Rose Disease Control

Portland is the City of Roses, but sometimes roses struggle here

Our rainy spring weather and temperate early summers contribute to conditions that support many fungal issues, and aphids are always problematic. The good news is that most common rose problems in the Pacific Northwest don’t kill plants, they just make them look bad. On this page we review best practices to prevent pests and diseases, and offer solutions for when they occur.

Best Practices

Choose disease resistant roses! Many roses have been bred to be disease resistant, but disease resistant does not mean disease-free. It means that a rose is resistant to at least one of the fungal diseases common to roses. Research disease resistant roses here.
Air circulation is incredibly important! Roses should be planted at least 3 feet away from other plants. Be aware of fences or walls that block air circulation as well. Prune out shoots that grow toward the inside of the plant – branches should grow up and out in a vase shape.
Plant in a site with ample sun. Roses need at least 6 hours of sun every day, and they love heat.
Add a layer of mulch on the ground to cover the roots. Mulch keeps roots wetter and cooler in summer, and drier and warmer in winter. It also reduces weeds and adds nutrients to the soil as it breaks down.
Try not to wet the leaves while watering. Some wetting is okay, but ideally leaves should be dry before nightfall.
Monitor for diseases. Remove diseased leaves as soon as they appear to reduce spreading.
Rake up leaves in autumn and dispose of them.
Prune off diseased leaves or branches in mid-late winter.
Follow basic care advice found here.

Spraying

Keep in mind that fungi evolve quickly, so treating them with the same chemicals every time will not be effective. We recommend rotating treatments between two different types of fungicides.

To prevent disease from occurring, start spraying when shoots and flower buds are emerging – fungal spores are active at this point, but haven’t yet bloomed, so aren’t visible.

• Be sure to follow labels carefully when diluting and applying chemicals.

• Do not apply when temperatures are of 80f or on windy days.

• Treat early in the morning to avoid harming beneficial insects.

Call us or come in to our nurseries to get recommendations on specific sprays.

The information on this page has been compiled from the Pacific Northwest Pest Management Handbooks.

Aphids – small soft bodied insects cover flower buds and hang out on new leaves. Aphids suck sap from plant tissues and excrete sticky honeydew that attracts ants. They don’t cause a lot of damage to roses, but heavy infestations can reduce the quality of the flowers (besides just being gross).

Ladybugs are a great insecticide as are little birds, and aphids can be knocked off of roses with a strong stream of water. If that’s not enough, there are plenty of effective treatments.

Insecticidal soap – this is a low toxicity choice that is an effective way to treat soft bodied insects. It is a true soap – potassium salts of fatty acids is the active ingredient.
If you want to make a home remedy, check the ingredient list of the soap you plan to use to be sure that the active ingredient is present in your soap and that there aren’t a lot of added chemicals. To be safest, buy insecticidal soap from a garden center.
Oils – Neem oil and horticultural oils are effective treatments as well.

Both treatments kill adult aphids when they come into contact with insects. Neither works effectively to kill eggs, so they must be reapplied periodically through the growing season.

Find more info about rose pests and treatments in this handout.

Black Spot Diplocarpon rosae

Looks like: black spots on leaves and stems, yellowing around the spots and eventual defoliation. Black spot is a fungal disease.
How it spreads: rain and splashing spreads black spot spores onto fresh new leaves from stems and branches that were infected the previous year. After spores spread onto leaves, it takes 9 hours of wetness for leaves to become infected and another 11-30 days for spots to appear. Spores spread mainly in winter, but spreading continues during any rainy period if temperatures are between 50-80f.

Treatment

Follow best practices for growing roses. Pay particular attention to monitoring for the disease and picking it off as soon as you see it, trying not to get water on the leaves when watering, cleaning up any infected leaves that fall on the ground and pruning out infected leaves and branches in late fall.

Powdery Mildew Podosphaera pannosa

Looks like: white powder on buds, new shoots and both sides of the leaves. When young growth is infected, it distorts leaves, and if the infestation is severe, growth is stunted.
How it spreads: powdery mildew is a fungus that overwinters on diseased plants. It’s released when humidity decreases, and is carried by wind to new plants. Germination happens mostly on nights with high humidity or heavy dew, or when temperatures are around 70f.

Treatment

Follow best practices for growing roses. Pay particular attention to monitoring for the disease and picking it off as soon as you see it. Flowing water (rain or irrigation) causes spores to burst, so watering the leaves and canes thoroughly in early afternoon, so they have a chance to dry off before nightfall, can limit the spread.

Rust Phragmidium

Looks like: rusty orange spots on the leaves, yellow spotting on opposite side of leaves that are affected. In late summer or fall, pustules turn black.
How it spreads: There are nine species of rust that are not host-specific – they can spread to any plant that is susceptible to rust. Rust overwinters on diseased plants. Spores are spread by wind. Germination occurs when leaves are wet 2-4 hours and humid weather favors disease development.

Treatment

Follow best practices for growing roses. Pay particular attention to pruning and cleaning up diseased material and removing affected leaves as soon as they occur.

Rose Mosaic

Looks like: yellow markings on leaves that can be in patterns or blotches. It often appears in late spring or early summer, but may not be evident on mid-late summer growth.
How it spreads: Rose mosaic is viral. It’s caused by one of several viruses and is transmitted through grafting. It does not spread naturally or because of any known insect. Roses aren’t killed by the virus immediately, but are weakened over time. Smaller distorted flowers, reduced blooming, skinnier stems, reduced ability to survive cold temperatures are symptoms.

Treatment