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.
|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. 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, 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.
|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
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, 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
||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
|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, 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
|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, 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.
|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
|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, 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,
||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.
|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
||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.
|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. 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
Leaves and flowers have angular holes chewed from the
|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
||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 Control Methods" (Word
Doc) for help.
|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, 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.
|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.
||Give the plants more water.
|Canes turn brown inside and
out. New growth may appear, but soon shrivels and
|Winter freeze damage.
||Prune out damaged canes.