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Questions, Answers and Comments by Category
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Can you please tell me what disease this is and if i am dealing with one or two diseases at the same time? The symptoms seem to point to downy mildew and stem canker. But I have never encountered a description of the white circles which always appears on the sick canes. They drive me crazy. Just a few, and not grouped together. I can scrape them off with my fingers easily. Then the cane becomes yellow and develops cankers. At first, the leaves might not show symptoms, or they have different kind of symptoms like in the pics. It affects all canes, old and young. Oh, and sometimes the base of the leaflet develops a dark growth around it. I think the problem has started with a diseased potted rose I have brought into the garden last year. The symptoms appeared by the end of july and august. I had to cut most roses to the ground, then sprayed with fungicide. In spring, they rebounded nicely and I was happy, but now the same thing is happening (at the same time as last year) and the decline is very fast. Have I been blessed with the two most dreaded diseases at the same time? It looks like I will lose all my roses to this and it's heartbreaking. Almost all the roses are affected, in different parts of the garden.
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#1 of 3 posted
1 AUG by
 Johno
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Based on your photos the small white dots I assume is scale which is easily treated by eco-oil or whatever the equivalent product is for your country. The leaves are showing Black Spot (Photos 3, 4, 8) and the last image is what I would expect to see with Spider Mite. Spider mite number increase rapidly following warm weather.
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Thank you so much for replying! I thought about scale too, but the puzzling thing is that the white dots appear singly on the canes, sometimes 2 or 3, but often at great distances one from another. And there is always a canker, spiral streaks or a swelling of the cane in their proximity, it can't be just a coincidence! I think I found the answer, though: I think the white dots are the eggs of the rose stem girdlers, Agrilus Cuprescens. It seems I have quite an infestation :-( And I think I managed to find the larvae in one of the swellings. It seems there is nothing to be done but destroy all the canes, which means I will lose more roses because my roses are all young and they don't have many canes to begin with :-(
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#3 of 3 posted
6 AUG by
Johno
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Luckily the beetle is not present in this country. I had to google it to find out more information. Control is only possible at the adult beetle stage with insecticides. Have you seen evidence of the beetle and its leaf chewing habit? I can only suggest cutting back your roses to healthy growth, check the end of the stem for larva presence, burn cuttings, and apply insecticide when the adult beetle is present. You may have to check your roses daily. I found this fact sheet from Utah State University to be useful:
Google - digitalcommons.usu.edu/extension_curall/773/
Maybe one of our American members with experience of the Rose Stem Girdler beetle could comment.
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