An Overview of Rose Pests
What To Look For And How To Control Them 

By Kitty Belendez
Master Rosarian
Santa Clarita, California

We are lucky here in Santa Clarita in that we are located inland and therefore have fewer rose diseases. This is because our microclimate is much hotter and drier than the coastal areas. Powdery mildew, rust, blackspot and downy mildew are not as much a problem as they are elsewhere. 

In this article, we will give you an overview of the rose pests to look out for. Pests include a variety of common diseases and insects found in our area.

THE DISEASES

Powdery Mildew
In the early spring and early fall we will get some powdery mildew on the more disease-prone rose varieties, as the new and tender foliage begins to grow in anticipation of the two major bloom cycles. Ortho's Funginex works well on powdery mildew. One tablespoon per gallon of water applied every seven days is the recommended dosage. I have also found that washing the rose foliage with plain water several times per week in the morning, helps to control powdery mildew.

Blackspot
This disease is rare in Santa Clarita. But if you see it, Ortho Funginex can help. Pulling off the infected leaves and immediately placing them in the trash also helps to keep blackspot under control. 

Rust
Rust is usually not a major problem in our dry area, but it has been known to appear sporadically in pocket microclimates. When it does hit, it is hard to eradicate. You will need to pull off the leaves and destroy them, then apply a fungicide. Rust looks very similar to the rust found on metals as it is orange-red, but rust is a fungus.

Downy Mildew
Another fungus disease that is not real common in our hot, dry area, but it is worth mentioning. It is a major problem in the cooler, coastal areas. Downy mildew is indicated with purple splotchy markings on the rose stem and foliage. The plants can become completely defoliated, and spreads like wildfire throughout the garden. It is more common in the spring than in the fall, and usually disappears in the warmer, drier weather.

Anthracnose
It can look similar to downy mildew or blackspot. We do get a touch of anthracnose in Santa Clarita. Usually  disease appears each spring when we have a lot of rain, and the plant can defoliate. Anthracnose disappears when the weather becomes drier.

Crown Gall & Root Gall
This common rose disease is caused by bacteria. Galls can occur on the crown of the rose plant as well as on the roots and stems. It looks like a tumorous growth. Remove the growth with a knife or cutters, and treat the infected area with 10% solution of household bleach mixed with water (1-1/2 cups of bleach mixed with one gallon of water). For a bareroot plant, soak the entire bareroot bush in this solution for a few minutes. If the bareroot bush does not appear to be infected, it is suggested to soak the entire bush in a 1% bleach solution for 24 hours as a general preventative (4 teaspoons of bleach per gallon of water.) Be sure to disinfect your garden tools after using around infected bushes as the infection can spread to other rose plants in your garden. Severely infected plants should be discarded along with the surrounding soil. Gall bacteria can live in the soil for many years, therefore the planting hole should be treated with a bleach drench before replanting with a new roses.

Mosaic Virus
Symptoms of mosaic virus are yellow streaking or mottling in the leaves of the rose. The plants are weakened and not as vigorous as virus-free plants. Viruses are spread by infected grafting and are already in the infected plants when you buy them. Viruses are incurable so the only way to get rid of virus is to destroy the rose plant. However, mosaic virus does not spread to other plants, so if your rose plant has just a touch of it, symptoms appear only sporadically, and the plant still performs well, there is no need to destroy the plant.

THE INSECTS

Leaf Cutter Bees
There really isn't anything you can do to prevent leaf cutter bee damage on your rose foliage. These bees don't eat the rose foliage, but rather, they use the clippings to make their nest. In my yard, their favorite roses are 'Gold Medal' and 'Double Delight'. I know they've been visiting, when I see circles cut out of the rose foliage.

Spider Mites
This is one tiny little critter that can be a major problem in our hot area, as they love it when it is hot. Fortunately, spider mites are less of a problem as the weather begins to cool down. The spider mite is actually a relative of the spider family. Spider mites look like tiny flecks of salt and pepper on the backsides of foliage. Watch out for roses that are growing against the house or block wall where they get afternoon sun, as this is a prime condition for the spider mite. The plant can defoliate quickly when spider mites are out of control. I have very good success with keeping spider mites minimized by using the water wand on a daily basis during very hot weather. Avid is a great product for controlling spider mites.

Aphids
These little critters love the tender new foliage of the rose plant. They can suck out the juices, and they will leave behind a sticky secretion that can attract ants. One spray of Orthene early in the bloom cycle will usually combat aphids, and then spray again later on an as needed basis. For those with many roses, a better produce for aphids is Merit, available from rosemania.com.

Caterpillars
Sometimes caterpillars can be controlled simply by picking them off of the rose bushes. This takes vigilance. If they get out of control, apply a spray of Orthene, or Diazinon in the cooler months.

Thrips
Thrips are so tiny that they are barely visible on the rose blooms. They favor light-colored rose blooms. They suck the juice from the petals and the damage they create makes the petals look dirty. Orthene works well for control of thrips. Avid works well too. Here is a very educational link to UC Davis that tells about thrips.

Chilli Thrips See this link.

Beetles
Luckily, we do not have Japanese beetles in Southern California. However, we do get June beetles and cucumber beetles during the summer, which only stay for a short time and are easy to control with Diazinon.

Cane Borers
A tiny wasp-like insect that drills holes in the rose canes and lays its eggs inside. Seal newly-pruned canes with white glue for prevention.

Scale Insect
Relatively unknown in most areas for many years, this insect has recently surfaced. San Jose Scale is particularly destructive to roses and is very difficult to get rid of. Use Orthene at times when they are active. In the winter dormant period, you will need to make at least two applications of a dormant horiticultural oil such as Sun Spray brand.

OTHER ROSE MALADIES
Herbicides such as Round-Up can cause severe, long-lasting damage and even death to rose plants if used improperly and too close to the rose plant. It is much better to pull the weeds from around the rose plant instead of applying a herbicide on the rose plant. A pre-emergent weed preventative such as Preen is very effective in keeping weeds away from the garden, and should be applied twice annually.

Keep a Clean Garden
Never use rose leaves for mulch, and never let diseased foliage lie on the ground. A clean garden is a healthy garden!


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© Copyright Kitty Belendez. All rights reserved.

Originally published in "Rose Ecstasy," bulletin of Santa Clarita Valley Rose Society, Kitty Belendez, Editor.

Photos © Copyright by Kitty Belendez

For questions about Santa Clarita Valley Rose Society, contact: Rose Society


Updated January 1, 2016
Cane Borer Damage on Rose Stem
Spider Mites on the front of rose foliage. Notice the damage to the top of the foliage.
Spider Mites on the back of rose foliage. This is a greatly magnified photo, and usually the spider mite is barely visible with the eye.
Rust on Rose Foliage
Powdery Mildew on Rose foliage.
Powdery Mildew on the rose bud and 
peduncle (neck and stem).
Mosaic Virus on rose foliage
Anthracnose on rose leaf
Leaf Cutter Bee Damage on rose leaf
Herbicide Damage on rose plant. Product such as Roundup was sprayed on or too near the rose bush.
APHIDS: This enlarged photo shows aphids on a miniature rose bud.
This is a Thrips (the word thrips always has an "s" on the end even when there is only one. The photo above is magnified at 250%, inside of an Evelyn bloom. 
Green worm and its damage to rose bud. Photo enlarged about 200%.

Root Gall on rose bush.
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Rose Mosaic Virus on a large bush of 'Gemini'.