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Past Articles Library | Plant Diseases & Control | Plant Diseases of Roses



Boy, I love roses.  They can be used in formal gardens to create an air of sophistication and at the same time used in cottage flowerbeds as a touch of country.  While they are flexible in their uses, all roses have problems.  Below are the common plant diseases shared by all types of roses.  The key to control is twofold.  First, one needs to be able to identify the problem and two; one must be able to act quickly when the problem is diagnosed. 

Black Spot

This rose disease is caused by a fungus called Diplocarpon rosae.  Black spot will appear in the scene when the environment is wet and warm for a long period of time in the spring.  As the season progresses, diseased leaves begin to turn yellow and eventually drop.  Immature canes develop purple-red blotches that can feel raised when rubbed.  If the disease is allowed to continue, the plant will have stunted growth and the flowers will be less vibrant.  By the end of the season, all the leaves on the plant can drop off and cause the canes to become prematurely bare. 

To prevent and/or reduce the damage caused by this plant disease, one must act quickly.  First, cut back affected canes and remove leaves with the symptoms.  If leaves have fallen, rake them up along with the mulch.  Throw this in a trash bag and dispose of in the trash.  Replace the mulch to conserve soil moisture and reduce splashing up of water.  Plant your rose in full sun and space properly according to the variety.  Finally, plant only disease resistant varieties in your garden space.  

Botrytis Blight

This rose disease is called by Botrytis cinerea.  This fungus thrives at temperatures that range from 62 to 72 degrees Fahrenheit.  It requires a moist environment. 

While other diseases can affect just one part of the plant, this blight can be found in the flowers, buds, can canes.  Flowers with this fungus will have spots that have a red ring around them.  As the spots grow, they will eventually take over the flower and in doing so making the petals fall off prematurely.  Buds with this disease, on the other hand, will not open and fall off.  Canes with develop sores or cankers and will dieback. 

To prevent this plant disease, keep the area clean.  Prior to any pruning, make sure the tools have been sterilized with bleach or rubbing alcohol.  Remove any diseased leaves and canes.  Throw them in the trash.  Since this disease loves moisture, make sure to check the moisture level before watering and only water in the morning.  To speed up drying of the leaves, take time to plant the roses the correct distance apart.

Powdery Mildew

This plant disease is also caused by a fungus.  It attacks shoots, buds, and young leaves.  This attack comes in the form of a gray-white, powdery substance that grows on these surfaces.  It causes shoots and leaves to be distorted and buds not to open.   While this disease can appear anytime during the growing season, it thrives when the temperature is between 70 and 80 degrees Fahrenheit and the humidity is high at night and low during daytime hours.

To prevent this and/or control the situation if you have it, one must keep the garden space clean by removing canes, spent leaves and flowers.  Also, when choosing your roses, make sure they are resistant to powdery mildew.

Rose Rosette Disease

Rose rosetta virus is what causes this plant disease but this is not the end of the story.  This virus is carried by the rose leaf curl mite, which is a very small mite that sucks on the young shoots and leaves of the rose.  The process of sucking forces the virus into the plant, which in turn will cause stems to produce an unusual number of thorns, side branches will develop a witch broom type of structure at the end and the terminal end of the main, effected stems will have a red growth.  The number of flowers on these plants will be fewer and those blooms that do form will be distorted.

There is no cure for this virus but there are a few things one can do to reduce the chances of developing this problem.  First, remove all wild multiflora rose bushes in the area.  This just happens to be a favorite food of the rose leaf curl mite.  Second, once you see the first symptom, remove the plant immediately. 

 Rose Mosaic

This rose disease is caused by a virus by the same name.  The symptoms are varied and can range from spots that are shaped like rings to mottling and/or a yellow, wavy pattern that appears on the leaves.  There is no cure for this plant disease and in doing so the best approach is to just remove the rose from the garden space. 

Crown Gall

A Crown Gall is a swollen growth that is created by a soil-inhabiting bacterium called Agrobacterium tumefaciens.  This s bacterium causes galls to form on the roots and/or on the stems or canes where they touch the soil.  They start out as small white to green areas and can grow to an inch or larger.  What happens when a gall takes hold is food and/or water on the side of the gall is blocked.  The side of the stem where the gall is located will either have stunted growth or will die.  While there is no cure for disease once it has taken hold, there are a few things you can do to prevent it and/or stop the spreading of this disease.

First, only plant rose varieties that are resistant to this plant disease.  Second, avoid damaging the canes.  This means properly planting roses and understanding correct pruning procedures for your rose variety.  One will also need to follow a strenuous sterilization procedure of all tools used on roses.  While this sounds complicated, it really is not.  All you need to do is to wipe down your tools with bleach before working on your roses.  Lastly, remove any rose plant that show symptoms along with the soil associated with the root mass. 

Stem Cankers

Stem cankers are caused by several different fungi and these same fungi set into areas of the rose that have been damaged.  This can come from improper pruning to Mother Nature damage from such things as wind and hail.  How do you know if you have stem cankers?  Take a look at the stems of your rose.  Is there beige to dark purplish-brown spots on the canes?  If the answer is yes then you have stem cankers.   While the beginning stage of this disease only causes visual damage, this damage will eventually go all the way around the stem and kill the cane.  This death is caused when water uptake is interrupted by the canker.  In doing so, the tissue dies. 

How do you prevent developing stem cankers?  Well, there are some things you can control and others you can but start off by keeping your roses healthy.  A healthy plant is one that can fight off a fungus.  Also, give your rose a good start by planting properly, and pruning correctly.  Third, while Mother Nature may throw you a curve ball with hail and damaging winds do not wait to address the damage.  Remove damaged canes correctly as soon as they happen and if you have a diseased rose, remove it.   

Rust

Rust is caused by a fungus called Phragmidium.  This fungus attacks all the parts of a rose except the roots and the petals.  It appears as orange spots and as the fungus grows a powdery, rust colored substance can be found on the ground underneath roses.  In response to this fungus, rose leaves will turn yellow and drop. 

To prevent this fungus from taking hold, one must keep the center of the rose open along spacing roses properly according to the variety.  To keep the leaves dry as possible, water before noon so that the sun has time to dry out the environment.   Finally, remove any plants that show rust symptoms.










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Gardening-tip:



When to Water

If you can, it is always best to water early in the morning. This allows the plant's leaves and flowers to dry off as the day warms up.

If you water at night, the plant stays wet for hours in the cool, which are prime conditions for fungi and other problems to set in.


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