Black Spot thoughts

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.

 

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.

 

@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.

 

This is really interesting @Sandy Ground ...

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.

 

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."

 

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

 

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.

 

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

 

In spite of having sandy soil I have no success with roses at all.I have now, as a last resort planted climbing roses from David Austin roses ( expensive!) called Strawberry Hill.Any advice?
In the past roses have gone spindly and full of blackspot.

 

Thanks Amandii for the mention. As many gardeners may have done. Fallen leaves can be a benefit when simply dug in. Over time it became evident that digging in rose leaves, especially in rose beds, that a very noticeable increase of Black Spot often appeared in future seasons. So soil analysis was undertaken and the proof was right there. The problem infection lived on, despite having been buried. So this in a way meant that the rose bush was now planted within it's own supply of the dreaded Black Spot. The rest is history.

 

Starting with black spot. Once upon a time, I did have trouble with this disease. That was depite diligently removing infected leaves, picking those that had fallen up off the ground. Everything, including prunings, was put into black bin bags, and taken to the tip for incineration. It still returned. That, however, was then! Since I started putting down the thuja cuttings, thare has not been any signs of it since! So, in my opinion, that seems to be a cure for it. I do realise though that not everyone has access to thuja cuttings. To that end, I am aware that a company in the Netherlands has developed a liquid based on conifers that is supposed to keep Black Spot at bay. I dont know much about it, other than it has to be used twice a year. Once in Autumn, once in Spring. As things stand, I am under the impression that it cannot be imported to the UK.

@ARMANDII like you, I too have light sandy soil. Initially, I was going to disagree with you, as my roses do quite well. Then, I rememembered something. Back in 2006, I installed geothermic heating. The two bores for it were in the area now occupied by my roses. As the area was a mess afterwards, it was levelled off by using copious amounts of compost. I always have more of that than I need in a normal year! It got to 2010 before I planted the roses. Whether or not the compost was still doing any good, I have no idea. Obviously, now that I am giving the rose beds an annual dose of shredded thuja clippings, ie more organic material every year. In conclusion, I would say this. Roses dont need clay soil, just lots and lots of organic material added regularly.

 

Thank you so much for the very informative replies, I will take up all the suggestions and hope to have a thriving Strawberry Hill rose this year.It’s planted in the ground and being trained up a tripod.I will post a pic of it in Summer.No wonder my roses haven’t done well!No compost and very little feed.The pic of the rose is amazing Armandii, and it’s grown in a pot as well.
Mine hasn’t dropped many leaves this Winter, some are green and some are yellow with black spot, I will take off those yellow leaves today and put them to burn.
Roses are lovely but do seem to need a lot of time and care, but if SH rose does well, I shall be encouraged to try more.