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Will some roses develop resistance to black spot if not sprayed?

Posted by roselee z8 SW Texas (My Page) on
Fri, Feb 4, 11 at 14:50

I stopped using fungicides and insecticides several years ago and have been surprised at how healthy my roses are now. Beneficial insects must be keeping things like thrips and spidermites under control as I've had none that I can detect.

Nor have the roses been affected by enough black spot to speak of. This could be because I have gotten rid of the most susceptable varieties, but I seem to remember Michaelg, and maybe Henry K., saying that their roses seemed to have developed their own resistance after a few years of not spraying fungicides.

This year I'm bringing in several new (for me) roses which are hopefully naturally resistant varities, but am wondering if some of them get more black spot than I'm willing to tolerate if by just waiting it out they may eventually acquire resistance as my other roses seem to have done.

Perhaps 'aquiring' resistance is not what happens, but rather beneficial fungi balance out the disease causing fungi.

I don't post much, but read most of what is posted here so I feel like I know you all and am looking forward to any discussion on this subject.

Follow-Up Postings:

RE: Will some roses develop resistance to black spot if not spray

  • Posted by hoovb z9 Southern CA (My Page) on
    Fri, Feb 4, 11 at 15:58

Or a strong, well-established plant with a large root system and lots of carbohydrates stored up in lots of canes can better regulate foliage moisture, and thus fight off infections better than a baby plant?

RE: Will some roses develop resistance to black spot if not spray

  • Posted by bboy USDA 8 Sunset 5 WA (My Page) on
    Fri, Feb 4, 11 at 16:42

I do see black spot on potted roses at outlets, that is not so bad on the same kinds in the ground. And I've seen mention of a supposed link between infestations of the three major leaf fungi and specific soil minerals - think with black spot it was supposed to be potassium that needed to be tweaked. You can probably find out doing a web search.

Otherwise, where site conditions remained the same, if anything a particular specimen might become more heavily infested as time went on - due fungus strains being able to mutate rapidly, become more virulent.

RE: Will some roses develop resistance to black spot if not spray

Oddly enough, I just wrote a blog on a book that discusses some of these resposes, particularly regarding soil fungi. See the link for more info.

Here is a link that might be useful: Garden Musings blog on book

RE: Will some roses develop resistance to black spot if not spray

Plants have immune systems. An active present research area is how to optimize the plant immune system. A 2007 paper on this subject concerning blackspot on roses is at the link in the "Here is a link that might be useful": box,

Notice the word "biocontrol". Another subarea under the general term biocontrol is to allow the friendly fungi to reach equilibrium with the blackspot. The following may be useful in putting this area of biocontrol into layman terms:

"Disease-suppressive soil microorganisms have been found in many places. In monoculture wheat the severity of "take all" disease often decreases within three to five years. This phenomenon is known as "take all decline," and is considered an effective natural control. Although the mechanisms are not completely understood, the decline is associated with changes in soil microorganisms that compete with and prey on the fungus. Melon plants grown in the Chateaurenard region of France do not show Fusarium wilt symptoms even though the fungus is present in the soil. Soils with suppressive characteristics tend to develop slowly and are usually found in fields where perennial crops or monocultures have been grown for many years.
Suppressiveness may be lost if the monoculture is interrupted even for one year, or if pesticides are applied. For example, researchers first recognized soils suppressive to cereal-cyst nematode when nematode numbers increased after application of a broad-spectrum biocide. Many species of fungi and bacteria in the genera Trichoderma, Streptomyces, Bacillus and Pseudomonas suppress diseases, but at this time only a few strains are commercially available. Additional commercial products may be available soon, however, as this is currently an active research area."

The quote was taken from:


Here is a link that might be useful: 2007 research paper

RE: Will some roses develop resistance to black spot if not spray

IMO absolutely yes. I stopped spraying some 15 years ago and every year my roses get better and better. New ones get some disease but as they get older and more established they get less. When I first stopped spraying everyone got it but the older ones sorted themselves out quickly. I grow some 400 roses from Antique to Modern.

Now that being said I do believe there are some varieties that will never do this. Likely a combination of a particular climate and/or the variety not being inherently healthy to begin with.


Here is a link that might be useful: Here is a blog post I did about it.

