Roses forum: Black spot

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rmbryant38
Apr 5, 2015 12:41 PM CST
What is the best spray for black spot?
Name: Elaine
South Sarasota, Florida (Zone 9b)
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dyzzypyxxy
Apr 5, 2015 2:16 PM CST
Once you have black spot, nothing will take it away, but if you remove the affected leaves religiously, (pick up any that fall, too) and spray regularly with a baking soda and water solution (1/2tsp. soda to a quart of water) it can slow down or halt the spread of the fungus. Some people swear by spraying with milk, too. (but baking soda is cheaper) Both these just change the pH on the leaf surface to make it inhospitable to the fungal spores. They both will wash off if it rains, so be sure to re-apply after rain.

The other really important things especially in humid summer areas are to make sure there is plenty of space between your plants for air movement, and be sure to water early in the morning, so that the leaves will dry. Watering in the evening is a sure invitation to fungal diseases, if you wet the leaves and they stay wet over night. If you feel you need to water in the evenings, try not to wet the leaves! A drip or soaker system that does not wet the leaves is good, or using the hose to soak the soil without wetting the foliage will help to prevent the spread of the black spot.
Elaine

"Success is stumbling from failure to failure with no loss of enthusiasm." –Winston Churchill
Name: Celia
West Valley City, Utah (Zone 7a)
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Zencat
Apr 6, 2015 8:38 PM CST
Everything Elaine said. I agree
Name: Rita
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Newyorkrita
Apr 8, 2015 7:02 PM CST
I personally would recommend a systemic spray. Not the ones that contain insecticide but the fungal ones only. Bonide makes a fungal systemic rose spray and so does Bayer.
Name: Annie
Waynesboro, PA (Zone 6a)
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LysmachiaMoon
Apr 9, 2015 7:12 AM CST
Because black spot fungus drops its spores onto the ground around the roses, it's important to pick up fallen leaves, etc. as everybody else has suggested. I've also found that planting groundcover that comes right up to the rose stems but is loose enough to allow stuff to fall through seems to work to reduce blackspot. I have violets planted below/around my roses and those seem not to have as bad blackspot. Roses that I have with bare ground or bare mulch around them seem to suffer more.
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Name: Lyn
Weaverville, California (Zone 8a)
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RoseBlush1
Apr 9, 2015 9:32 PM CST
Annie ...

From what I've read and from my own experience, once a leaf that has black spot on it drops, the spores on that leaf die with the leaf. BS spores are in the air and when conditions are right, depending upon which strain of BS is in your garden, you are going to get BS on your roses. Doing that extra clean up won't make a difference, but you can help the rose by spending your time pulling off infected leaves.

There are a lot of right ways to grow roses. Since I don't spray, the roses I grow have to be able to recover from an infestation of BS. Also, I don't get summer rains, so my climate isn't BS friendly and I have less disease pressure.

Studies have shown that there are five active strains, or races, of BS active in the US and not all roses are susceptible to all of the different strains of BS. So, it depends on the rose and the strain of BS you have in your garden as to whether or not a rose gets BS.

In my climate, which can be wet during the spring, but very hot and dry in the summer, under-planting has little or no impact. The BS goes away when temps reach 85F because the spores are not active above those temps unless they are on a moist/wet leaf for more than 6 hours at night.

Smiles,
Lyn
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Name: Sue
Ontario, Canada (Zone 4a)
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sooby
Apr 10, 2015 4:47 AM CST
I agree with Annie, the fallen leaves are a source for continuation of the disease, quoting from University of Illinois Extension: http://web.aces.uiuc.edu/vista/pdf_pubs/610.pdf

"The black spot fungus overwinters in fallen leaves and in lesions on infected canes. White, slimy masses
of microscopic spores (conidia) produced in diseased tissue are splashed by water or wind-blown rain
from fallen leaves and cane lesions to the opening leaves in the spring......."

"Carefully collect and compost or burn all fallen leaves in the autumn and again before the new growth commences in the spring. Where feasible, removing infected leaves, especially during early to mid-season, will further reduce the spread of black spot."





Name: Zuzu
Northern California (Zone 9a)
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zuzu
Apr 10, 2015 1:38 PM CST

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I agree with RoseBlush1. The link provided by sooby is dated 1987. More recent research has proved that the spores cannot live on dead leaves. Removing affected leaves while they're still alive is useful, but removing them after they fall from the rose bush is unnecessary.
Name: Sue
Ontario, Canada (Zone 4a)
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sooby
Apr 10, 2015 2:25 PM CST
Zuzu and Lyn, that article I quoted is only one of several that say fallen leaves are a source of inoculum for the next year. The American Rose Society site says: "Make sure to clean up the beds completely of all leaves or stems to help keep the disease from wintering over." The Missouri Botanical Garden page for black spot says "Remove diseased leaves. As soon as diseased leaves are noticed, remove and dispose of them. Dead leaves on the ground should also be collected and destroyed. Compost only if this material will not be used back in rose beds. To minimize overwintering of the fungus, collect and remove all leaves from the ground in the fall, mulch with 2–3 inches of leaf mold or fine bark, and prune diseased canes before growth begins in spring."

