Red Spots on Rose Stems

Black Spot

• Black spot disease begins as black, fuzzy spots on rose leaves, according to Bob Bauer at Roses and Everything About Them, but when it spreads to the canes, dark red spots appear. This rose disease is caused by the Diplocarpon rosae fungus and is most common in humid areas like Florida. Black spot spreads quickly and kills any of the leaves it infects after turning them yellow. Fast pruning may save the plant, and antifungal sprays should be applied to any unaffected areas.

Stem Canker

• Roses require pruning, but each cut made creates an open wound on the plant. These wounds are susceptible to fungal infections like stem canker. Stem canker, also known as brown canker, forms as small red spots that cover all parts of the plant including the stem of a flower, according to the Ohio State University Extension Office. These cankers turn brown or black as time passes and the infection worsens. Stem canker requires heavy pruning of infected plant material and an antifungal spray.

Rust

• Another fungus that likes to attack rose bushes, Phragmidium mucronatum, also first appears as small red spots on rose canes or stems, says Bob Bauer's Roses and Everything About Them. Bright brown or reddish warts grow in the woody stems. The fungus only spreads when temperatures are warm, but quickly destroys plants by killing off leaves. Trimming infected leaves and stems off prevents reinfection; antifungal treatments have little effect. Catch rose rust early and don't spread the fungus by using contaminated pruning tools.

Aphids

• Aphids are a pest and not a fungal infection. These tiny insects are the most common pest found on American rose bushes, says the University of California Integrated Pest Management Program, and may be bright red. Aphids suck sap and usually live on the stems or under leaves on a rose bush. They have little effect on a plant by themselves, but they secrete a special substance that encourages damaging fungus infections. Spray them with a rose-safe pesticide or insecticidal soap.