Downy mildew

Are there any species roses known to breed a resistance factor against Downy Mildew?

palustris, perhaps?

There is a recent article titled: Downy Mildew in Roses: Strategies for Control by D.F. Schulz and T. Debener in Acta Hort 870:163-170.

Out of what they surveyed, they found genotypes of the following resistant and also made some crosses of these with more modern roses and documented segregation for resistance as well.

R. agrestis

R. arvensis

R. corymbifera Borkh.

Thanks for the information, David. Some of these I have not even heard of!!!

Theyre mostly either glandular types, types with high gloss foliage or types with some really gnarly canes.

None of the rugosa (‘Scabrosa’, ‘Pink Grootendorst’, ‘Alba’, ‘Frau Dagmar Hastrup’ seedlings, and an unknown double red one), are affected atm. Rugosa ‘alba’ x ‘Golden Emblem’ seedlings are unaffected and growing strongly. ‘Scabrosa x modern minis’ are unaffectted by it but are copping it from powdery mildew. ‘Ann Endt’ (rugosa x foliolosa) is unaffected. 'Ann Endt’s OP progeny are unaffected and will be used extensively here in the future. The ‘Commander Gillet’ OP seedlings I have left after the cull are very strong and unaffected. These are the main ones. ‘Temple Bells’ and all its progeny have <5% infection. ‘Immensee’ has <5% infection. Rosa setigera has <3-5% infection. The gallica I have (‘Tuscany’, ‘Tuscany Superb’, ‘Charles de Mills’), are largely unaffected. R. banksiae seem unaffected. R. bracteata is 0% affected. R. laevigata is 0% infected on both my plants (though ‘Anemone’ is a mess). Clinophylla was 50% infected (which makes sense as it’s a rose from the tropics… it has no need to develop immunity to DM which is more of a problem in cooler weather), but is recovering well. Clinophylla x bracteata seems pretty good. The spin. seedlings I have got over it the fastest. Roses like ‘William III’ got it, dropped about 10% of their leaves and got over it real fast without skipping a beat. All bush teas except ‘M. Tillier’ and ‘Lorraine Lee’ were universally bad though ‘Marie Van Houtte’ is coming back more strongly than the others. ‘Safrano’ completely dropped her bundle… underwhelmed by this rose. ‘Knockout’ is 0% infected. ‘Carefree Wonder’ is so far free of it. The mosses don’t seem affected; ‘Henri Martin’ is <5% affected whilst ‘William Lobb’ is 0% infected. Multflora species plants are <3% affected… hybrids less so but are pretty good (a lot of the hybrid musks are mostly clean too. ‘Trier’ and ‘Felicia’ show very minimal infection and ‘Cornelia’ is almost as good). R. gigantea has so far been free of it, which I found interesting and ‘Nancy Hayward’ seems unaffected so far. Flower Carpets scarlet, yellow, pink, and appleblossom are <5% affected. FC White, Red, Amber… don’t even bother… The Fairy is <2-3% infected. I mention these because they are near to wichurana. Wichurana itself is about 15-20% affected but they are newly struck and planted, unestablished cuttings. ‘Fortune’s Double Yellow’ seems unaffected. ‘Mutabilis’ is <5% infected. ‘Golden Chersonese’ seems <2-3% infected (ecae x Canary Bird (hugonis x xanthina)). Longicuspis var. sinowilsonii broke down only on the older foliage. New foliage existing prior to the outbreak was unaffected at the height of the problem. The 88-390 (80-358 x ‘Bayse’s Blueberry’) OP seedlings are mixed but most are <5% infected. Surprisingly, a few of the ‘Ebb Tide’ op seedlings seem pretty resistant to it, whilst others are a mess. ‘Sympathie’ is unaffected. Get this rose if you can IMO! ‘Reve d’Or’ is unaffected. ‘Euphrates’ is unaffected but PM is loving it.

That’s a bit of a run-down of things that seem to be doing ok here atm when DM is running rampant.

Hi Simon.

Is it possible to show us a picture of the DM lesions of some of your damaged roses?

I have never seen DM in my climate, this would be a good opportunity to see what it actaully looks like!!

Here is a pic from online. Its of the leaves, but this is the same color/pattern the stems of mine will get when theyre infected. The cane usually then goes yellow then black in slow motion over the summer. The splotches grow slowly over time, and I never found pruning it off to be helpful. Its always an irregular, chaotic yet rounded purple mess on them stems. Its a weird disease.

