6 Steps to Healthy Roses

For many gardeners, the flower that's beloved more than any other is the Rose.  But it can also be one of the most challenging to grow. The reason? Roses are prone to several diseases, including black spot, powdery mildew, rust and crown gall. If they all sound ugly, it's because they are.

However, diseases can be controlled. The key is to recognize them early and treat plants quickly. Ohio State University Extension offers help in identifying diseases by offering clear descriptions and many color photos. You'll also find a variety of suggestions for controlling rose problems, from good watering and fertilizing practices to pruning technique and spray application recommendations.

Powdery Mildew

 

Identifying Symptoms
Powdery mildew, caused by the fungus Sphaerotheca pannosa, appears as a white powdery growth on rose leaves, stems, buds, or flowers. It usually first appears on new growth in periods of warm, dry days followed by cool, damp nights. The new leaves may become curled or twisted and the shoots may look badly deformed. The fungus may also infect older leaves. Often, the upper surface of the leaves appears normal, but there is extensive fungus growth on the underside of the leaf.
Control
Although detailed information is lacking, some rose varieties are more resistant to powdery mildew. When planting new roses, find out from local rosarians which varieties are most resistant. The planting site can also be made less conducive to powdery mildew development. Do not plant roses in shaded spots, especially those areas that tend to dry out slowly in the mornings. Surrounding hedges or shrubs should be pruned or thinned to allow for more air movement over the roses. Finally, a regular, preventive spray program with fungicides should be carried out (see below). You must spray with a fungicide proven to kill powdery mildew. For an organic gardener, the two best choices are a 10% milk-in-water solution or the Cornell formula (2 Tbsp of horticultural oil and 1 heaping Tbsp of baking soda in a gallon of water). Both will work, but the Cornell formula is more effective and immediate than the milk spray.

 

Black Spot

Identifying Symptoms

The fungus, Diplocarpon rosae, produces round black spots with fringed margins on leaves or stems. On some varieties, yellowing may show up around the spots. These symptoms are often seen on the lower leaves first. Infected leaves will drop off and may leave the plant almost completely defoliated except for a few leaves that have recently grown at the tip of the canes. Such plants are badly weakened and may die over the winter.

Control

As with powdery mildew, some varieties of rose are less susceptible. Select and plant resistant varieties whenever possible. The fungus survives the winter in fallen leaves. Raking and removing these leaves each fall may provide some control. Avoid watering the plants by splashing water on or about the leaves. Plant in sunny locations where plants will dry quickly after watering, rains, or damp nights (dew). And if you're looking for a chemical-free way to control black spot (see photo), try milk! Studies have shown that one part milk (not chocolate) and two parts water help to control black spot when spray-applied to leaves every week. Any treatment, organic or otherwise, aimed at preventing/controlling rose mildew and blackspot must be started early in the spring just as the rose buds begin to swell to be effective. If our CA. friends will begin treating for their mildew problems early in the season they should be able to stay ahead of the mildew. This method also applies to any other plant fungus that troubles you annually. Begin your treatment early with the Cornell formula, or cormeal or copper sprays; whatever treatment you plan to use for a particular plant problem. Beginning treatment early in the season is the key to success for many of the stubborn plant fungi that plague mature plants. Do not use cornmeal on young seedlings until they have developed four sets of new leaves.

 

Rust

Identifying Symptoms

Rose rust caused by the fungus, Phragmidium sp., appears as orange or rust colored growth on the underside of the leaves. Older leaves tend to show symptoms before younger leaves. Under favorable conditions rust can cover the entire leaf and stem of the rose plant. Severe infections can cause premature defoliation.
Control
Whenever possible plant resistant varieties. Any practice that prevents the leaves from remaining wet for extended periods of time is beneficial for control. Never spray leaves with water in the evening so that the leaf surface is wet over night. Plant roses in areas that have full sun and allow air to flow freely around the plants. As a last resort, fungicides should be used (see below). Gardenville sea tea seems to combine the best of the best... it contains fish emulsion, compost tea, and seaweed (contains all these nutritional "goodies")...

 

Stem or Cane Cankers

Identifying Symptoms

Several fungi cause stem cankers on roses. The different fungi cause slightly different looking cankers, but they usually produce brown, oval shaped, sunken or shriveled areas anywhere on the cane. When the canker completely surrounds or girdles the cane, the cane dies and the leaves wilt from that point outward. Sometimes small black specks of fungus spore forming structures can be seen erupting on the cane surface within the cankered area.
Control
Always plant disease free material. Each year, prune out and destroy all diseased canes, making sure to cut well below the obviously cankered areas. Protect the plants from cold or freeze injury in the winter. This can be done either with mulch or another kind of cover. Keep the plants vigorous with proper fertilization, good watering practices, and black spot and powdery mildew disease control programs.

