Here it is folks. NUMERO UNO! More roses are decimated by these garden nemeses than any other cause.
Anthracnoseby Bob Bauer
Fungus: Diplocarpon rosae Black Spot. This is the name that sends shivers down the spine of rose lovers who live in humid climates. Black Spot is a fungus that starts by growing in colonies on rose leaves. The result is the characteristic fuzzy edged rounded blackish or purplish spots on the rose leaf. The spots are about 1/8 to 1/4 inch in diameter. On the canes it appears as raised dark reddish or black blotches. Blackspot usually doesn't occur in early spring, but starts to show up in mid summer. The fungus then spreads across the leaf until it finally kills it. The leaf turns yellow and drops to the ground. A rose can quickly become completely defoliated by a bad attack of black spot. Whenever black spot is sighted in the garden it is very important to immediately remove and destroy any infected leaves, including those on the ground and remove them from the garden into the trash. Even if you have to remove the majority of the foliage, it will pay off to do so. The sooner you attack the problem the better off you will be. Spraying after you see symptoms has little effect. Any spraying for black spot must be preventative in nature.
Preventitive treatments for blackspot:
1. The least toxic spray for blackspot is baking soda. Combine 1 tablespoon baking soda and either 2 tablespoons horticultural oil or a few drops of dishwashing liquid detergent with 1 gallon of water. Mix as thoroughly as possible, and try to spray both sides of the leaves once a week. The liquid detergent helps the baking soda stick to the leaves. Reapply after a rainstorm. Baking soda changes the P.H. of the surface of the leaves, making a hostile environment for the fungus to grow.
2. Another treatment is sulfur dust, sulpher or lime sulpher used every 7-10 days.
3. As a last resort there are the more toxic chemical fungicides. They are often used once every week or two and are very effective, provided it is used as a preventative and is not used after the disease is present. As with all toxic chemicals, follow the label directions carefully. Be especially careful to spray properly in hot weather to avoid leaf burn. Spray the undersides of the leaves first.
4. Since a single fungicide may not completely wipe out all the fungi, using that fungicide over and over may actually cause fungus to build up a resistance to that fungicide. Alternating between two fungicides, such as Triforine (Funginex) and Daconil, is recommended to keep fungal resistance from occuring.
In general, fungicides generally can prevent blackspot and other fungal diseases, but do not work too well on an existing case of the affliction.
'Paris d'Yves St. Laurent'
BACK TO:
Rose Problems Page
Fungus: Sphaerotheca pannosa Powdery Mildew shows itself as a bumpy blistery surface on young leaves, followed by a white powderlike substance occurring on the top surface of leaves and flower buds and even ocassionally the stems. Infected leaves crumple and become distorted. This fungal disease doesn't occur everywhere, but is present in MOST rose growing areas. This is not as insidious as black spot for roses, in that it will probably not kill the plant, but it will make it very unsightly and affect the look of the foliage and the blooms. Powdery Mildew is most likely to occur when there are cool nights and warm days, or when there is a lot of moisture present in and around the roses that is not able to evaporate completely during the day. Unlike blackspot, conditions that are too wet actually inhibit the development of powdery mildew. It can not reproduce IN water. It thrives during high humidity but forms on dry leaves. This fungus favors 'soft'(thin, matte textured) foliage, and stays away from 'hard' or waxy thick (shiny) foliage.
Treatment and Prevention: In its early stages, it can actually be washed or sprayed off the roses, but once it takes hold, this is not possible.One of the best ways to avoid powdery mildew is to keep things as airy as possible. Roses planted too close to a wall or to each other may not get enough airflow. You can prune away crossing canes and open the center of the bush to allow sunlight and airflow, if you have a bush that is troublesome. Also, spraying the foliage with a mixture of 1 T. baking soda per 1 gallon of water and a few drops of dish detergent can be effective. If used as a preventative. Sulpher or lime sulpher sprays can be used early as a preventaive also. There are other more toxic chemicals available, such as copper based poisons among other things, but as with all toxic chemicals they should be used only when absolutely necessary and strictly according to the directions provided witht he product. These treatments as with most anti fungal agents are best applied as a preventative in the early spring or late winter when the plants are still dormant.