Thanks for your assistance Michael, that is a real clear picture of lesions.

No I can’t say I have seen this scurge round here on any roses, if I have, I missed it…although thinking back now, perahps I have seen it on some tomato plants (in previous seasons), so I am suspicious that DM outbreaks might also happen in my warm climate, as well.

I posted a bunch of photos on the thread below when we got hit rather badly last year. When the conditions are right, it can be devastating. The upside is as Simon points out, that you get to see what shines.

I have a multiflora selection that I use for root stock that did very well and have begun to use it in my breeding.

Jim Sproul


It moves really quickly too. This is after only a day or so on ‘Paul’s Scarlet Climber’:

On ‘Nahema’ after about 3 days:

Climbing White Iceberg after a few days:

‘MME Isaac Pereire’ almost dead:

I live in the middle of farming land and my neighbour grows opium poppies right next to my roses. Apparently it’s a serious disease of poppies too and this year has been very bad for it. It is all through the poppies. I have no chance of escaping it with this next to me, taken from my place looking into next door’s paddock:

A farmer friend of mine has given me some of the stuff he uses on his poppies that will kill the downy mildew on and in the plant… I hate using it at all but if I don’t I’m going to lose a lot of stock. The fungicide is called Ridomil Gold. It’s expensive stuff too… almost cheaper to rip the roses out and start again :frowning:

Good shots Simon, and Jim.

Simon it is obviously a devastating attack you have there, I feel for you.


Fara has a point with Chlorox. There really has to be some sort of solution that maybe only damages the roses but kills the downey. Personally, a preventiative would be helpful too. I know that Chlorox is highly alkaline – like 10 or something lol.

The ingredients for Chlorox Clean Up is


Sodium hypochlorite


Lauramine oxide

Silicone emulsion

Sodium hydroxide

Sodium silicate

Unfortunately, Sodium hypochlorite has a massive salt byproduct, which destroys rose/plant cells if they are exposed directly to it. Pre-hydrating the rose plant would obviously help but its pretty strong. I have personally only used Chlrox for gall treatment in mid-winter, which is when the soil is massively water saturated and the plants are “kind-of dormant” (ie. eye buds are hiding). So this solution can work in some scenarios, but downy mildew is more active dring the growing season :frowning:

One partial solution is to minimize morning watering onto the plant or its leaves. This is its prime time to take off and take hold. The spores travel by air, thrive in humid yet air mobile spaces but degrade easily (as spores only) in UV. It tends to take off when conditions are between 50 degrees and 75 degrees F with 90%+ humidity for a few consective days of long day light hours. It massively thrives between 65 to 75 degrees F.

One difficulty with downey mildew is that it is not specifically a true fungi. Its more closely related to algae, sudden oak, phytophthora, die back, etc. This is why it is so nasty to deal with.

A note of interest is that this class of “fungi” are composed of cellulose, which is what real fungi thrive on. Fungi are highly simple whereas this class of critters are highly complex and immune to adverse conditions because of their efficiency and complexity. In contrast, fungi are mostly made of chitin, which is similar to cellulose but is stronger and capable of water binding. However, baking soda can change its properties by making it immobile. Hence baking soda is more useful on mildew, blackspot, etc.

There are 10 or so different sea salt sources. These false fungi can thrive in both fresh and salt water, depending on type of pathogen. Prsonally, I’d try various salts on test spots to see reactions, especially salt types that are alkaline and not acidic. Unfortunately plant cells often hate alkaline too but maybe there is a optimal salt dose that most roses can tolerate that can kill this stuff. One thing that I do know is that a lot of the nitrogen sources we use to feed plants are acid-based chemicals. Even though fungi and these false fungi are different, they both seem to love nitrogen-enriched plant cells, but I am guessing for different reasons. Calcium Chloride is safe-ish to plants and found in some De-Icer crystals. It can burn plant roots easily though. I wonder what would happen if one took diluted liquid de-icer thats calcium based and mixed it with petroleum jelly w/ aloe to slick it on rose canes.

Again, one of the known fungicides to work on this type of pathogen is called Metalaxyl, which is also alkaline. Unfortunately, especially since they are both asexual and sexual, they change resistances easily. So Metalaxyl can easily become useless.

