The Leaves of My Knockout Roses Are Yellowing & Falling Off

Pests

• Yellowed, falling leaves can be a sign of spider mites when accompanied by webbing. Spider mites have many natural predators, including ladybugs, predatory mites, minute pirate bugs and predatory thrips. One reason for the increase in the spider mite population is that many of the natural predators have been killed off by broad-spectrum pesticide usage. Wash leaves with insecticidal soap or neem oil to reduce spider mite populations.

Environmental Factors

• The leaves of a rose bush can turn yellow and drop because of heat stress. Radiant heat also is a common problem for roses. Radiant heat is the reflection of too much heat from the ground or foliage surface. Causes include dark and rock mulches or over-saturated leaves. Shredded cedar mulch and careful watering can reduce radiant heat stress. Keep rose bushes moist but not soaking wet; water the base of the plant only.

Black Spot

• Black spot (Diplocarpon rosae) is a fungus that begins with black spots on the leaves, then the leaves begin to turn yellow and drop to the ground. As the disease spreads, it defoliates the entire bush. If black spot is found on roses, prune off the damaged parts of the plant and burn them. Do not put diseased branches in the compost pile. After pruning the diseased parts of the plant, treat the rest of the bush several times with a fungicidal soap or sulfur. The Knockout rose series is resistant to black spot, so the yellowing and dropping leaves are more likely to be a sign of pests or environmental stress.

Preventive Measures

• Prevention is the best way to avoid trouble with rose bushes. Proper spacing is important for good air circulation. Plant smaller bushes three feet apart, and larger ones four feet. Light-colored mulch will decrease the amount of watering needed, solve radiant heat problems and deter competitive weeds. Treat the rose bush with sulfur in the spring to prevent fungi. Fungus spores cannot thrive in a sulfuric environment. Always water at the base of the plant to prevent reflection of water from the leaves and to avoid creating a fungus-friendly environment.