Despite their delicate appearance, roses are generally hardy plants. Sometimes, however, even with the best growing conditions, yellow leaves can appear on the rose. Several things can cause this discoloration of the leaves and, with the exception of one disease, early identification and treatment will help the rose recover.
Causes and Symptoms
Overwatering can cause light yellow leaves on a rose. Generally, these leaves will appear on the lower portion of the plant.
If the yellow leaf has circular black spots on it, this could be the disease, black spot.
If there is yellow speckling on the leaf’s surface, according to Clemson University, it is an early sign that spider mites might be attacking the rose. The undersides of the leaves will have fine webbing.
A yellowish veining or variegated look on the leaf could be a mosaic disease.
Over watering reduces the oxygen in the soil, which weakens the rose. It will lead to soggy roots, which sets up the conditions for more diseases.
Left untreated, black spot will move from the leaves to the stems and then to the main canes of the rose bush. The disease will eventually kill the plant.
Spider mites will infest the whole rose bush and, if not treated, can destroy the whole plant.
Mosaic virus not only destroys the rose, it can spread to nearby flowers.
Allow the soil around the roses to dry out before watering again. To determine when the ground is dry, stick a finger an inch deep into the soil to tell how moist the ground around the roots is. During periods of drought or intense heat, if the rose has drooping leaves between waterings, it needs more water.
Remove all leaves infected with black spot from the rose bush and pick up any fallen rose leaves and petals. Spray the entire rose bush with a fungicide.
If the spider mite infestation is small, Clemson University recommends a strong spray of water; be careful to get both sides of the leaves. This removes the spider mite adult, nymph, larvae and eggs. If the infestation is pervasive, use insecticidal soaps or horticultural oils. During periods of drought, these soaps and oils can stress the rose bush. Water the plants well before treating for spider mites and apply the soaps or oils when the temperature is less than 85 degrees F.
Mosaic virus cannot be treated. Remove the infected rose bush and throw it away; do not add it to the compost.
Give roses one inch of water once a week. Since watering systems vary, there is a simple means to determine how much water is needed to make one inch. Measure and mark one inch on an empty tuna can and place it next to the rose. Turn on the water sprinkler and set a timer for half an hour; then check to see how much water is in the can. Continue watering until it has reached one inch.
Keep the area under the rose cleaned up from fallen rose leaves and petals, which creates conditions not only for black spot, but also for other diseases and pests.
Thoroughly spray the rose bush with water twice a week to prevent spider mites, according to the Santa Clarita Valley Rose Society.
Since the mosaic virus is spread from one plant to another, practicing sanitary gardening methods can reduce the occurrence. Purchase roses from a reputable nursery, clean gardening tools after each use and sanitize them by dipping in a solution of 10 percent alcohol and 90 percent water.
Early morning watering is better than later in the day. The midday summer heat can scorch the freshly watered rose bush. Late afternoon or early evening watering prevents the water on the rose bush from drying, which makes the bush susceptible to disease.
Using broad scale pesticides to treat for spider mites can also destroy beneficial insects, such as ladybug beetles. Check with the local agriculture extension agent for specific products to use.