Why Disease Prevention Matters in the Pacific Northwest
The Pacific Northwest’s wet climate creates ideal conditions for plant diseases. Fungal infections thrive in our cool, moist environment, and our mild winters allow many pathogens to survive year-round. Preventing disease is far easier and more effective than treating it—once plants are infected, options become limited.
Understanding common diseases and implementing preventive practices keeps gardens healthy without relying on sprays and treatments.
Common Pacific Northwest Plant Diseases
Late Blight (Phytophthora infestans)
The most feared tomato and potato disease. Spreads rapidly in cool, wet conditions and can destroy crops within days.
- Symptoms: Water-soaked spots on leaves turn brown/black, white fuzzy growth on leaf undersides, brown lesions on stems, fruit rots
- Conditions: Cool temperatures (60-70°F) with moisture; spreads during rainy weather
- Prevention: Resistant varieties, good air circulation, avoid overhead watering, remove infected plants immediately
Early Blight (Alternaria)
Less devastating than late blight but common in Pacific Northwest tomatoes. Weakens plants over time.
- Symptoms: Brown spots with concentric rings (“target spots”) on lower leaves first, working upward
- Conditions: Warm, humid weather; spreads by splashing water
- Prevention: Mulch to prevent splash, remove lower leaves, stake plants for air circulation
Powdery Mildew
White powdery coating on leaves of squash, cucumbers, and many ornamentals. Nearly universal in Pacific Northwest gardens.
- Symptoms: White powdery patches on leaves, starting on older growth
- Conditions: Moderate temperatures, high humidity but dry leaf surfaces
- Prevention: Resistant varieties, good spacing for air circulation, remove infected leaves promptly
Downy Mildew
Affects lettuce, spinach, and brassicas. Different from powdery mildew despite similar name.
- Symptoms: Yellow patches on leaf tops, gray-purple fuzzy growth on undersides
- Conditions: Cool, wet weather—perfect Pacific Northwest conditions
- Prevention: Morning watering so leaves dry quickly, good air circulation, resistant varieties
Clubroot
Devastating soil-borne disease of brassicas. Can persist in soil for 20+ years.
- Symptoms: Wilting on hot days, stunted growth, swollen distorted roots
- Conditions: Acidic, wet soil
- Prevention: Raise pH with lime, 7+ year rotation, resistant varieties, start transplants in clean soil
Damping Off
Fungal disease that kills seedlings, often before or just after emergence.
- Symptoms: Seeds fail to emerge, seedlings fall over at soil line with pinched stems
- Conditions: Cool, wet, poorly ventilated conditions
- Prevention: Sterile seed starting mix, good air circulation, avoid overwatering, don’t crowd seedlings
Botrytis (Gray Mold)
Common on strawberries, beans, and many flowers during wet weather.
