Answer: The white powdery substance on the leaves is probably a fungal disease called powdery mildew. It appears in several ways. The most common is a gray-white powdery dusting on the leaf surfaces. The actual color ranges from a white to brownish-white (almost a tan color) and there are few other problems that appear to be similar in the garden. If you see this dusting, it is almost 100% sure you have powdery mildew.
The real tipoff to this problem is when your young leaves start to curl and twist as they develop and do not fully unfurl. Roses twist the entire new shoot. Other plants simply twist the leaves.
Older leaves are pretty much immune to this twisting and usually don?t show any other signs other than a dusting or small spots where the outbreak is severe. The will also brown off once the mildew has developed.
Leaves are usually attacked on the lower surface first and then the mildew moves around to the top of the leaf. If you?re in the habit of turning over leaves, you might notice a small, raised blister on the leaf surface (you have to be looking pretty carefully and regularly) and some slight purple mottling with leaf edge curling. The white powder develops after that and will eventually cover the entire leaf.
Sodium bicarbonate (baking soda) is recommended by many gardeners and when it is mixed at the rate of 1 teaspoon to a quart of water (add a small dash of liquid soap as a wetting agent).
There are also products on the garden center shelves featuring jojoba oil and neem oil. I can?t speak to these but some gardeners swear by their effectivness for controlling powdery mildew.
I don't know what bug is feasting on your plants so I don't really have a recommendation for controlling it. Try going out at different times of the day, including early evening, to see if you can capture a few of the pests. If so, put them in a plastic baggie and take it to your local garden center for identification. Helpful folks there can make recommendations for control.
Best wishes with your roses.
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