Disease in roses


As with any other garden or crop plant, roses are sometimes
susceptible to specific types of diseases, infestations of
harmful insects and “critter damage”. Diseases and pests that may be common here in the Carolinas may not be common in the dry climate of the American southwest or the cooler climates of the upper Midwest or northeast.

Many old garden or heirloom roses are moderately to highly
disease resistant. Newer cultivars and varieties bred specifically for disease resistance are quickly coming to market each year. Most of the concerns gardeners have heard regarding rose diseases and the need to spray relate to roses bred and introduced during the early to middle 20th century, particularly the Hybrid Teas, some Floribundas and Grandifloras. Much of the propensity for fungal infection some roses exhibit results from incorporation of genetic material from rosa foetida, a species native to Persia. However, it is a vital species for breeding and growing contemporary roses as it is the main source for yellow pigment genes in roses.

There are no native yellow European or American species of roses. It would be helpful for any gardener interested in rose culture to become familiar with common rose issues. There is vast information on these topics as roses are American’s favorite flower.

Here, I will try to address the most common issues
affecting roses grown in the Carolinas. Given the fairly humid climate of the Carolinas, some roses will inevitably affected by fungal diseases. Some are highly resistant while others moderately so. Some can be completely defoliated by fungal issues. These would be considered poor choices for the
Carolina garden.

I will detail disease resistance separately for roses in their respective listings. Further along in this post, I will review prevention and control methods.

Fungal Diseases Blackspot:
caused by the fungus, Diplocarpon rosae. Blackspot is, by far, the leading fungal issue affecting Carolina grown roses.
It appears as small, irregularly shaped black spots (as you
might have guessed) on the foliage of susceptible roses. These spots generally become surrounded by yellow leaf tissue. This is the plants defense response. It will shut off connection to the affected leaflet, causing it to eventually drop. The fungus is harbored in the soil and is generally transmitted to plants by splashing rain or water. It is also spread from plant to plant via wind borne spores. It is most common in mild, damp weather.

Mildew
two main types of mildew fungus affect roses. The first is powdery mildew, caused by Sphaerotheca pannosa, var. rosa. It is more common than its cousin, downy mildew. It appears as a diffuse, thin, dry, white-grey covering on foliage of affected plants.   It is encouraged by cool, damp weather and poor air circulation between plants.
Thankfully, it is easily treatable and rarely a serious threat to the plant. Downy mildew is not often seen in gardens as it primarily affects nursery grown cut rose varieties. It appears as brown-grey pustules on the undersides of plant foliage. It is caused by the fungus, Peronospora sparsa.

Anthracnose
is a less virulent fungus than those responsible
for blackspot and mildews but it can be an issue on some rose types.   An affected plant’s appearance is similar to that of one experiencing blackspot. However, any damage caused by Anthracnose is usually minimal. The family of fungi, Collectotrichum are responsible for the disease.

Rust
is a somewhat common problem on susceptible roses. Many
rose types are resistant. It appears as orange or yellow-brown spots or pustules on the underside of plant leaves. Like most of its fungal cousins, it is common in mild, damp weather. It is harbored in the soil and can be spread by wind borne spores. Cane or Stem Canker is caused by any number of pesky fungi. Affected stems or canes show a darkened oval “sore” that can be brown, dark grey or black. Seriously infected canes, those completely girdled by a canker will eventually die and require
removal.

Crown Gall
generally affects grafted roses. It appears directly
on the bud union which will become swollen, corky or wooden looking. Healthy plants are rarely affected.   It too is a fungal disease. Take care not to mulch heavily over a bud or crown as this can encourage the disease in our warm, humid climate. A severely infected plant should be removed from the garden and destroyed.

Botrytis Blight
this can affect certain varieties of roses but is only somewhat common. It can affect leaves, blooms and fruits of affected plants. The disorder is caused by the fungus, Botrytis cinerea. Infection will appear as a downy, dark grey
covering that distorts leaf or bloom formation. Do not confuse simple “balling” which can occur on some rose blooms in wet weather with a Botrytis infection.

Rose Mosaic
is caused by a complex of viruses that also affect plum, apple and other fruiting and ornamental plants. Infected roses show yellow veining, a wavy yellow pattern or yellow splotching on foliage. Viral infections are not as common as fungal ones but do pose a significant threat as there is no effective treatment or cure. The viruses are most often transferred in breeding processes involving infected stock.

Rose breeders and nurseries have worked diligently in recent
years to ensure they offer virus free stock. Affected plants should be immediately removed from the landscape and destroyed. If you inadvertently bought a virally infected rose from a reputable breeder, nursery or retailer- tell them about the issue and request a refund.

Rose Rosette
is also believed to be caused by a viral infection.
Affected plants show abnormal elongation of canes, hyper-growth of rusty colored foliage, malformed leaflets and a “witch’s broom” appearance. There is no treatment or cure. Remove and destroy infected plants. Again, if acquired from a reputable seller- advise them of the issue and request a refund.

Prevention & Treatment
The climates of North and South Carolina, from the mountains to the coast are humid and relatively warm during rose season. Black spot and mildew problems commonly, if not universally affect susceptible roses grown in our region. If there is a pet rose that you must have in the garden that is prone to infection (like me, I have plenty!) then prevention and control of fungi are very important.

The first step to prevent and control fungal diseases includes
buying or acquiring healthy plants or roots that have exhibited documented vigor and disease resistance. Secondly, roses should always be sited well, ensuring adequate sunlight, spacing for air movement and appropriate soil moisture. A healthy, well-nourished and vigorous rose will often shrug off fungal issues in stride, even if it is particularly prone to disease. Often, disease prone roses are also those that require the heaviest feeding.

Supplemental fertilization can make a big difference in the health of these roses. Pruning or snipping then destroying infected foliage can offer a bit of control.

In addition, thorough removal of leaf and plant debris during late fall and winter can help reduce fungal soil harbors in the rose garden. There are a plethora of effective chemical products available from garden centers, nurseries, big box centers and online retailers which treat, prevent or control fungal diseases common to roses.

Please refer to the NCSU Rose Information link later in
this post for some good listings of chemical controls. Many are available in spray form and others available as systemic granules or concentrates. Always follow label directions and only use chemical treatment methods as a last resort.

Lynn Cochran member of American Rose Society.


Helpful Links

Plant Pathology
IPM for Roses
What is Blackspot?
NCSU information on Blackspot
Royal Hort Society information
American Rose Society/Powdery Mildew
Clemson information/Powdery Mildew