Rose leaves are a integral part of a healthy plant. Many diseases and pests can be headed off by properly inspecting a leaf. When they are strong, good rose leaves can really make your flowers shine. They capture the sunlight and provide energy for big, strong blooms. So get into the habit of looking at the leaves - they can tell you a lot about the health of the plant.
Healthy leaves should alternate positions on the stem, and be a nice strong green color. Dropped leaves may indicate problems with the plant. You should also take care when handling leaflets. Some may have sharp edges, and a few breeds may even have a couple of prickles or thorns underneath.
Mildew. A common issue is mildew, which can be caused by excessive moisture on the leaves. Moisture combined with a damp or humid climate is a recipe for mildew. While you may not be able to avoid it altogether, you can at least take precautions to minimize it. Plant your rose where it gets 5-6hrs or direct sun every day, water the ground rather than the leaves, and do your watering early in the day so the plant has time to dry off.
Mosaic. Leaves with a yellow hue in the veins may be infected with mosaic. Unfortunately, there is little you can do as this is caused by using infected rootstock. It won't kill your plant, but it will result in less blooms, smaller flowers, and an overall lack of vigor. The disease often shows early in the growing season... just because it appears to improve over the summer, don't be fooled. The disease is systemic, and once a plant is infected, then it's infected for life. No amount of pruning of the infected areas will help.
The best thing you can do, is never take cuttings from the pant, and take care to check that you have certified virus free stock when buying new roses.
Blackspot. This fungal disease is one cause of yellowing leaves. Treatment is easy... pick off affected leaves, collect fallen ones and discard them (not in your compost pile as that will only spread the disease). Then treat the plant with a spray of copper, or any of the commercial preparations for blockspot available in your garden center.
Chlorosis. There are two other main causes of yellowing leaves. Lack of oxygen (caused by either poor draining soil or over watering), or a lack of nutrients. When I say "lack of nutrients" I don't mean for you to go out and start feeding the plants. It will (most likely ) be either a low ph, or a lack of iron. There are testing kits for these available at any good gardening center.
Remember, most roses tends to do better in a slightly acidic soil. 6 is neutral, so somewhere around 6.5 is ideal, though a little either way of that should be fine.
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