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Powdery Mildew


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WANTED FOR: DEFACING TREASURED PLANTS, BRINGING DISEASE AND DEATH TO PLANTS

Powdery Mildew Powdery Mildew

Aliases Dusty; Podosphaera spp.; Erysiphe spp.; Sphaerotheca spp.

Description
Victims: Fruit and ornamental trees, especially roses; garden plants
Hangout: Fruits and leaves in areas that are cool, shady and humid but not wet
Physical Features: Fruits and leaves affected by powdery mildew will look as though they've been sprinkled with baby powder. If the infection is bad, the mildew may also be speckled with small brownish-black dots. Eventually the threads may look yellowish green. Only yellow spots, not white ones, will appear on some plants, such as tomatoes, peppers, and eggplant. Rhododendrons can have yellow or purplish brown spots.

Life Cycle
Powdery mildew spores survive the winter by taking leaf buds hostage and using them as a cozy home. In the spring, they assault tender new plant parts by stabbing them with root-like organs. These organs suck out nutrients from the plant. After stealing food, the rascals reproduce rapidly and continue to raid healthy tissue.

CAUTION
DURING MILD WINTERS, POWDERY MILDEW SPORES CONTINUE TO BUILD UP THEIR FORCES BY REPRODUCING ALL YEAR LONG.
Take the POW out of Powdery Mildew!

1. Patrol the area

  • Stake out new growth in the spring. Powdery mildew spreads rapidly on humid, but not rainy, days.
  • Be more attentive to shaded areas.
  • Carefully search for white powdery marks and curling leaves.
  • Make a record of any encounters with powdery mildew. Be sure to record who its victims were and the extent of injuries.

2. Make a positive I.D.

  • Powdery mildew is usually whitish, but remember it can cause colored spots. Cooperative Extension has Master Gardeners that will help you identify the suspect for free! Just bring in a fresh sample.

3. Do a thorough background check

  • Powdery mildew is one of the only fungal diseases that doesn't benefit from plants being wet. These spores die in water, which also inhibits growth of the powdery strands. Humidity helps powdery mildew, but liquid water is lethal.

4. Determine the danger level

  • Healthy plants can withstand most damage from powdery mildew. However, it doesn't hurt to take some precautions, like pruning.
  • Seedlings and water- or nutrient-stressed plants are not as able to defend themselves from this prowler.

5. Make a plan

  • Do-Nothing Method
    -Letting nature run its course without intervening can be a good strategy. -The inevitable rain and cool weather of the Lake Whatcom area will help plants fight off powdery mildew.
    -Select resistant varieties. See Passive-Aggressive Plants for rose and rhododendron recommendations.
  • Manipulative Measures
    -Get the S.W.A.T team to ward off powdery mildew! Hack off infected parts. Prune out young diseased tips anytime, but older infected parts should be removed only in the spring.
    -Let air circulation keep humidity between plants down. Remember to space plants with plenty of breathing room and, if possible, plant on the windy side of your property.
    -Hose down the intruder. Use a forceful spray of water to kill powdery mildew. But beware-wet leaves can be a breeding ground for other fungal infections.
    -In garden beds, don't put plants from the same family in the same place every year since diseases usually assault plants from the same family. Rotate plants from different families.
  • Secret Agents
    -Many lady beetles help in the fight against other bugs, but there is one kind of lady beetle, Psyllobora spp., that actually eats powdery mildew. It's very common in the Lake Whatcom area. Look for small beetles mottled with black and cream colors.
  • Armed and Dangerous
    -If you've got a powdery mildew invasion going on, use sulfur products to keep it from expanding. Most home and garden stores sell sulfur products to combat fungi.
    -When using a chemical weapon, be sure to read and follow directions for use, storage, and disposal.

6. Evaluate the results

  • After documenting the types of plants assaulted by powdery mildew, note if you took any steps to retaliate or if you just let Mother Nature handle it.
  • Recording the outcome of the strategies you choose will help you get better each year at fighting off powdery mildew using as few chemicals as possible.

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Lake Whatcom Cooperative ManagementWSU Whatcom CountyWhatcom County IPM
For more information, contact Scarlet Tang or Todd Murray
WSU Cooperative Extension (360) 676-6736
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