I have the same type of damage/lesions occurring on two different roses. One is La Reine and the other Mme. Louise Odier. I am in Zone 5B and both plants have been indoors (kept in a sunroom) since I rec'd them. I plan to plant them outside this year. Both roses were purchased from Heirloom Roses late last summer (2007). Is this blackspot or something else? Please see attached pics.
Hard to say. The spots on the leaflet in the picture on the left, and the leaflet in the upper part of the right picture looks like blackspots but are probably not. Black spot lesions are actually black, round lesions, with a central necrotic centre which tends light brown when it dries up. There is usually a rim of yellow around the lesion. This yellow region grows larger until the whole leaf turns yellow and drops off. The brown patchs on the lower leaflets in the picture on the right are not blackspots - they are brown, symmetrically distributed with sparing of the leaves adjacent to the ribs. Black spot does not spare any parts of the leaves. These lesions could also be due to leaf burns - either by unaccustomed direct sunlight, excessive concentration of "salts" in the soil, inefficient watering, or even drafts.
Wow, all of those pretty much apply -- yikes!! I guess I should start collecting rainwater (actually I can start today -- darn weather!). These 2 roses are in smallish pots (8" dia.), and I water them with tap water, so there could definitely be a mineral accumulation problem. I didn't even think of that before.
The succulents in the pot with it may indicate you are keeping it too lean and dry. Garden roses love fertility and moisture. Old garden roses specifically are liable to be infested with viruses that may make it necessary to provide additional stimulation to get strong growth. From what I can see here I would first guess your plant is sun-burned. It is also possible it is being discolored by a mite infestation, although usually when you get to the point of yellowing and browning there has been plenty of the characteristic stippling (areas of fine yellowish dots) beforehand. You might want to take a lens and inspect the leaf undersides for these anyway.
Thanks Ron, I will check the undersides tomorrow but I suspect you are right on with the water and sunburn. We had a long gray winter, not particularly cold, but very little sun, and then in the last 2 weeks or so we've gotten lots of sun and these plants are getting hit directly with it. I thought that was good for roses, but these ones in particular might be having a hard time adjusting. (The leaves were uniformly green until the past week.) As for the water, I've been watering them once every 3-4 days. I usually do it when I notice the top of the soil has dried out. I put the succulents in there to act as a mulch, but they haven't spread out enough to be any help yet.
When I referred to salt build up, it's not so much from tap water than from fertiliser applications. If those leaves developed out of direct sunlight, and are suddenly subjected to direct sunlight, then sun burn could be the problem. In which case, you do not have to do anything - new leaves more adapted to the direct sun will develop and the damage will drop off. One way of looking for mites is to hold a piece of dark coloured paper (black, if you have one) under the affected leaves, and give the leaves a good shake. You should see the tiny creatures scurrying around on the paper.