 

Mosaic

Identifying Symptoms

Rose mosaic is caused by a virus. Bright yellow patterns made up of wavy lines may appear on the leaves of some varieties. Other varieties may show no yellow lines, but may be stunted and weak due to virus infection.
Control
Virus infected plants cannot be cured. Plant virus resistant roses if possible. Try to control insects, particularly aphids, since they help spread the virus. If you are pruning virus-infected plants, don't prune healthy plants unless you have disinfested your pruners. Pruners can be disinfested by dipping in a 10% solution of chlorine bleach in water. Severely infected plants should be removed and destroyed.

 

Rosette and Witches Broom

Identifying Symptoms

Rapid stem elongation may be an early symptom of this disease. Later on, certain branches of the plant will develop thickened, thorny stems. Many short, deformed shoots will form, often with red pigmentation and tiny misshapen leaves. These shoots give the appearance of a witches broom. Plants die within one to two years as symptoms spread from branch to branch.

Control

The exact cause of this disease is unknown. Infected plants cannot be cured. Try to control insects, particularly leaf hoppers and plant hoppers. To protect other plants against the possibility of an infectious agent, symptomatic plants should be dug and discarded as soon as the disease is noticed.

Crown Gall

Identifying Symptoms

Irregularly shaped, bulbous masses of tissue (galls) appear on stems near the soil line. These can appear as small swellings, or be several inches across. Severely infected plants become stunted and fail to produce acceptable flowers.
Control
Avoid buying infected material with suspicious swellings or galls on lower stems or crowns. Protect plants from injuries on the stems. Maintain vigor with fertilization and watering. Pull and destroy badly infected plants. There is no chemical control for this disease.

 

Organic Remedies

Fertilizer-- get alfalfa pellets or alfalfa meal from a feed supply store. Don't get the kind with added sugar-- rabbit food. Cottonseed meal is also good if you can find it. Also plenty of composted manure.

Safer's Fungicide is considered organic, since it's just sulphur and it is supposed to control powdery mildew and rust. Just keep spraying it per the instructions. Cornell mixture (2 Tbs Sunspray oil and 1 Tbs baking soda in 1 gal water) is supposed to be a blackspot and mildew prophylactic, but doesn't seem to do much on existing disease, plus you shouldn't use it with the fungicide, it burns the leaves, so it's either/or.

Corn Meal is also considered both a fungicide and a fertilizer and is in every grocery store under the name Polenta, just scratch it in around the rose bushes but it won't do anything to cure disease once it's there, again more of a prophylactic. If you have any of the Whole Food stores or the Rainbow or Berkeley Bowl-type stores around, they sell cornmeal loose by the lb for about 49cents/lb.

Alaflfa pellets can be found in some of the good nurseries some sell it just in small bags (boxed E.B. White's is the most expensive) Some pet food stores also carry it. I was lucky and found a feed store that carries 50lb bags for $10, but that was in the 'burbs. Sprinkle 1 cup around the bottom of each rose bush. Worm Castings (WormGold) is also really good for all plants, but again, expensive unless you make your own (see the Vermiculture Forum)

Roses that show a resistance to Black Spot

Resistant hybrid teas:	Resistant floribundas/grandifloras:	Resistant shrub roses:
Carla	Angel Face	All that Jazz
Cayenne	Betty Prior	Carefree Wonder
Charlotte Armstrong	Carousel
Chrysler Imperial	Cathedral
Duet	Europeana	Resistant miniatures:
Electron	Fashion	Baby Betsy McCall
First Prize	First Edition	Gourmet Popcorn
Forty Niner	Gene Boerner	Little Artist
Granada	Goldilocks Impatient	Rainbow's End
Miss All-American Beauty	Ivory Fashion	Rose Gilardi
Mister Lincoln	Love
Olympiad	Mirandy
Pascali	Montezuma	Resistant Rugosa hybrid:
Peace	Pink Parfait	F. J. Grookendorst
Pink Peace	Prominent	Polyantha
Portriat	Queen Elizabeth	The Fairy
Pristine	Razzle Dazzle
Proud Land	Red Gold
Smooth Lady	Rose Parade
Sutters Gold	Sonia
Tiffany	Sunsprite
Tropicana

 

If you have tried all the recommended organic methods and still are having disease problems below is a list of fungicides sprays selected by the  Ohio State University Plant Pathology Extension.

Fungicide Spray Programs

Fungicides generally recommended for powdery mildew control include: Triadimefon (Bayleton, Strike); Triforine (Funginex), Thiophanate-methyl (Cleary's 3336, Domain). Propiconazole (Banner) and sulfur fungicides. Frequently used fungicides for black spot include Folpet (Phaltan), Captan, Mancozeb (Fore), Chlorothalonil (Daconil) and Triforine (Funginex). For rust, Triforine (Funginex), Mancozeb (Fore) and Chlorothalonil (Daconil) are effective. Follow labeled instructions regarding dose and frequency of application. It is important to spray on a regular schedule.

Follow all labels carefully. Be sure and spray both surfaces of all leaves thoroughly. With some wettable powder materials, it may be well to add about a teaspoon per gallon of a spreader-sticker to properly wet the rose foliage.

 

Remember Healthy roses are happy (and beautiful) roses!