DISFIGURING BEAUTIFUL ROSES AND OTHER PLANTS, INFECTING
PLANTS WITH A POTENTIALLY DEADLY DISEASE
Leaf Spot; Marssonina rosae; Diplocarpon rosae
Victims: Certain rose varieties, ornamental plants, garden
and warm areas; found on flowers, fruits and leaves
Physical Features: the name says it all!
scoundrels seek shelter in fallen leaves and diseased branches through
the winter, waiting for warmer days to launch their attack. Raindrops
splashing on soil under plants and brisk spring winds give these nasty
fungal spores a lift, catapulting them directly onto unsullied, budding
leaves. Besides being unpleasant to look at, they can damage leaves and
inhibit blooms. Black spot spores spend the spring and summer beefing
up their troops through reproduction and continuous movement to new plant
IF CONDITIONS ARE RIGHT, BLACK SPOT SPORES CAN BECOME INFECTIOUS IN JUST
ONE DAY, WITH NOTICEABLE EFFECTS IN LESS THAN A WEEK.
Stop Seeing Spots!
1. Patrol the area
- Start scouting for spots
in spring when temperatures begin reaching 60F and the days are wet.
Before black spot can infect plants, the temperature must be in the
mid-60's and leaves must be wet for 6-9 continuous hours.
- Carefully search leaves
for spots, being sure to check densely clustered leaves since humidity
will be higher where leaves are closer together. Also examine leaves
close to the ground.
- Take note of what you see
and record changes in numbers or size of spots. Some spots may be left
over from last year.
2. Make a positive I.D.
- Sometimes other problems
like nutrient deficiency, water stress, and too much or too little sun
can look like black spot. If you're not sure your problem's black spot,
let a WSU Master Gardener help you out.
3. Do a thorough background
- Black spot is difficult
to get rid of after it has set in, even with chemicals.
- Your best chance at warding
it off is by making your plants an undesirable place to set up camp.
Prune to decrease humidity and increase sunlight around susceptible
4. Determine the danger
- Think about your aesthetic
standards. Black spot is an ugly assaulter and will deface your plants
mercilessly, but the plants will still most likely survive.
- Continued leaf loss weakens
plants and invites aphids-a one-two punch.
- The health of the victim
is key in determining if it will survive the assault. Plants that are
weak or under stress may be killed by black spot.
5. Make a plan
- Do-Nothing Method
-Vigorous plants will courageously fight the fungus.
-As the summer progressively gets warmer and drier, nature will check
the advance of black spot invasions.
- Manipulative Measures
-S.W.A.T. will do in this criminal! Slash off areas where black spot
is trespassing. Prune out invaded leaves and canes. Do not compost infected
parts. For roses, prune victimized canes down to two buds.
-Give your plants an advantage by planting in well-drained, sunny places.
Also, plant on the windy side of your house to let breezes dry out leaves
and lower humidity near plants.
-Don't buy wimpy plants. Purchase varieties that can fight off black
spot. See Passive-Aggressive Plants
for a list of resistant rose varieties.
Give plants a chance to air out. Don't plant them close together.
-Destroy black spot hideouts by raking up leaf matter around plants
frequently, especially in the fall. Don't compost these leaves.
-Smother the villainous spores in the spring by putting mulch around
-Avoid overhead watering. If that's not an option, do it in the morning
so plants can dry out during the day.
-Don't put plants from the same family in the same place every year
since diseases usually assault plants from the same family. Rotate plantings
from different families.
- Secret Agents
-Being gobbled up isn't a worry for black spot.
- Armed and Dangerous
-Use fungicides in early spring to prevent black spot attacks. Be sure
to treat vulnerable new growth. Usually treating once per week is advised,
but read and follow the directions on the label.
-If your plants are already victims of black spot, you might try sulfur
products to keep the attack under control. Use them when the temperature
is between 65-85F. Do not use within 30 days of an oil spray. Always
read and follow the directions for use, storage, and disposal when using
6. Evaluate the results
- Keep an annual record that
you can refer to each year. If certain plants have been seriously assaulted
in the past, your notes will help you decide if preventative steps against
black spot should be taken in the spring and which strategies have the
highest success rates