I would like to see a controlled study of cleaning up fallen blackspot leaves vs. covering them with fresh mulch twice a year. Cleaning up is a tedious chore and I would bet it has little if any effect. BS spores are being produced from the bark and from leaves that haven't fallen yet. These spores are likely to reach the new growth by splashing water, whereas spores from fallen leaves are less likely to.

I had heard that some studies showed that once the leaves had died and dropped the spores on those leaves died with them. But I also have to agree with Michael, the spores are out there all over the place anyway. So I can't think that cleaning up the dropped leaves would make that much difference.

This controlled research study looked at the question from the opposite side (infected canes and buds not fallen leaves).

"Since D. rosae survives in fallen leaves, removal and destruction of infected plant debris, done prior to bud break in the spring, should help reduce primary inoculum.
However, D. rosae also overwinters in infected canes and buds of the plant (7,13), which would explain increasing disease pressure on roses from year to year, as observed in a previous study (3). We hypothesized that it might be possible to reduce D. rosae survival if rose canes were treated with fungicides during plant dormancy.
Thus, the objective of this study was to evaluate the effect of winter applications of fungicides on the onset and season- long development of black spot disease on hybrid tea rose plants."

In the actual full article linked to below they mention reference 3. Here is the web address for reference 3.

http://www.cabdirect.org/abstracts/19952307481.html;jsessionid=03F013B81A7B2EE62427EAD18174D15B

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

I wish they made a special BS eye-glass that allowed a person to see living BS spores in the garden... lol
Well on second thought that may be a bit too much to handle...lol
The nightmares!

Henry - thanks for the link and the study, but let me understand something.

I buy a rose or propagate a rose. It has no blackspot. I plant it in the garden and it starts growing.

Then it gets blackspot.

Now I'm supposed to remove the leaves because they may cause the rose to get blackspot??

It seemed to get it the first time with no leaves in sight. When I planted my first roses, there were no dead leaves around because there had not been any roses growing here. In fact, I even removed a lot of the original soil. So I have a hard time with the logic of leaf removal - if the roses got BS in the first place with no leaves around, what does removing the fallen leaves do for me?

I suppose it may reduce the places where spores might be hiding, but isn't BS a fungus. And aren't those kind of like mosquitos? In other words, you can smash them and kill them all night long, but if you leave one or two alive, you'll have thousands a couple of days later.

So ignoring the fact that the BS blew in from somewhere in the first place, if I leave one or two leaves on the ground, and the BS takes hold somewhere, won't it reproduce like crazy? Here on the east coast, it's humid all the time anyway, so I imagine the environment is really great if you're a blackspot spoor.

The study talked about the timing of sprays, etc., and in that sense was interesting. But their most important suggestion was to find roses that were more disease resistant in the first place.

I grow all kinds of herbs mixed in with the roses and I'm not about to start spraying in the winter so I can continue doing it in the summer. If a rose doesn't do well, I dig it up and throw it away, or give it to someone who wants to spray it every week.

Interesting study though.

Most fungal spores are five microns in length (give or take a few microns).

The coats on fungal spores are very thick and protective. I cannot imagine any chemical that can penetrate that coat....heck, all the acids we used in palynology to remove fossil acid-resistant pollen and spores from solid rock didn't penetrate the fungal spores.

I rely on "good" fungi to keep the "bad: fungi in check in my no spray garden (I also do not remove fallen leaves). For many years I had about a thousand roses. Now in final stages of independent living I have around 60 roses.

According to a Vintage newsletter there was a study at The University of California Berkeley which showed that while green leaves harbor fungal spores, when the leaves die the fungi die on them. Thus simply let the leaves decompose or put some mulch over them. See p.5.

Here is a link that might be useful: Vintage Gardens Newsletter

Unfortunately Gregg didn't give a link to support the claim that dead leaves don't produce spores. Does anyone have a link? Since this was a California study it may have been more concerned with rose rust and mildew than blackspot and cercospora. Many scientific sources say that BS can live in dead leaves. The question is whether these can efficiently transmit spores to the new growth from under a mulch. Blackspot spores are relatively heavy and sticky.

Blackspot does not "blow into the garden" to any significant degree. Roses come into the garden with BS already established on the canes, or they catch it from neighboring plants in splashing rain or carried on the gardener's hands and gloves. Once established, blackspot will tend to persist on a particular plant indefinitely, but not systemically, rather as a series of local infections in the bark and leaves, reproducing asexually.

BS does have an alternate life cycle with sexual reproduction and airborne spores, but this appears so rarely it is of no practical concern to gardeners.

OK-- just coming off of a 12 hour day at the office, so maybe that's why I feel like most of this doesn't make sense...