RE: Will some roses develop resistance to black spot if not spray

I heartily endorse what's been said thus far because my own experience has lead me to the theory that just as predatory animals are observed to feed preferentially on the weak or defenseless so too the composters of the herbaceous.
I first observed the phenomenon on my fig tree. Its foliage is lush and green until about a week to two before autumnal leaf drop. In those last weeks the leaves are covered in rust.
I planted an OGR Tea which completely defoliated with black spot early in its first season. Subsequently, as with my other roses, I observe that only the old foliage about to be shed gets black spot.
Most of the roses I've planted developed black spot to some extent before and then again soon after planting. I believe the before planting development is due to the plant's stressed state from its commercial existence. And the latter I attribute to what used to be called transplant shock.
I now view black spot as part of the natural cycle. So much so that I don't fearfully remove the dropped foliage. I compost it in place as would occur naturally in the wild to no ill effect.
I am constantly adding organic material to the beds so I am convinced that soil health is critical to plant health. And I believe fungi are soil creators and therefore good indicators of soil development.
There's no need to buy fungi usually. If you build it they will come. They will come along in your compost, composted manures, and mulches.

RE: Will some roses develop resistance to black spot if not spray

has led me to the theory (ed)

RE: Will some roses develop resistance to black spot if not spray

Thank you ALL for your excellent informative comments and experiences! The experiences match my own and there is also science to back them up. I'm very much encouraged in the practice of growing roses organically!

RE: Will some roses develop resistance to black spot if not spray

i had read, and the information could have been wrong, that pine needles under a rose bush prevents black spot. anyone care to comment?

i do like the idea to leave it alone and they will adjust

RE: Will some roses develop resistance to black spot if not spray

I am no expert so take my thoughts for what they

I use pine needles under most all of my roses. I think I have around 70. I don't spray. I have selected a lot of roses that are BS resistant, but I also grow roses that do get some BS.

I have two observations on pine needles.

1. During rain or watering, you tend to get less "backsplash" on the roses from water bouncing off the dirt and back up on the rose. Perhaps this is effective in some way.

2. I always use thick pine needle layers under my roses. Last year, I was so immersed in my graduate papers that I didn't get the pine needles down until mid summer and then could only do a light refresh since I only had the use of a pick-up truck for one hour (this equated to one truck load of pine bales--I usually use 3-4 truck loads). I had RRD this year. I had it on the roses that we low growing or had branches touching the ground. I can't help but wonder if this made it easier for the mites to get on, stay on, or crawl on my roses. I never had it before, and the ONE year I don't use thick pine straw, I lose 5-6 roses. Clearly this is not a scientific study, but I won't test the theory again.

Thinking of getting a semi to bring in the pine straw this year ;o) Any of you drive a flat bed?? j/k


RE: Will some roses develop resistance to black spot if not spray

My 2 cents-
I stopped spraying 4 years ago. Some of my roses went on to become big beautiful plants. Other roses got BS to the point that they were eventually so weak that they died. It was mostly moderns that kicked the bucket. Some of Austins suffer pretty badly, but most live through it. In all, though, I probably lost half of the 70+ roses that went into my first rose garden, because it was a good mix of antiques and moderns. So no, the disease suceptible ones didn't develop a resistance, not even in my good, ammended soil.

RE: Will some roses develop resistance to black spot if not spray

I haven't sprayed my roses in many years, and sprayed rather infrequently back when I sprayed at all. When I stopped completely, many of my roses started weakening and declining, a good number of Hybrid Perpetuals and some Austins and Bourbons. I have noticed; however, that some roses that were spotty when they were younger developed better resistance and vigor as they matured. They still lost some of their lower leaves, but that didn't markedly affect the overall health of the rose.

I think blackspot resistance is a combination of genetics, culture, and plant maturity. If the genetics of a rose are particularly susceptible, only spraying fungicides will keep it from declining. For roses that are only moderately susceptible, the culture and age can make a difference. That's why I usually give my roses several years to get established before deciding to shovel-prune them or not.

RE: Will some roses develop resistance to black spot if not spray

I have no experience with roses developing BS resistance, but this may be because I don't have the patience to let it happen. I have had the experience of well established roses that were resistant becoming susceptible, almost certainly because a new strain of BS reached them.

However, I have two plants with verticillium wilt disease which apparently came from the vendor's fields--at least, the disease showed in their first season. I let the roses stay because I assumed they had already contaminated the soil in that part of the garden. They lost many canes for the first few years, but eventually shook it off. They became large, vigorous plants that haven't showed symptoms in at least five years.

Immune responses cost the plant energy. It seems likely that a well-established plant can more afford this expense, or perhaps it has learned from experience that it needs to make the investment.

(Pardon the anthropomorphism.)

RE: Will some roses develop resistance to black spot if not spray

From the original post: "This could be because I have gotten rid of most of the susceptible varieties..."
Because this seemed to be already stipulated, I didn't say that I too have planted only roses that are reported to be both suitable to my climate and disease resistant.

In another of my favorite threads, someone brought up the fact that some roses in certain climate zones are DAWGs. Of course, a dawg is a dawg, but I think it also works as Definitely Ain't Worth Growing. Maybe some roses are DAWGs in any climate.

I do my research before purchase.

