Pests & Diseases

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This will help you identify solutions for Fungus Diseases, Unwelcome Guests, and Other Problems.

Note: A good rose reference book with pictures will greatly assist your identification of rose problems. This information is offered as a general guideline for the south Puget Sound area. This material has been adapted from the Time Magazine Book, How to Grow Roses, pages 94 - 97.

There are no cures for most rose diseases.  Fungal diseases in particular must be prevented.  Fungicides cannot cure infected plants, but only stop the spread of fungal diseases.  That is why we recommend a preventative fungicide program, beginning as soon as leaves appear in spring. If you're very good, and keep up the fungicides through spring, you might be able to cut out spraying over the summer when the weather dries.  Blackspot in particular needs water on the leaves to germinate, although powdery mildew needs simply humidity and does not germinate well in free water.  That's why blackspot shows up in spring, but powdery mildew not til summer.

 

Fungus Diseases

Problem/Symptom Cause Solution
Circular black spots 1/4 inch in diameter appear on upper leaf surfaces. Each black spot has a feathery margin and is surrounded by a yellow halo. As the spots enlarge and coalesce, the entire leaf turns yellow and falls from the plant.  Purplish or brownish spots and streaks may appear on canes.

Blackspot on leaves

Blackspot on leaves

Blackspot. This fungus disease is most common in humid and rainy conditions; the fungus spores germinate in water. Once a plant is infected, the fungus will remain in the canes through the winter and reappear on the next season's growth. VERY common in the Pacific Northwest. There is no chemical cure for infected plants.  Plan on spraying with fungicides on a regular  basis to prevent infections. Prune out affected canes in your early spring pruning, and apply a fungicide before the leaves open. To prevent blackspot from infecting new plants, spray with a fungicide as soon as leaves appear in spring, and repeat according to label directions.  If symptoms appear, remove and destroy all infected leaves, including those on the ground. Do not water plants from above; wet leaves are hospitable to germinating spores.
Leaves, especially new leaves, become twisted or curled and are covered with a white powder. Flower buds and canes may also be affected.

Powdery Mildew

Powdery mildew, a fungus disease carried by wind. The problem is most severe when nights are cool and humid, and days are warm and dry. Remove and destroy all infected leaves, including those on the ground.  You will probably only have one or two varieties of roses that are susceptible to powdery mildew. To prevent mildew or to arrest its spread, spray with a fungicide starting in early summer.
Small red, brown or purple spots develop on upper leaf surfaces. The center of each spot eventually dries out, turns white and may fall out of the leaf. Leaves eventually turn yellow and fall from the plant.

Anthracnose spots on leaf

Spot anthracnose, a fungus disease. The fungus spreads in water.  This disease is somewhat similar to blackspot in appearance but appears in cool early spring weather, where blackspot needs warmer, late spring weather. Regular preventative fungicide applications for blackspot will be the best you can do for anthracnose.  Do not water plants from above, since the fungus spreads in splashing water.
Rosebuds fail to open and are covered with a grayish brown, fuzzy mold. Open flowers are flecked with yellow or brown and lower petals are wilted and brown. The stems below infected flowers become discolored.

Botrytis blight on flower

Botrytis blight, also called gray mold, a fungus disease that spreads in moist air and cool temperatures.  It can be widespread in spring, and often appears on mail-order roses that have been kept in a dark damp box during shipping. There are no chemical cures for blight once it occurs. If symptoms appear, cut off and destroy all infected plant parts. Spray with a fungicide to keep the disease from spreading.
Red or brown sunken spots with dark margins develop on canes. Cracks may appear within the spots. The spots enlarge and eventually encircle the cane. Leaves and stems above the damaged area turn yellow, wilt and die. Canker, a fungus disease. The fungus spreads in water and enters the plant through cuts or wounds in the canes. The problem is most severe in early to middle spring, when plants come out of dormancy. There is no chemical preventive or cure for canker. When symptoms appear, prune infected canes below the canker. Use sharp pruning shears and make the cut just above a node at a 45 degree angle.
An orange, powdery substance appears on the undersides of the leaves. Eventually, yellow or brown spots appear on upper leaf surfaces. Infected leaves may wilt or curl.