- Symptoms: Gray fuzzy mold on fruits, flowers, and dying tissue
- Conditions: Cool, humid weather with rain
- Prevention: Good air circulation, harvest promptly, remove infected material
Prevention Strategies
Site Selection and Preparation
- Sunlight: Full sun dries foliage quickly after dew and rain
- Drainage: Well-drained soil prevents root diseases; raised beds help in heavy clay
- Air movement: Good air circulation dries foliage and prevents fungal growth
- Soil pH: Test and adjust pH—some diseases thrive in acidic conditions
Plant Spacing
Crowded plants create humid microclimates where diseases flourish. Follow recommended spacing even when it seems excessive:
- Tomatoes: 24-36 inches apart minimum
- Squash: 3-4 feet between plants
- Brassicas: 18-24 inches apart
Crop Rotation
Many diseases overwinter in soil on plant debris. Rotating crop families prevents buildup:
- Don’t plant tomatoes/potatoes where either grew the previous 3 years
- Rotate brassicas on a 4+ year cycle, longer if clubroot is present
- Keep records of what grew where each year
Resistant Varieties
Disease resistance is the most effective prevention tool. Look for resistance codes on seed packets:
- V: Verticillium wilt resistance
- F: Fusarium wilt resistance
- N: Nematode resistance
- TMV: Tobacco mosaic virus resistance
- LB: Late blight resistance
Water Management
Most fungal diseases need moisture to spread:
- Water at soil level: Drip irrigation or soaker hoses keep foliage dry
- Water in morning: Foliage dries before evening when fungal infection often occurs
- Avoid overhead watering: Especially on tomatoes, roses, and other disease-susceptible plants
- Don’t work in wet gardens: Walking through wet plants spreads diseases
Mulching
Mulch prevents soil from splashing onto leaves—a major pathway for many diseases:
- Apply 2-3 inches of organic mulch around plants
- Particularly important for tomatoes (prevents blight spread)
- Keep mulch slightly away from stems to prevent rot
Sanitation
- Remove infected material immediately: Don’t wait—diseases spread rapidly
- Clean tools: Disinfect pruners between plants when disease is present (rubbing alcohol or 10% bleach)
- Fall cleanup: Remove all plant debris at season’s end; don’t compost diseased material
- Wash hands: Tobacco users should wash hands before handling tomatoes (tobacco mosaic virus)
Building Plant Immunity
Healthy plants resist disease better than stressed ones:
- Proper nutrition: Balanced fertilization—excess nitrogen creates soft, disease-susceptible growth
- Consistent watering: Water stress weakens plants’ natural defenses
- Healthy soil: Compost-rich soil supports beneficial microorganisms that suppress pathogens
- Avoid injury: Wounds provide entry points for disease
Organic Disease Management
When prevention isn’t enough, these organic options help control disease:
Copper Fungicides
Broad-spectrum protection against many fungal and bacterial diseases. Apply before disease appears or at first sign. Can burn foliage if applied incorrectly—follow label directions.
Sulfur
Effective against powdery mildew. Don’t apply when temperatures exceed 85°F or within 2 weeks of oil sprays.
Baking Soda Spray
Home remedy for powdery mildew: 1 tablespoon baking soda + 1 tablespoon vegetable oil + 1 gallon water. Spray weekly. Somewhat effective but not as reliable as commercial fungicides.
Neem Oil
Provides some fungicidal activity along with insect control. Preventive more than curative.
Biological Controls
Products containing beneficial bacteria or fungi that compete with pathogens:
- Bacillus subtilis: Prevents various fungal diseases
- Trichoderma: Soil-applied beneficial fungus
When Disease Strikes
Act Quickly
At the first sign of disease:
- Identify the problem accurately
- Remove and destroy infected plant parts (don’t compost)
- Improve air circulation by pruning or spacing
- Reduce moisture on foliage
- Apply appropriate organic fungicide if necessary
Know When to Remove Plants
Sometimes removing a diseased plant protects the rest of the garden:
- Late blight: Remove and destroy infected tomatoes and potatoes immediately
- Clubroot: Remove infected brassicas and the soil around their roots
- Viral diseases: No cure—remove infected plants to prevent spread
Season-by-Season Prevention
Spring
- Start with disease-free seed and transplants
- Clean and disinfect tools and pots
- Ensure good drainage in planting areas
- Apply mulch after soil warms
Summer
- Monitor plants weekly for disease signs
- Remove infected leaves promptly
- Maintain air circulation through pruning
- Water in morning at soil level
Fall
- Remove all plant debris—don’t leave diseased material over winter
- Note problem areas for next year’s planning
- Add resistant varieties to seed orders
- Apply lime if soil is too acidic
Winter
- Plan crop rotations
- Research resistant varieties
- Sterilize seed starting equipment
- Cover bare soil with mulch or cover crops
Disease prevention in the Pacific Northwest requires constant attention to moisture management, air circulation, and plant health. While we can’t eliminate disease pressure in our wet climate, implementing these practices dramatically reduces problems and keeps gardens productive throughout the season.
