Yellow leaves and brown spots on roses may be caused by pests, nutrient deficiency, environmental factors and diseases. Yellowing leaves may also be caused by excess water. Brown and black spots may be caused by fungus. Too much rain, irrigation or poor drainage can lead to yellow leaves. There are multiple reasons for yellow leaves and brown spots on roses, and treatment must be started after properly diagnosing the cause of the problem.
According to the Olympian Rose Society, the growth of fungus on plant leaves can cause brown spots and yellowing leaves. The spots start at the bottom of the plant and work their way upward. Sometimes, removing the spotted leaves helps to stop the spread. Too much moisture may cause the fungus to spread. Watering early in the day and letting the leaves dry completely in the sun prevent the spread of the fungus.
Heat stress is one reason for the yellowing of leaves. When spring turns into summer and the weather turns from cool to hot, the sun's heat scorches the leaves, therefore turning them yellow. This type of yellowing usually occurs on new growth or in new plants. Symptoms of the scorching that occur along with the yellow leaves include scorching leaf margins, misshaped blooms and darker petal edges. Cooler weather, moisture and an established root system can help combat the symptoms.
Salt stress and clogged pores
Salt stress and clogged pores are also factors that cause yellow leaves or brown spots. Salt stress and clogged pores may come from using too much fertiliser or chemical sprays on the rose plants. The excess salt in the fertilisers build up in the soil until they suck all the water away from the plant. The lack of water causes yellow leaves and potentially kills the plant. Clogged pores cause the plant to shut down from an inability to produce chlorophyll. In addition to yellow leaves, symptoms of salt stress or clogged pores include reduced plant growth and scorched leaves.
Too much cultivation
According to Garden Grapevine Magazine, cultivating too closely to root systems causes yellow leaves or brown spots as well. Often, root systems grow very close to the surface and can become damaged even in routine weeding. These damaged root systems cause stress for the rose plants that can result in yellow leaves and brown spots. Check carefully for exposed roots, and replant if roots are sticking out of the ground.
To prevent yellow leaves and brown spots, use sunscreens on new plants or plant the new roses where they will be protected from the sun when it is hottest in the afternoon. Use lots of mulch around the root areas of the plants. Spray the plants with water when the sun is hottest to help reduce temperatures for the plants. Improving the plants drainage system, reducing the amount of water the plant receives in wet conditions, and increasing the water in dry conditions will also be helpful to prevent the yellow leaves. Also, avoid using too much fertiliser or pesticides on the rose plants.