The daytime temperatures are getting warm and it’s cool at night. It seems like the perfect scenario for great growing weather. Your peas are up and growing like a storm. What’s that on the leaves and pods? There are white fuzzy spots growing on my peas. Powdery Mildew strikes again!!

Powdery mildew is the most common occurring plant fungal disease. Its appearance, when first noticed, is characterized by spots or patches of white to grayish powdery growth on the upper surface of leaves and their stems.

• It impairs photosynthesis
• Stunts growth
• Increases the rate of plant decline

Advanced stage of powdery mildew:

• Foliage turns to yellow, leaves curl or turn brown.

How does this happen? There are three requirements for disease to flourish. A host, the pathogen/disease and the right environmental conditions.

• Powdery mildew (thin layers of fungal tissue on the surface of the leaf which produce spores) requires new and young living plant tissue to grow.
  • The disease grows as mycelium (fungal tissue) on the surface of the affected plant as white patches where the spores are produced.
    • Spores make up the white/gray powdery growth visible on the leaf surface.
    • Spores  of powdery mildew are carried by the wind, splashing rain drops and/or insects to new plant tissue.
    • The spores can produce in 48 hours.
• Warm, humid days and cool nights are the perfect conditions for the fungus to grow.
  • Moderate temperatures of 60° to 80° (F) are favorable temperatures for powdery mildew growth.
  • Temperatures of 90° (F) and above inhibits the growth of the disease.
• High humidity for spore germination common in crowded plantings, where air circulation is poor and damp shaded areas invite this plant fungal disease.
• Crop debris and host weeds are important to the survival of powdery mildew.
• The spores of powdery mildew over winter attached to plant parts and plant debris.

These are the common preemptive strategies to avoid/combat powdery mildew.

• Plant powdery mildew resistant varieties if available.
• Plant in full sun.
• Properly space plantings to allow good air circulation.
  • If plants are overcrowded, prune to allow increased air circulation, reduce humidity and cross infection.
• Arrange your rows or beds east to west to eliminate shade possibility.
• Tallest plantings should be to the north.
• Avoid overhead watering. Best time to water is mid morning allowing the plants to completely dry.
• Stay out of the garden when wet!!! Otherwise, you become a carrier of the disease.
• Avoid the late summer application of nitrogen fertilizer (fish emulsion, composted poultry litter, ammonia nitrate, urea) which limits new growth the disease attacks.
• Remove ALL diseased plant parts and burn or bag and remove from property.
  • If possible, remove diseased plant parts on a sunny, hot, wind free day.
    • Spores are killed by heat and direct sunlight.
    • In the absence of wind there is less of a chance for spore dispersal.
• Remove ALL plant material and plant debris (mulch included) in the fall.
  • Survival rate of any overwintering fungal spores is decreased.
• DO NOT COMPOST ANY PLANT MATERIAL OR DEBRIS where this disease was active.

When fungus pressure/threshold is too great you can pull all diseased plants or are there fungicides to use? The answer is YES!!

• Horticultural oils – Saf-T-Side Spray Oil®, Sunspray Ultra Fine Spray Oil® – follow label directions
• Plant Based Neem Oil (Amazon Link), Plant Based Jojoba Oil (Amazon Link) – follow label directions
  • Never apply when temperatures are 90° (F) or above or with drought stressed plants.
  • Never apply an oil spray within two (2) weeks of a sulfur application as plants may be damaged.
• Wettable Sulfur is most effective when applied before disease symptoms appear. – Safer Garden Fungicide® (Amazon Link) – follow label directions
  • Never apply when temperatures are 90° (F) or above or with drought stressed plants.
  • Never apply within two (2) weeks of an oil spray.
  • Be careful when spraying squash and melons as there may be damage.
•  Sodium Bicarbonate (Amazon Link)
  • Combine with horticultural/dormant oil with liquid/insecticidal soap
    • 1 TBS Baking Soda, 1 tsp horticultural/dormant oil, 1 tsp insecticidal/liquid soap to 1 gallon of water.
    • Spray every one to two weeks.
    • Use sparingly to avoid sweetening (alkaline) the soil.
• Potassium Bicarbonate – Kaligreen® – contact fungicide killing spores of powdery mildew quickly.
  • Approved for organic use. Follow label directions.
• Mouthwash – generic ethanol based – spray 1 part mouthwash to 3 parts water.
  • Caution use on new foliage as it may be damaged.
• Vinegar – spray 2 -3 TBS apple cider vinegar (5% acetic acid) mixed with 1 gallon water.
  • Caution use on new foliage as it may be damaged.
• Milk – natural occurring compounds in milk attack the disease while improving the plants immune system.
  • Spray 1 part milk to 2 parts water weekly.

Biological Fungicides – beneficial microorganisms when sprayed on plant tissue destroy fungal disease. Serenade®, Actinovate SP® (Amazon Link), Cease®

• Active ingredient  – Bacillus subtillis – prevents powdery mildew from infecting the plant. Follow label directions.
  • Non toxic to people, pets and beneficial insects
    • Not proven to be as effective as oils or sulfur

Here are a few examples of powdery mildew.

Remember, fall clean-up is essential!! Be on your guard!

Papa