from the plants neem oil is applied to.

3. by repelling insects that spread and open wounds for disease entry

4. The fatty acids in liquid fish and neem fuel the beneficial fungal/bacterial presence that out competes the harmful types.

I should say too - Fungal defense has to start with healthy plants. Your soil needs to be supportive of beneficial mycorrhizal and saprophytic fungi so that your rose is actually able to feed properly. There are a lot of fungal/bacterial innoculants available and good mulching and regular fish/neem applications keep them fed and happy and thereby keeps your roses fed and happy.

It's surprisingly not far off from how pro-biotics benefit people when consumed and/or how proper diet can keep them in balance in people. Therein lies the problem with systemics/chemical means that indiscriminately wipe out fungal presence and/or bacterial and makes it harder for your rose to do what it wants to do - grow healthy. You're killing off the most basic part of the supply chain and creating a blank slate where disease pressure can actually be worsened and becomes dependent on regular killing applications instead of developing a culture that out competes disease pressure.

Another good example is how a sourdough starter doesn't go bad when fed regularly. The fungal presence (yeast), bacteria (lactobacillus), and enzymes (amylase,maltase, etc) stabilize the pH and overall culture and outcompete/create a hostile environment to other forms within the starter as long as they are fed regularly.

^That's the basic idea of what you want going on in your soil but (obviously) with different fungal/bacterial/enzymatic types (because I hope you aren't baking bread with your soil) and if you kill one of them off with a indiscriminate fungicide you're opening yourself up to bigger problems.

    Bookmark   March 27, 2014 at 2:43PM
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