I also checked on Google Scholar for scientific articles and found this one from the American Society for Horticultural Science dated 2008 in HortTechnology (Evaluation of Griffith Buck Roses for Resistance to Black Spot) http://horttech.ashspublications.org/content/18/4/588.full - it says "Black spot of roses, caused by the fungus Marssonina rosae (teleomorph = Diplocarpon rosae)........... M. rosae survives unfavorable periods on fallen leaves and diseased canes."

The American Phytopathological Society says "Remove fallen leaves and other infested material and prune out infected stems during the dormant season" http://www.apsnet.org/publications/apsnetfeatures/Pages/Rose...

Perhaps I'm missing something, in which case I'd like to know Smiling - might you have references? Thank You!
Name: Zuzu
Northern California (Zone 9a)
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zuzu
Apr 10, 2015 2:45 PM CST

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From a 2009 article by Gregg Lowery of Vintage Gardens:

"We are indebted to a scientific study done at the University of California at Berkeley in recent years. It showed that while green leaves can harbor fungal spores, once the leaves die, the fungi die on them. This leaves us with a simple cleanup process, and one that obeys the basic rules of recycling in the plant world. All leaves when dead decompose and add to the soil the nutrients that were stored in them. They become compost. If we want to avoid the possibility that some of those leaves will stay green and keep fungi alive, we need only cover the leaf litter on the ground with a light covering of decomposed organic matter—mulch. They will then die completely and become an additive to the soil to enrich it."
[Last edited by zuzu - Apr 10, 2015 3:12 PM (+)]
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Name: Larry
Enterprise, Al. 36330 (Zone 8b)
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Seedfork
Apr 10, 2015 3:01 PM CST
I find it so common that what we think we know as fact is constantly changing. Now you would think with all the money and time spent on studying black spot on roses there would at least be a consensus on such a basic thing as whether or not the dead leaves with black spot are or are not a future source of infection. Even that seems to be still in controversy. I live in black spot central I think, almost all roses here seem to get it to some degree. I have had almost no luck with any of the above mentioned sprays. They may work in areas where black spot is only a slight problem, but not here. Now I have learned that I can seldom attribute any action I take to any event with my plants. It is so easy to assume that if I have black spot and I spray with something and the black spot goes away, then the spray must have worked. Too often it was just a climate change, of some unknown that actually destroyed the black spot, maybe even the plant itself resisted it. I was thinking about the possibility of using a vacuum to suck all the infected leaves off, avoiding the thorns and all the stooping, but that might seriously damage the plant. I was out picking off black spot infected leaves today, some of them I really had to tug on and jerk to get them off, but on one rose I think I could have shaken the bush and they all would just have fallen to the ground.
Now not long ago when black spot was just beginning to show and starting to look like a serious problem on several of the plants I used Bayer 3 in one rose granules, and now the new growth is coming on and the black spot is almost vanished. I used the product three years ago with the same effect, I used sprays the past two years and just ended up with burnt leaves with black spot. Baking soda and milk sprays were of no use at all.
I have tried to grow roses and vegetables organically, but obviously I am not knowledgeable or energetic enough to constantly battle disease and pests organically, so I have had to resort to using chemicals. Most organic solutions I have tried have totally failed. Organic solutions for squash bugs and borers and organic solutions for black spot require way more time and dedication to a few plants than I have. Slugs and snails, whiteflies and aphids I can get by with organic solutions most of the time. I am just saying that for me some battles need heavy artillery and others can be fought with lighter armament.
Name: Zuzu
Northern California (Zone 9a)
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zuzu
Apr 10, 2015 3:16 PM CST

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I doubt that there ever will be a consensus. Some gardening myths live on forever, in spite of all the scientific evidence disproving them.
Name: Elaine
South Sarasota, Florida (Zone 9b)
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dyzzypyxxy
Apr 10, 2015 5:39 PM CST
I wonder if the actual status of the leaf itself might confuse the issue even more. Angel What I mean is, what constitutes a "dead" leaf?

If a leaf drops but still has some moisture content, and/or green tissue, is it considered dead, or does it have to be brown, dry and dessicated? Seems like the leaves when they first drop might still have tissue that could support fungal growth, but once they're brown and really dead, they might not.

Think I'll still pick up all the dropped leaves to try to keep the fungus from spreading.
Elaine

"Success is stumbling from failure to failure with no loss of enthusiasm." –Winston Churchill
[Last edited by dyzzypyxxy - Apr 10, 2015 7:04 PM (+)]
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Name: Sue
Ontario, Canada (Zone 4a)
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sooby
Apr 10, 2015 5:49 PM CST
zuzu said:From a 2009 article by Gregg Lowery of Vintage Gardens:

"We are indebted to a scientific study done at the University of California at Berkeley in recent years. It showed that while green leaves can harbor fungal spores, once the leaves die, the fungi die on them.


Thanks, I now found that article. He doesn't, unfortunately, give the scientific reference (nor, I think, does he specifically refer to black spot unless I missed that). I'd really like to see the scientific study referred to, but I've so far been unable to find anything or even anything that agrees with the statement.

