While roses often do succumb to diseases of various kinds, some varieties of rose are less susceptible than others. So, if you are planting a new rose garden try to stick to resistant species if you can.
Rose gardening diseases are generally categorized as being either fungal or viral; however, there are also several deficiency diseases roses can get.
Deficiency diseases relate to the main nutrients all plants require, namely:
- nitrogen deficiency where older leaves turn light green to yellow,
- phosphorus deficiency where the leaves tend to be undeveloped and dark, with a purple tinge,
- potassium deficiency where stems are weak and leaves turn brown and papery around the edges,
- magnesium deficiency where patches of dead tissue form between the veins,
- iron deficiency where the young leaves turn yellow between the veins (usually because the soil is too alkaline), and
- deficiencies in trace elements that also cause weak, stunted growth of plants.
Common rose diseases
While deficiency diseases may be fixed with proper feeding, you need to be constantly on the lookout for other common rose diseases. If you can identify the most common rose diseases you should be able to control them and prevent the rose from deteriorating unnecessarily. These are the ones to look out for:
Powdery mildew is a fungal disease that attacks the young shoots, leaves and buds of rose plants. It appears as a white or grey powdery covering on new growth, usually when hot, dry days are followed by cool, damp or wet nights. New leaves and shoots often look deformed. While old leaves might appear normal, if you look on the underside, the telltale fungus will be there too.
Downy mildew is not nearly as prevalent as powdery mildew, but it does occur in some coastal areas during humid months of the year. It causes rose buds to turn brown, while a white down-like substance forms on the underside of leaves.
Black spot is a very common rose disease that often appears after the first rains of the season. Like powdery mildew, black spot is a fungus. Severe attacks can cause all the leaves to fall off (defoliation). Older leaves – particularly those towards the bottom of the plant – develop purplish-black spots (hence the name), and the area around the spots often turn yellow.
Rust, another type of fungus, causes small, reddish-brown (rust colored) pustules to form on the underside of rose leaves, and it cause yellow spots on the upper surfaces. It is usually more severe in rainy weather or where humidity levels are very high. In severe cases, rust can cover an entire plant and cause rapid defoliation.
Canker affects the stems or canes of roses, usually in summer in areas where the weather is hot and dry. If the canker (which is a fungus) forms right around the stem, the leaves will wilt from this point and you will need to cut the stem off at this point. The first sign of canker is small black spores that look like little flecks forming on the surface of the cane.
Mosaic is a virus that causes bright yellow, wavy patterns to form on the leaves of some rose varieties. It also sometimes causes leaves to be weak and stunted.
Rosette and witches broom causes stems to get long very quickly and then some of the branches of the plant get thick, thorny stems. Shoots are often deformed and shorter than they should be. Leaves appear to be red and leaves are tiny and also deformed. The shoots look like a witch’s broom, which is where the name of the disease originates.
Crown gall results in great bulbous bits of tissue on stems near to the ground. They vary in size from quite small swellings to irregularly shaped lumps that are several inches wide. Plants are stunted and they normally won’t flower, or if they do the flowers aren’t very pretty.
How to manage diseases that affect roses
Once you have identified the symptoms of a particular disease, you can treat it accordingly. But as always, prevention is better than cure, and it always pays to plant and maintain roses in such a way that diseases will be avoided.
Powdery mildew can be controlled by spraying with a fungicide or dusting with sulfur. If the disease gets out of control, you will have to cut away the affected parts of the plant. To avoid roses getting powdery mildew plant those that are particularly susceptible to the disease in shady areas of the garden that will dry out slowly in the mornings. Don’t plant hedges or large shrubs too close to these roses because this has the effect of restricting air movement around them.
Downy mildew can be controlled by spraying with a fungicide.
Black spot can also be controlled with a fungicidal spray program that should start when new leaves appear early in spring time. Roses that are susceptible to black spot should be planted in a sunny location where the plants will dry out quickly after rain or watering. It is also good practice to avoid splashing rose leaves with water. If the plants do get this fungal disease, rake up the leaves and burn them to prevent further infection. Be warned that the fungus does survive winter in fallen leaves.
Rust can be controlled by preventing leaves from remaining wet for extended periods. Plant roses in full sun and make sure there is plenty of air around the plants. Don’t water in the evening. A fungicide may be used as a last resort.
Canker must be removed by pruning and the diseased stems should be burnt. Don’t try to compost any diseased plant material. Protect plants, particularly in the winter months when it is very cold, because canker usually enters through ‘wounds’ in the stem. If you keep your roses well fed and healthy, they aren’t likely to get this disease.
Mosaic can be severe and if it is, the plants that are infected should be removed and burnt. You can prevent mosaic by controlling insects, especially aphids that spread this particular virus. If the disease is mild, mix a solution of 10% chlorine bleach and water and dip affected bits into the liquid.
Rosette and witches broom is a mysterious disease that cannot be cured. The other problem is that nobody is quite sure what causes it. It may be that it is spread by insects, so control these, and if you see that plants are affected, dig them out and destroy them immediately. If the disease spreads, all your plants will die.
Crown gall cannot be controlled chemically or otherwise. Try to keep all your roses clean and healthy and if you spot diseased plants, dig them out and burn them.
Popular disease resistant roses
When you buy roses, always ask your nursery which are the most disease resistant species they stock. Different species go in and out of fashion, so you may not always find every variety you want. However, here is a list of roses that are said to be resistant to black spot, the most common rose gardening disease of them all, taken from http://ohioline.osu.edu/hyg-fact/3000/3063.html:
Resistant hybrid teas include:
Carla, Cayenne, Charlotte Armstrong, Chrysler Imperial, Duet, Electron, First Prize, Forty Niner, Granada, Miss All-American Beauty, Mister Lincoln, Olympiad, Pascali, Peace, Pink Peace, Portrait, Pristine, Proud Land, Smooth Lady, Sutters Gold, Tiffany and Tropicana.
Resistant floribundas and grandifloras include:
Angel Face, Betty Prior, Carousel, Cathedral, Europeana, Fashion, First Edition, Gene Boerner, Goldilocks Impatient, Ivory Fashion, Love, Mirandy, Montezuma, Pink Parfait, Prominent, Queen Elizabeth, Razzle Dazzle, Red Gold, Rose Parade, Sonia and Sunsprite.
Resistant shrub roses include:
All that Jazz and Carefree Wonder.
Resistant miniature roses include:
Baby Betsy McCall, Gourmet Popcorn, Little Artist, Rainbow’s End and Rose Gilardi.
Resistant Rugosa hybrids include:
F. J. Grookendorst, Polyantha and The Fairy.