Simon, whats your soil pH? Plant pH? I wonder if Rosa californica is immune or highly prone, lol? I also if pH dependant colors are more prone, too. Yellows and mauves prior to some of the work of those like Kordes seem to get hit the hardest. And what are the days like before infection hits there?

From what I could tell re: patents of these chemicals, the 2ndary “synergistic half” used as a site preventitive is often from chlorides/chlrorines/etc, which probably tells us that this is a key fact for preventitive culture in areas prone to these types of pathogens. I do not know chemistry well enough to understand what is being used as the short-term curative. Whatever it is, though, is is toxic to birds, mammals and fish, as well as being highly persistant and leachable. It can kill earthworms, which I specifically hate. Destroying soil biomes is dumb. Its LD50 is 375 on rats. And for whatever weird reason, mites find it beneficial… ? LOL, lovely for us. No pH value is listed (example: C15H21NO4). Maybe one of you all out there studied in advanced chem can tell us. However, what I specifically want to know is how this formulation gets around the plant cell burning properties.

btw, no offense to anyone from Ireland out there, but ya’ll seriously need to consider a different crop and let areas not prone to these pathogens grow taters, lol.

Hydrogen peroxide is a safe alternative to chlorine bleach provided it is applied in a very dilute solution, 1000 ppm to 2000 ppm on intact plants (about 30 ml to 60 ml of 3% peroxide to a liter of water). They work the same way, by free radical degradation, but hydrogen peroxide is much less ionic than chloride so puts less osmotic stress on roots.

I thought of that but wasnt sure. I also thought, “I wonder what oxiclean would do?” lol. I’m no longer allowed to use it in the home =( (or use kitchen utensils as tools, haha).

Seriously though, It’d be cool if we could come up with solutions that are safe (like hydrogen peroxide) and easy to find in terms of both products and culture.

Another query: What role can Neem oil play, if any?

I forgot to ask. Simon, do you use dolomite on your soil?

Simon, I just finished reading the Arts section of the New York times. Then I saw your kindred poppies photo. That one is fantastically near perfect. If you could photoshop the utility pole no one would believe it is a photo. I especially like the framing in the upper corners and the flow of colors left to right.

Add your name and market it under panoramas of Tasmania. It would look great as a mural on office walls, especially for those who recognize the genus and species of plant portrayed. BTW, such stuffs are banned in U.S. Though photos are probably not illegal, they draw attention of local narcs.

On a more practical note, the downy mildew on roses, may be totally different species from that on potato or poppy. In 2009 a massive outbreak of late blight (potato) took down most of the amateur tomatoes in the north-east U.S. Many pros lost out too, except those big enough and quick enough to get their Ridomil going. It was about 4 days from first signs to total death of a patch. It was spread from seedlings shipped in and sold at big box stores. No signs on roses.

In the U.S., chlorothalonil (Fungonil or Daconil brand names) are recommended, along with mancozeb. Ridomil is specifically disrecommended because resistance to the active ingredient arises easily. A hydrogen peroxide- releasing material going under the name of OxyDate is used on food crops, and some materials containing potassium phosphite ( not phosphate) are used systemically (see ProPhyt or K-Phite). But you can’t mix some of these with copper compounds which are used as adjuvents in some fungicides. And OxyDate is probably not good with fungicides like daconil.

Overall, the literature is a complicated mess, depending on crop, state, indoor vs outdoor. Downy mildew on basil is the big threat this past year.

Yeah, it just LOVES cool crops and night shades. I imagine it has something to do with whatever cellular matter is rich in what it needs since it works as an aggressor (almost like a spider to its prey) by forcing necrotic tissue) rather than a decomposer like most fungi.

I can remove the 4 utility poles in the pix if Simon desires. Thats easy.

Larry, I believe they are different species. I think there are more than strains of blackspot! Its crazy. Sudden Oak Death scares me here cause the Oregon White Oak is so symbolic of the PNW. If anyone has sat underneath one on a summer day or experienced the smell of the autumn leaves they produce under these massive, almost “Edgar Allen Poe” feeling canopies, then it would be understood.

However, they all operate on similar principles.

The photo of ‘Mme. Isaac Perriere’ illustrates Blackspot, not Downey Mildew I believe. Opinions?