Rust

Rust, a fungus disease that spreads in moist air and moderate temperatures. The problem is most severe in the Pacific Northwest, where the climate is cool and humid. Remove and destroy all infected leaves, including those on the ground. Avoid overhead watering because the fungus spreads in water.  Rust prefers plants growing in hot, dry, nutrient-deficient soils, and is not common in the Olympia area.
Black patches appear on upper leaves, then spread to rest of the plant.  Unlike blackspot, these spots are angular or squared-off, stopping at leaf veins, and don't have a yellow halo.  Leaves fall off within a few days of patches appearing.  There may be long black streaks on the canes.

Downy mildew

 

Downy mildew, a fungal disease that is not at all common in the Puget Sound area, but does show up once in a great while.  Spray damage can look similar - blackened or browned patches on the upper surfaces of the leaves, but the leaves don't fall off.

If you think you might have downy mildew, call one of our Consulting Rosarians for a confirmation before embarking on control measures.

Prune out and destroy (don't compost!) all affected parts of the plant.  Spray remaining plants with Stature, Subdue, or Alliette.  You will probably have to get these mail-order.  Follow label directions to the letter.  Do not add anything else to the spray mix.  Downy mildew can kill a rose plant within weeks so act fast.  You may want to simply dig out and burn the affected plants.  Clean all tools before using on unaffected plants.

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Unwelcome Guests

 
Problem/Symptom Cause Solution
Leaves curl, rosebuds and foliage wither or become distorted in shape. A clear, sticky substance that attracts ants appears on foliage. Aphids, semitransparent insects 1/8 inch long that cluster on new growth and flower buds. They suck the juice from the plant and secrete the sticky substance. Aphids can carry and spread diseases. Aphids may be knocked off plants with a stream of water. In severe infestations, spray with an insecticide or an insecticidal soap.
Small, rounded holes appear in leaves. Eventually, the entire leaf surface between the veins disappears.

Rose slugs, the larvae of sawfly wasps. The slug is light green with a dark brown head and up to 1/2 inch long. Some species have shiny bodies; others are covered with hair. Rose slugs generally feed on the undersides of leaves; they do not eat buds and flowers. Spray with a systemic insecticide and make sure the insecticide covers the undersides of leaves.
Buds do not open, or flowers are deformed. Petals have brownish yellow streaks and small dark spots or bumps. White and pastel roses are particularly susceptible. Thrips, tiny orange insects with elongated bodies. Thrips feed at the bases of rosebuds and on the petals of open flowers. They seem to be attracted to light-colored blossoms. To discourage thrips from attacking, spray plants with a systemic insecticide just before the buds open. If signs of thrip damage appear, remove and destroy infected flowers and buds. Spray infected plants with an insecticide. If the infestation is severe, repeat applications may be necessary.
Roses fail to blossom or existing buds suddenly turn black and die. The foliage and the stem surrounding affected buds may also blacken and die.

Rose midge, a fly larva that is white and 1/12 inch long. The larvae feed in clusters at the bases of rosebuds. A soil-applied insecticide applied to the ground around the plants is your best bet.  Imidacloprid is the most widely recommended insecticide.  Foliar sprays won't help, since the midge spends most of its 2 week long life either in the soil or deep inside rose buds.
Leaves become dry and have a bronze sheen. Tiny specks may be visible on the undersides of the leaves. Eventually, thin webbing appears on the foliage.

Spider mites on leaf

Spider mites on underside of leaf

Spider mites, nearly microscopic pests that may be red, black, yellow or green. To confirm their presence, shake a leaf over a piece of white paper; the mites will be visible moving against the white background. Mites proliferate in hot, dry weather. Knock adults off plants with a strong stream of water.  Most infestations can be controlled with regular showers of water.  Only if that doesn't work, spray with a miticide three times, three days apart. Use different miticides; mites may build up resistance to a single miticide. Mites produce new generations in a few days, so repeat treatments will be necessary.

Avoid insecticide use, as insecticides kill the beneficial insects that keep mites under control.  Reducing reflected heat will go a long ways to preventing mite infestations - use cool mulches, underplantings of low-growing annuals and perennials, or  use overhead sprinklers and hose down surrounding hard surfaces in hot weather.