About pine needles/straw: Someone wise recommended using a local product for mulch. It turns out that many fungi have developed specialized symbiotic relationships to specific hosts. Truffles are a great example. So for those of us in the southern pine forest region, pine needles should in theory be top notch because the associated fungi should be abundantly available.

RE: Will some roses develop resistance to black spot if not spray

  • Posted by bboy USDA 8 Sunset 5 WA (My Page) on
    Fri, Feb 18, 11 at 1:20

Note that Henry was talking about soil conditions changing in a way that suppressed disease fungi after spraying was discontinued. This is the pathogens being diminished, not the plants becoming more resistant.

RE: Will some roses develop resistance to black spot if not spray

Again -- thank you for all the replies!

Perhaps the question would have been better expressed as, "Will some gardens develop resistance to black spot if the roses are not sprayed?" While it is of maximum importance to choose our roses for their natural immunity if we don't intend to spray, research that Henry K. sites say that if good fungi is allowed to live in a garden an equilibrium with disease causing fungi will be reached so that the disease causing fungi is repressed.

Paul Zimmerman says in his blog post linked above that the plants themselves develop resistance if exposed to a wide range of blackspot strains. Other posters collaborate that this has been their experience.

It seems plants are more active in resisting attack than we might think. It's been scientifically shown that if a tree is coming under attack from caterpillars, or browsing, then that tree, and all the trees in the vicinity, will begin to pump out poisons in their leaves to combat the attack. So perhaps other plants like roses will do something similar in response to leaf diseases if we give them a chance. Of course well nourished, well sited, and well chosen roses are better equipped to do this so I'm learning that there's lot of things to be taken into account in keeping our gardens looking good. It will be interesting to see what scientific observations will reveal in this arena of research in the years to come.

Much appreciation to you all for your experiences and input on this subject. It's great to be able to pose questions and receive your excellent replies.

RE: Will some roses develop resistance to black spot if not spray

Plants have an immune system. I would expect that an "organic" rose (not being grown in soil that resembles a chemical dump) will be better able to resist diseases.

There are research papers on the effects of "growing organically" that detect differences in the amounts of certain chemicals that could be involved in the pathways that are immune system related. "•The organic strawberries had significantly higher antioxidant activity and concentrations of ascorbic acid and phenolic compounds."

"Side-by-side comparisons of organic and conventional strawberry farms and their fruit found the organic farms produced more flavorful and nutritious berries while leaving the soil healthier and more genetically diverse."
"Our findings have global implications and advance what we know about the sustainability benefits of organic farming systems," said John Reganold, Washington State University Regents professor of soil science and lead author of a paper published today in the peer-reviewed online journal, PLoS ONE. "We also show you can have high quality, healthy produce without resorting to an arsenal of pesticides."

The above quotes are from the link below.

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

RE: Supporting research Will some roses develop resistance to bl

A 2011 USDA Beltsville, MD scientific paper supports the findings of the strawberry paper presented earlier in this thread.

Title:Effect of cultural system and storage temperature on antioxidant capacity and phenolic compounds in strawberries

Authors: Peng Jin a,b, Shiow Y. Wang c, Chien Y. Wanga,*, Yonghua Zheng b

Authors affiliation: a Food Quality Laboratory, USDA-ARS, Beltsville, MD 20705, USA
b College of Food Science and Technology, Nanjing Agricultural University, Nanjing 210095, PR China
c Genetic Improvement of Fruits and Vegetables Laboratory, USDA-ARS, Beltsville, MD 20705, USA

Abstract: "The effects of cultural systems and storage temperatures on antioxidant enzyme activities and non-enzyme antioxidant components in two cultivars (‘Earliglow’ and ‘Allstar’) of strawberries were investigated. Fruit samples were hand-harvested from organic and conventional farms in Maryland, USA, and were stored at 10, 5 and 0 °C. The results from this study showed that strawberries grown from organic culture exhibited generally higher activities in antioxidant enzymes. Moreover, the organic culture also produced fruits with higher level of antioxidant contents. Strawberries stored at higher temperature (10 °C) had higher activities of antioxidant enzymes and antioxidant capacities than those stored at lower temperatures (0 or 5 °C), in both organic and conventional cultural systems. In conclusion, strawberries produced from organic culture contained significantly higher antioxidant capacities and flavonoid contents than those produced from conventional culture, and even though low storage temperatures retarded decay, they also reduced the increase in antioxidant activities."

Here is a link that might be useful: USDA paper

RE: Will some roses develop resistance to black spot if not spray

J. Jennings mentioned that Old Garden Tea rosebushes show an increased immunity to disease when mature.
I documented a couple dozen Tea culitvars over 2 years, both band sized and mature in the same location, where they received the same cultivation means and methods,
and I believe this to be true; Old Garden Tea roses become more resistant to blackspot and powdery mildew as their immune systems became stronger with age.


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