'Traviata'
Fungus: Phragmidium mucronatum or Phragmidium subcorticatum or Phragmidium sp. Rose rust is a VERY serious affliction that is capable of killing smaller roses quickly if left unchecked. Rust is a fungus that has bright reddish brown, sometimes even bright orange powdery looking spots that resemble warts. These appear in colonies underneath the leaves and on the wood. As the infection worsens, the color can appear on the upper surface of the leaves as well. As the spores age they turn to dark brown and then to black pustules and in the process, the leaves are killed and will wilt and then fall off. Rust is pretty easy to spot. Spores only remain viable below temperatures of 85 degrees F. These colonies grow and spread into each other and can eventually cover entire large areas of the plant. It is most likely to occur in the spring in areas when it is very wet for a period of time. In the USA it is most prevalent in the Pacific Coast regions, but can occur in other areas. In areas with hot summers and cold winters rust is not much of a problem. When you first see an outbreak of rust it is very important that you cut off the infected leaves and canes that have the rust present and carefully take them away from the area and dispose of them in the garbage. DO NOT put this into your compost pile. The spores overwinter on leaves that fall to the ground and not on the wood. Dead spots are left on the canes where rust has once been. These areas can be the entry point for other fungal infections, and so the canes should be cut off below where the damage is.
Treatment: Prune before bud break in the spring to reduce the quantity of spores still present on old leaves and stems. Remove these prunings from the garden and destroy them. Sulpher and lime sulpher applied in early spring when the plants are still dormant seem to work well. Chemical control after leafing out is difficult, because the spray has to be directed upward to the underside of the leaves. But that said, the same fungicides as are used to treat blackspot do have a preventative effect. According to Peter Beales,"...if an attack is diagnosed at an advanced stage, oxycarboxin, a chemical available under several trade names, is quite effective. Some sources say that too much potassium in the fertilizer will favor the production of rust. Another reason to not overfertilize. For what it is worth Rosa multiflora rootstock has shown itself to be relatively immune to rust.
Fungus: Peronospora sparsa Also known as Black Mildew. Not to be confused with Powdery Mildew. Downy Mildew is a more serious affliction.
Symptoms: Previously vigorous leaves commence to wilt turn yellow and fall off the plant. The young shoots also wilt and die back. It can also affect the calyxes of the bud and cause the bud to look distorted. Blackish or dark red/burgundy stains are seen on the infected upper surface of leaves and stems. These sometimes resemble the strokes of a paintbrush. Sometimes there is a downy fungus on the undersides of the leaves. The leaves then turn yellow and fall off. Downey Mildew usually occurs when the humidity gets high (over 85%) and the temperature is below 75 degrees F. Moist humid summers.
Treatment: Good ventilation and immediate removal of diseased tissues is necessary. The basic fungicide treatments are the same as for blackspot or powdery mildew above. Dormant oil sprays can be used to destroy the overwintering spores. After cleaning up the rose the addition of frest compost overlain with mulch will help greatly. This will separate whatever spores are left in the ground from the plant in addition to providing fresh nutrients.
Fungus: Botrytis cinerea also known as Bud Blight or Flower Blight or Grey (Gray) Mold, and also when the cane is affected: Cane Canker. The first symptom you will probably notice is a characteristic set of pink spots on white and lighter colored blooms. Sometimes there is a pink ring that forms around these spots. These can also appear as yellow or brown flecks as they age. At times this can even look quite artistic. Other main symptoms will be balling of the buds or failure of the buds to open. The petals can then turn brown at the bottoms of the bud like shreds of a paper bag. In serious cases, infected leaves can appear as if they are rotting away. Sometimes there can be a grayish brown fuzzy growth on parts of the infected portions. Lower petals on these blooms dead and brown. A gray mold can also occur at the base of canes that are covered by leaves or mulch. The problem is most severe in humid and cool (60 degrees F) conditions. The disease tends to start high and run down the stem so that over time, more and more of the plant becomes infected. The blight survives winter as black lumps in the soil or on dead infected canes and buds that have not been pruned off.
Treatment: Cut out and destroy all infected plant parts. Also open up the plant so it has good ventilation and air circulation. Preventative spraying isn't very effective, but will work in some climates and cases. Chemical spraying is with copper hydroxide or lime sulpher early in the season.
Fungus: Sphaceloma rosarum or Elsinoe rosarum also known as Purple Spotting or Spot Anthracnose. Characteristic small red, purple, pink or brown spots appear mostly on the leaves and sometimes the stems. Blooms are not affected. The center of the spot dries out turns white and sometimes leaves a hole in the leaf or petal. Leaves turn yellow and drop. Usually showing up in the middle to later part of the season. Some say that it affects climbers and ramblers more than other rose varieties. Overwinters on infected canes.