Small circles or ovals appear in leaf margins. Leafcutter bees, which are shiny black, blue or purple bees. The bees do not eat the foliage; they use leaf material to build their nests. Prune out canes that have damaged foliage. Since leafcutter bees are pollinators of several crops, the use of chemicals to destroy the bees is not recommended.
Round or oval masses appear on stems and canes. Foliage wilts, turns yellow and drops from the plant. Growth is stunted and flowers are not produced. Rose scales, 1/8-inch, white, gray or brown insects with crusty shells. Scales usually appear in clusters. Prune out and destroy heavily infested canes. To prevent scale infestation, spray plants with horticultural oil in early spring.
Holes appear in unopened rosebuds. Leaves and stems may also have holes or may be chewed off.

Caterpillar damage

Caterpillars, the larvae of moths and butterflies. Most are yellow or green and up to 1 inch long. Some, such as budworms, attack only the flowers; others eat the leaves and stems. Spray with Bacillus thuringiensis, called Bt, a bacterium fatal to caterpillars but harmless to plants and other animals.
Upper surfaces of leaves are covered with small yellow specks. Leaves may curl.

Leafhopper damage

Leafhoppers, which are triangular, white or light yellow insects 1/8 to 5/8 inch long. They feed on the undersides of leaves and suck the sap from the foliage. Leafhoppers can carry and spread diseases. Spray with an insecticide or insecticidal soap. In the fall, rake up leaves and remove weeds that can harbor leafhopper eggs through the winter.

Rosebuds have holes drilled in them, or unopened bud falls over after something drills a hole in the stem right below the bud.

Rose curculio weevil damage on flower

Rose curculio.  Curculio weevils chew on rose buds. Annoying, but whaddya gonna do. Usually they are very small, dark reddish weevils, with a long dark snout. Hand pick these small weevils.  Hold a largemouth jar full of soapy water below the weevils as they will drop to the ground at the slightest disturbance.
Leaves and flowers have angular holes chewed from the edges.

    

Root weevils.  There are a couple of species whose adults will chew on flowers as well as leaves, such as the raspberry bud weevil and clay-colored weevil, and the Fuller rose beetle in Oregon. And of course the ubiquitous black vine weevils that decimate rhodies can also chew on rose roots and leaves. Hand pick the weevils (usually gray), or dust with carbaryl or Spinosad.  Some slug control products include one of these for soil insects.  Larvae feed on roots, so treat the soil too.
Growing tips, foliage and canes wilt. Swollen areas up to 1 inch long appear on canes. Borers, moth larvae that are white or yellow worms up to 1 inch long. Borers enter the canes through wounds and through pruned stem tips. Cut off the affected area. Make the cut below the swelling on the cane to be sure you remove the borer. To prevent borers from entering canes, apply shellac or white glue to the exposed tips after pruning.
Overnight, roses are chewed and trampled.  Buds and blooms are damaged the most.  Buds and new growth may appear to have been clipped off with pruners. Deer See "Deer Control Methods" (Word Doc) for help.

Other Problems

Problem/Symptom Cause Solution
Round growths about 2 inches in diameter appear at the base of the plant. The growths are light green when young and turn brown and woody as they age. Plant growth is stunted, foliage is abnormally small and few buds are produced.

Crown gall

Crown gall, a disease caused by bacteria that live in the soil. The bacteria enter a plant through the roots or through wounds at the root area. The bacteria cause abnormal cell growth, which produces the galls. There are no chemical controls for crown gall. Small galls may be pruned out with a sharp knife or pruning shears. Disinfect tools with alcohol or household bleach after each cut. In severe cases, remove the plant and the soil surrounding the roots to prevent the bacteria from spreading.
Leaves are mottled or streaked with yellow or the develop a pattern of yellow netting. Plant growth slows, flowers may be few and/or small.

Rose mosaic virus

Virused leaves

Mosaic, a virus disease. Although the virus does not affect flowers, it detracts from the overall health and appearance of the plant. There are no chemical controls or cures. In mild cases, symptoms often disappear by themselves. In severe cases, infected plants should be removed.
Leaf margins are brown and crispy.  Weather has been hot.

Heat damage on leaves

Heat damage. Give the plants more water.
Canes turn brown inside and out.  New growth may appear, but soon shrivels and dies.

Winter-killed canes

Winter-killed canes

Winter freeze damage. Prune out damaged canes.

 

 

?/font> 2008,  The Olympia Rose Society . This page last modified:  Saturday, November 12, 2011