I have found several scientific studies that show black spot winters over on dead fallen leaves. This article: Interactions of four pathotypes of Diplocarpon rosae with species and hybrids of Rosa from the journal Plant Pathology -2009, says:

"During the rose growing season, conidia of D. rosae are dispersed by rain splash and animal vectors that include insects and arachnids. The mycelium spreads between the epidermis and cuticle of the upper leaf surface and forms haustoria in the cells of the epidermis and mesophyll. The fungus overwinters on stem tissues and in dead leaves (Cook, 1981). In spring, new infections are initiated from asexual conidia produced in acervuli and sexually produced ascospores formed in apothecia but the sexual stages have rarely been found (Knight & Wheeler, 1977)." (Cook 1981 says: "When overwintered at Wisley, Diplocarpon rosae from Silwood Park alone and not the local strain produced apothecia on fallen rose leaves."

An article on black spot in Descriptions of Fungi and Bacteria 1976 said: "TRANSMISSION: Mainly by splash dispersed conidia formed in acervuli on infected leaves on the host or after they have been cast. A microconidial state (spermagonia) with a similar dispersal mechanism may occur on fallen leaves in the spring and autumn. Conidia lose viability rapidly, few surviving more than one month (24, 508). Overwintering is by saprophytic mycelium in cast foliage or infected stem tissues. The perfect state has been reported from Britain, North America and the USSR (51, 2590 and IMI 185129), where it is formed on infected cast foliage in the spring. Ascospores are forcibly ejected; they do not appear to be essential for the survival of the pathogen."

Even if we assume that it overwinters as saprophytic mycelium in cast foliage, that's still a reason to pick up the dead leaves even if existing spores did not survive that long. As I said, I'd really like to see the study he's referring to or something else that supports it, still skeptical Smiling . Do you have any other references?








Name: Larry
Enterprise, Al. 36330 (Zone 8b)
Region: Alabama Composter Garden Photography Garden Ideas: Master Level Plant Identifier Celebrating Gardening: 2015
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Seedfork
Apr 10, 2015 5:52 PM CST
I think the best thing is to keep black spot under control, then you won't have to pull the leaves or bother to pick them all up...now if I can only figure out how to do that.
Name: Lyn
Weaverville, California (Zone 8a)
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RoseBlush1
Apr 10, 2015 6:01 PM CST
Sue ...

We have a horticultural representative from UC Davis up here and I discussed this with her last year and she confirmed that spores cannot live on dead leaves.

As I said in my post above, it depends on the rose and the race of bs you have in your garden.

Not all roses are susceptible to all of the five strains/races of bs in the United States. That's why a rose might be very disease prone in one climate and the same rose be totally clean in another climate. The bs race in the second climate is different than the one in the first garden. The second garden may have roses that are bs magnets, too, but it's a different race of bs.

Thanks, Zuzu, for finding Gregg's article. I was looking for it this morning.

I was also going to post that in normal years, I always mulch my garden in fall and in spring because I had read that this prevents any pathogen that may have overwintered in the garden debris from surviving to infect plants in the new growing season. Just covering the old mulch in spring will catch any leaves that have dropped after I mulched in fall, so I know that the bs in my garden does not come from the dead leaves below the rose.

For some fun reading about bs, here is a link from the Rose Hybridizers Association.

http://www.rosebreeders.org/forum/viewtopic.php?f=2&t=39246&...

You can get a sense of what I mean about bs races.

Smiles,
Lyn
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Name: Porkpal
Richmond, TX
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porkpal
Apr 10, 2015 6:01 PM CST
I prefer to think that there is nothing much I can do about it and just enjoy my tough old rose that may lose some leaves but bloom anyway.
Porkpal
Name: Zuzu
Northern California (Zone 9a)
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zuzu
Apr 10, 2015 7:03 PM CST

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That's the spirit, Porkpal! Almost every rose in my garden suffers from black spot after our rainy season in spring. Some of Ralph Moore's roses and Ping Lim's roses are immune and never exhibit any black spot. In spite of this, the immune roses are virtually indistinguishable from the susceptible roses at any other time of the year.

Black spot does not weaken my roses. The roses that suffer from black spot lose many leaves, if not all of their leaves in severe outbreaks, but the new growth following the outbreak is always pristine and perfect. The roses susceptible to black spot are just as healthy and vigorous as the immune ones, they produce just as many blooms, and they live just as long, so I've never felt the need to use chemical or organic remedies or to take any other extra measures. There is an aesthetic consideration, to be sure, but my only concern is the quality of the blooms, and black spot never affects the blooms.
Name: Larry
Enterprise, Al. 36330 (Zone 8b)
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Seedfork
Apr 10, 2015 7:09 PM CST
A pretty bloom on a plant covered with black spot, or naked of leaves is just not for me, the foliage needs to be as pretty as the bloom. It's like a pretty girl with bad teeth, just takes the pazaz out of it for me. Smiling
Name: Zuzu
Northern California (Zone 9a)
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zuzu
Apr 10, 2015 7:15 PM CST

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I see your point, but I'm a confirmed flower floozie, so I can easily look past anything else and focus only on the blooms.

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