Treatment: Avoid overhead watering. Prune off infected canes before spring buds form. Spray with commercial sulpher or lime sulpher on dormant plants in early spring or late winter.
Fungus:Verticillium albo-atrum also known as Verticillium Wilt
Verticillium wilt is a soil born fungus disease that infects the plant and inhibits the plants ability to move water from the soil to the foliage. The disease becomes evident when temperatures are hot (late June to August). Initial symptoms begin with the wilting of new leaves and yellowing of older leaves. Permanent wilting occurs after a few days and leaves eventually turn brown and die Advanced cases result in defoliation from the base up and eventual death of the cane or plant. These symptoms appear during periods of stress.
Treatment: Verticillum is very difficult to control. Infected plants are candidates for removal and replaced by varieties with resistant rootstocks.
Canker is a garbage can term for several fungal diseases that attack the stems or leaves of roses. What these all have in common is the presence of dark spots or lesions that grow and kill the plant. They can all be treated in a similar way.
Fungus: Coniothyrium wernsdorfiae also known as Brand Canker The green bark of the previous years stems develops reddish brown or purplish black oval shaped spots or lesions. These later turn brown and spread down the cane as the disease develops. The stems will collapse and die unless it is dealt with. Sometimes you can see this disease in your local nursery. Don't buy a plant infected with this scourge unless it is on just one or two canes that can easily be pruned off without major damage to the overall plant size. Most common in the garden, in spring and again in late summer. This fungus enters through mechanical wounds in the bark or frost damage of canes. Some varieties of rose are more susceptible than others. Heavy applications of wet manure or compost left too long on the plants in the spring can contribute to this problem.
Treatment: The ONLY treatment is to cut off the cane below the infection and remove it from the garden. If any is left, it can reinfect the entire bush, so you must get it all. Make sure that you dip your pruning shears after every cut, in a bucket of 1:4 mixture of household bleach and water. You do not want to spread the disease with your pruners. Chemical controls are mostly ineffective for these canker diseases. All of the 'Canker' afflictions can be treated in this manner.
Fungus: Coniothyrium fuckelii or Leptosphaeria coniothyrium also known as Stem Canker Fuckelii.....that's pretty funny.... heh heh... That IS what it does to your rosebushes metaphorically speaking. They symptoms are large irregularly shaped lesions. Usually parched brown discoloration is noticed on the canes. They are sunken near the edges and swollen in the middle. The surface or edges may become wrinkled and then rupture, releasing spores. Canker ususally enters through a mechanical wound in the bark of the canes. This disease if left unchecked can kill a rose, especially if it is a one or two year old bush that is not yet fully established, OR an old less than healthy bush or climber. The stems will turn black and die from top to bottom.
Treatment: The canes MUST be cut off below the infected portions on all of the canes. If the infection reaches the bud union, the rose will die. Remove the cut off infected wood from the garden and sterilize your pruning shears after cutting the damaged canes as this disease is easily spreadable. When pruning DON'T leave long stems above the last bud, as the disease favors attacking the rose at these points. Cut 1/4 inch above the bud at an angle only. Chemical controls for stem canker are not known.
Fungus: Cylindrocladium scoparium also knows as Crown Canker or Cylindrocladium
Plants are usually attacked at the budding point. Discoloration of the bark which spreads slowly to very rapidly, to encircle the whole stem. Once the stem is encircled, the plant weakens and dies. This is more of a problem in newly budded plants. And is seen in nurseries more than in the garden.
Treatment: Cut off and discard the infected plant parts outside of the garden.
Fungus: Septoria rosae also known as Leaf Scorch Irregular blackish spots with reddish or purple edges 3 to 12 mm in diameter. These spots develop on the leaves.
Treatment: Cut off and discard the infected plant parts outside of the garden.
Fungus: Gnomonia rubi Large brown spots with purple edges. Yellowing and die back of the stems. Infection in spring on frost damaged buds. Possible connection of spreading from blackberries.
Treatment: Cut off bad stems and discard out of garden.
Fungus: Diplodia rosarum also known as Die Back Dark spots appear on stems and rootstalks.
Treatment: Same treatment as all of the other cankers. Cut and discard. Dip pruners after each cut in water with bleach in order to not spread the disease.
Cercospora Leaf Spot