What Is Causing Yellow Leaves on My Rosebush?

Insects

• Insects such as aphids, thrips and spider mites cause a rose bush's foliage to turn yellow. The tiny pests suck sap from the leaves with their piercing mouthparts, causing wilted and misshaped foliage. Large spider mite infestations cause webbing over the plant and premature leaf drop. Thrips are slightly difficult to control, requiring timely applications of soaps or insecticides every seven to 10 days. Spraying an infested rose bush with water from a hose removes aphids, mites and their eggs. However, a miticide may be necessary in cases of large mite infestations.

Diseases

• Fungal diseases such as powdery mildew and black spot cause a rose bush's leaves to turn yellow. Powdery mildew occurs in warm and dry climates with high levels of humidity, and causes different symptoms including distorted leaf growth, white powder on leaves and discoloration. Typically occurring in wet climates, black spot disease causes infected leaves to turn yellow and appear spotted. Treat them with regular applications of a registered fungicide according to label directions.

Nutrient Deficiency

• Nitrogen deficiency causes lower leaves to turn yellow, while magnesium deficiency causes yellowing between leaf veins. Long-term planting, low pH, crop rotation and heavy rains deplete nitrogen in the soil. To test the soil for the nutrients it lacks, mix soil samples collected from parts of the garden bed in a clean bin or bag. Let one cup of the soil dry then mix it with water and powder from a soil test kit. Compare the water's color to the pH value chart to determine the nutrients it lacks. Apply a magnesium- or nitrogen-rich fertilizer every six to eight weeks to help the foliage regain its natural color and vitality.

Improper Irrigation

• Excessive amounts of water causes root rot, preventing the roots from absorbing soil moisture and increasing plant stress. Conversely, insufficient water causes the bush to dehydrate and shrivel up, especially if exposed to direct sunlight, or during prolonged periods of drought. Ideally, water the rose bush deeply but infrequently, preferably two to three times a week for 30 minutes. Providing 1 to 2 inches of water encourages it to thrive and produce healthy foliage.