With all the wet weather Sydney has been experiencing lately, it has provided perfect humid conditions for the development of black spot on roses. I recently got this question from a reader and thought I would answer it here.
Q: have found black spot on my roses and have cut all the bad leaves off, now some one told me that soapy water will help this and affes can you please tell me pretty new at the game but love my roses.
A: The soapy water will help with aphid control but I have never heard of it helping with black spot.
You’re doing the right thing by removing the badly diseased leaves. Also be sure to pick up any fallen diseased leaves around the base of the plant. Unfortunately you usually need to use a fungicide to manage black spot. If you don’t want to use a fungicide then you could try this mix which a lot of organic gardeners use with some success, but you need to apply it weekly.
In a six litre sprayer put 4 level tspns bicarb soda, and 3 tspns of fish emulsion, fill it with water to the six litre mark. Agitate the sprayer as much as you can to dissolve the bicard soda and spray the underside of the leaves as well as the top of them.
I usually use this mix one week, then a fungicide the next week, but that’s for sixty roses all planted in the one area. If you’ve only got a few roses you might be able to get away with just using the organic mix weekly, and you might also want to adjust the measurements accordingly and use a small spray bottle if it’s just a few plants. Don’t spray this on a really hot day. This mix. along with good garden hygiene should help you manage black spot on roses.