Right now, my roses appear as through the blackspot fairy has doused their leaves in spots. It's awful and I feel really helpless. Totally did not have this problem last year (the only upside to a catastrophic labeled drought).

If I remove the partially affected leaves, that would mean removing 60-90% of their leaves. ACK! What do I do? Don't my roses need their leaves to live?

Bought some garlic fungicide off of Amazon. It got good reviews so hopefully it will work. Do I spray this all over the leaves? How often?

Ugh! I was so proud that my beloved rosies survived last summer, the snowy winter, and my first (clueless) pruning.. If I lose them to blackspot, I will be so devastated as these plants give me so much happiness. Sorry for being a bummer but if anyone can understand, I think it's you all because you seem to really love your roses too :-)

Oh, DPC! I know exactly how you feel. A month ago I had 80 beautiful roses. Now, several of them are almost bare. I think I OD'D them with Bayer, trying to get rid of BS or at least keep it at bay. I am trying to figure out what is ruining this season. So many variables, late start, too much rain, too much humidity, my over zealous feeding and spraying, improper drainage. And now we have super humid and rainy days . To be followed by 100 degree temps. In August!
Ugh.

DisplacedClevelander, going by the theory of why cleaning up leaves is probably not very helpful, and noticing that leaves feed the plant, I decided to not take off any leaves unless they can basically be blown off easily :) It's just what I do.

I'm mostly no-spray, and so I noticed that a lot of my roses with BS don't defoliate badly. Because of that, I'm glad I don't remove infected leaves (unless they are truly goners). They look spotty, but they still help the plant, I'd think.

Here where I am, though, the heat comes pretty early in summer, and my garden is dry, sunny and windy this time of year. BS isn't much of a problem for me even though our air humidity gets pretty darned high often, with thunderstorms that decide not to form. Maybe my leaves live on because the (hot and breezy) conditions don't foster further spread? Dunno exactly, but I don't strip leaves :)

Displaced,

Leaves infected with blackspot are going to die soon anyway. The exception is that, if small new spots are treated with tebuconazole (Bayer Disease Control for Roses) within a few days, the leaf will survive. Propiconazole (Banner, etc.) probably has the same effect, but I have not seen scientific research demonstrating that.

Spores infecting new leaves mostly come from infected leaves on the plant. Therefore, if you are not spraying tebuconazole, removing spotted leaves promptly should slow the spread of disease. If you are spraying tebuconazole, picking off leaves would make little difference.

The issue that I raised at the top of the thread was not about picking off spotted leaves, but about picking up dead fallen leaves, which was recommended in the linked article, but not by me.

Using Bayer every two weeks will prevent defoliation, but it will take two or three weeks before you notice the benefit, because leaves with old spots will continue to die, and new leaves take weeks to develop. To grow roses well in the East, South, and Midwest, you have to either spray fungicide or select resistant varieties. There are not a lot of roses that are resistant in the East. but there are some.

I live in the Midwest (north east Ohio), and grew 1000 roses in a no spray garden. The good fungi reach an equilibrium with the bad fungi. It may take 4-5 years.

Henry, weren't you growing mostly roses that had some resistance? I know you were breeding for resistance with rugosas etc.

Michaelg has admirably summarized our knowledge of the black spot fungus. Without getting technical in terms of mycology, the fungus DOES persist in various viable forms ("spores") on dead leaves, partially dead leaves, fallen leaves, canes, stems, etc. Not just during the growing season, but over-wintering as well. Specifically, the "spores" do NOT die with the leaves. All that has been known since the fungus was systematically studied by Wolf in 1913, and has been confirmed many times since. I'm truly surprised that anyone would doubt it.

Removing infected plant parts falls under the general agricultural practice of "inoculum reduction". It's obvious if you remove material containing a disease inoculum, you will be reducing the amount of inoculum available for re-infection of that disease. . It's also been proven that when that is done routinely over a large area ( as in a fruit orchard, vineyard, etc.). it does result in significantly reduced infection rates over the entire area. .

The only question is if it is done on a small scale such as a typical rose garden, will it result in significantly reduced infections.

I would say that based on the life-cycle of this disease, it can't hurt..

In the early years I used a lot of rugosas. However, this is the list of crosses from the last year that I kept records:
http://home.roadrunner.com/~kuska/hips sorted by seed parent-2006.htm.

Other year crosses are available at:
http://home.roadrunner.com/~kuska/rosepublicationsindex.htm

Pictures of my seedlings are at:
http://home.roadrunner.com/~kuska/seedlingpage1.htm

At first, I was interested in producing winter hardy, disease resistant roses. However, after a while I found that I did not have to select for disease resistant as they all passed that criteria. I do get blackspot in the fall, but I consider that as part of Nature's way of preparing the roses for winter.

