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Black Spot thoughts

Discussion in 'Roses' started by Sandy Ground, Oct 9, 2019.

  1. Sandy Ground

    Sandy Ground Total Gardener

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    As some of you are aware, part of my garden is a rosarium. This is divided into eight separate beds. Four larger, four smaller. Like many, I have been troubled by Blackspot. As I do not like to use chemicals of any sort, I've treated it in the recommended way. Remove affected leaves both from the plant and ground, and burn. Even so, it usually returned the following year.

    Going back to Autumn 2017, I spread some thuja cuttings as mulch on these borders. During the summer of 2018, I noticed that the black spot did not for once appear. That is despite the fact that summer should have been perfect for it to grow. Hot, dry days, and cool moist nights. A bit of online research came up with the fact that thuja contain what is known as a parasitic fungus that is known to feed of the fungus that causes black spot.

    In Autumn last year, I decided to conduct a small experiment. I again used cuttings on the four larger borders, but omitted to use them on the four smaller ones. The result? None at all on the "treated" borders, but plenty on the untreated ones.

    As I'm currently giving my thuja hedges there Autumn trim, this year I'll treat all of them, just to see if the results are the same.

    A few thoughts occur. How will the annual addition of cuttings affect the ground? My guess is that it will lower the pH. Will this do any harm to the roses? They've been thriving in a pH of 7,5, but is that a normal level?

    I do have some more to add, but will see what our resident experts have to say.
     
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    • noisette47

      noisette47 Total Gardener

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      Hi Sandy, not just a change in PH, but I suspect that eventually the soil will become deficient in nitrogen, as the decomposition of woody materiel depletes it.
      You could try sprinkling sulphate of ammonia or pelleted chicken manure around before adding the prunings. I think sulphate of iron will increase the acidity.
       
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      • wiseowl

        wiseowl Friendly Owl ADMIN Staff Member

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        Good afternoon @Sandy Ground most Roses like a PH of of between 6 and 7,but as I am of the old school rose grower ,my honest opinion is that a good rose will thrive in almost any PH soil if it is cared for correctly. I think that your observations on the Thuja mulch are excellent and they should lower the PH over time as you suggested,but in my limited experience the spores of blackspot can be spread from our clothes,gloves ,splashed and spread by the rain,and remain in the soil all winter,in fact all most any part of us,but please carry on with your experiment I for one will be very interested in your findings my friend,and for what its worth I don't think it will cause any harm to your beautiful Roses my friend:smile::blue thumb:

        A pH of 7.0 is considered neutral. An acid soil has a pH value below 7.0 and above 7.0 the soil is alkaline.
         
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        • Sandy Ground

          Sandy Ground Total Gardener

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          @noisette47 I'm not to worried about nitrogen depletion. My local rose breeder supplies me with an 8-3-4 fertiliser that according to him "should be put down when leaves on the birch are as big as the ears on a mouse" and then again after the first bloom period is ended.

          @wiseowl I will keep you informed as to the results, and will be continuing with the experiment. As things stand, I've just finished giving my thuja hedges their "autumn haircut" and have started shredding the cuttings to put on the beds. This year, it will be all of them. I was going to ask if the spores how the spores moved around. Now, I know.

          I did check the pH earlier. Its still 7,5, the same value as in Spring. I'll check again around next Easter.
           
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          • lolimac

            lolimac Super Gardener

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            This is really interesting @Sandy Ground :thumbsup:...

            I've tried many a way to obliterate Black spot and I notice with my Roses the ones I nurture in my little parterre that get fed and mulched from early spring every 6 weeks give or take ..have flourished where as others dotted around the garden amongst other plants where I can't mulch as much as I'd like do succumb ..the answer is in the mulch as @wiseowl says...it's the spores in the soil around them that do the damage.:dbgrtmb:
             
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            • Sandy Ground

              Sandy Ground Total Gardener

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              Something that would be interesting is that others that have both thuja and roses in their garden try this and get either the same or a different result. If the same, it would help prove things. If different, why is the result different?

              I've got so many questions about all this now. With luck I should be able to answer them, especially with the help of "GC Power." :)
               
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              • Sheal

                Sheal Total Gardener

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                It's an interesting idea Sandy, let's hope your experiment with the other rose beds has the same result. :blue thumb: Feeding the roses/soil if necessary will be a lot easier than fighting black spot.
                 
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                • Mike Allen

                  Mike Allen Total Gardener

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                  Hello Sandy. May I compliment you upon your experiments and trials. Like you. I am a rosarian in my own rights. Also like you, I have a certain questionable attitude via chemicals in the garden. However as an oldie....long retired, and plant pathologist. I find your contribution very interesting. Personally I'd like to further the intererest with you further. Mike.
                   
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                  • Sandy Ground

                    Sandy Ground Total Gardener

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                    By all means @Mike Allen I will welcome any and all input.
                     
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