I found that many/most of the U.S. roses were virused and died within 4-5 years (I did not winter protect, but I did bury the graft below ground). I settled on purchasing roses from Pickering, Palatine, and the company that went out of business and helped Palatine get started.

A relatively recent (2008) reference for a controlled scientific study of using "friendlies" is given in the link below.

Here is a link that might be useful: link to scientific study

This post was edited by henry_kuska on Wed, Jul 10, 13 at 13:36

I'll have to try to do a little home study and mark green leaves that are infected that I leave alone. I'm not talking about the roses I spray when I say that I have noticed that the infection does not necessarily kill the leaves that are spotty but green.

Of course, they die at some point as all leaves do, but they can stay infected and green for a long time, unless I'm just imagining things.

I'll try to mark some of them and record the dates. I can't get close-up pictures, but I'll use my phone, anyway :)

If I removed every leaf with spots in my garden, so many would suffer great defoliation from that so often. I just can't imagine that their time being green on the plant isn't possibly more beneficial to the plant's health than the amount of reduction of BS during BS season (I do believe in dormant spraying).

I do spray a few roses during the season, too, but these are the no-spray roses I have (the vast majority) that get varying amounts of BS each year.

In any case, that's just my personal opinion and what I do in my garden. Certainly if y'all all notice that the leaves die quickly anyway after being a bit spotty, it's a moot point :) I do have roses that do that, and those are the ones I do spray, personally.

I think I found a possible explanation for what I've observed often enough to keep bringing up that I don't strip on the first sign of leaf infection. If this does explain it (or if it's a good theory anyway), then it does seem to have to do with my choice of roses or the aggressiveness (lesion expansion rates) of the local strains of BS here.

From http://www.rosehybridizers.org/Fall_2012_Zlesak_long_version.pdf

"...We have blackspot resistance field data for the roses that are designated Earth-Kindî in the South Central US. As we correlate the mean lesion size in the lab with the percentage of defoliation due to blackspot in the field, r=0.62...So in this case there is a pretty strong trend that as lesion size goes up for a rose in the lab, the more defoliation we see for that rose in the field. Since different isolates of different races can be more aggressive and lead to different lesion expansion rates, it is very promising to see such a strong correlation in the midst of that variability. As more blackspot resistance data are collected from other ongoing Earth-Kindî trials involving some of these 70+ roses, it will be interesting to revisit the correlation between the laboratory and field data...."

And the RHS has seen what I'm talking about, so at least I don't feel so crazy ;)

http://apps.rhs.org.uk/advicesearch/Profile.aspx?PID=270

"Typically, a rapidly enlarging purplish or black patch appears on the upper leaf surface, with diffuse and radiating strands of the fungus sometimes just visible.

Leaf tissues may turn yellow around the spots and the leaf often drops, even though other parts are as yet unaffected.

At other times, the yellow colour does not appear, but infected leaves still drop.

Sometimes, the spots remain relatively small and the leaf does not drop. [!]"

I'm not trying to tell anyone how to do anything if it goes counter to their local experience, of course. I guess I'm saying that folks might wait a bit and see what happens if they don't strip leaves if the plant will be very defoliated if they did strip so many leaves that don't look too bad. See what happens, perhaps. Or not :)

Here is a link that might be useful: Strategies and resources for breeding blackspot resistant roses, DAVID C. ZLESAK

Meredith, a point of possible confusion in your observations is the similarity of cercospora spots to blackspot spots. Leaves with cercospora can live a long time, maybe a couple of months. It may help the plant to leave these alone, although fallen leaves are said to be a source of reinfection in spring.

Cercospora spots have smooth definite margins, not fringy or notchy or vague.

After a while, cercospora spots bleed through to the opposite leaf surface where they are brown. Blackspot spots generally do not.

After a while, on many varieties, cercospora spots will merge to form a large dead brown area, maybe half a leaflet.

Thank you, Michael :) That could well be it. I have never paid very detailed attention to how the two diseases look different, just assuming mine was more likely BS since you always hear about that in the South. Good to know!

This year so far I've seen more cercospora than blackspot, probably because it has been cooler than usual. I currently have leaves dying from infections that probably started in May. In your own garden, try flipping over a few spotty leaves and see if the spots go all the way through.

My roses are very clean this year, with few exceptions like Fragrant Cloud, in spite of the almost constant rain every few days. I removed almost every cane at soil level this spring, thus supporting the idea of spores overwintering on the canes causing the blackspot. I do not pick up leaves, nor spray. By autumn, I expect many will start showing some infection.

Steve