Summary

The 2025–2026 production season in Agadir presented an unprecedented challenge: instead of chronic drought, growers faced persistent and intense rainfalls between January and March 2026. This created the ideal conditions for fungal diseases. Four diseases dominated—Mildew (late blight), Botrytis, Cladosporium, and Powdery mildew—each requiring distinct prevention strategies.

Success depends on three integrated principles:

• Real-time climate monitoring, using dew point and relative humidity trends to predict diseases before symptoms
• Preventive fungicide strategies with active resistance management
• Precise irrigation management to reduce canopy humidity and substrate saturation

1. Production Context and Environmental Conditions

For decades, Agadir's greenhouse sector managed production around drought and high temperatures. The 2025–2026 season reversed this. Prolonged rainfalls (January–March) maintained relative humidity far above typical 40–50%, creating extended leaf wetness periods, that passive greenhouses struggled to control. Standing water accumulated in many facilities, elevating internal humidity to activate dormant pathogens and forcing rapid crop protection adaptations.

Disease	Affects	RH (%)	Mainly Occurs	Temperature (°C)	Key Trigger
Downy Mildew (Late Blight)	Young leaves, stems, growing points	>90%	Around sunrise	10–25	Rain + closed greenhouse windows
Botrytis (Gray Mold)	Old leaves, stems, fruits	>93% (for >4 h)	Night / rainy periods	5–20	Wounds caused by pruning
Cladosporium (Leaf Mold)	Old leaves	>88%	Throughout the day	12–32	Poor ventilation
Powdery Mildew	Mid-age leaves	Alternating wet/dry conditions	Midday	12–32	Excessive ventilation (windows too open)

Mildew (Phytophthora infestans)—The Greatest Threat

Mildew was the most destructive disease this season. Unlike foliar diseases that reduce photosynthesis, Phytophthora kills the growing point, stopping development entirely and causing total crop loss within days under favorable conditions.

Favorable Conditions: Relative humidity >90%, leaf wetness >3-4 hours, temperatures 10–25°C (optimum 18–20°C)

Symptoms: Brown lesions with yellow halos on leaves; white fungal growth on undersides; dark lesions on stems, leaves and fruits.

Control Strategy: Prevent via resistant varieties, excellent ventilation, and preventive fungicides. Metalaxyl-M is the only option with extended curative activity (5 days post-infection). Most other registered fungicides act mainly or only as preventive treatments.

Botrytis cinerea (Grey Mold)

It attacks old leaves, stems, fruits, when humidity exceeds 92-93% for >4 hours. Prevention: Minimize wounds, reduce night humidity, ensure air circulation. Fungicides effective if applied preventively or within 2 days of infection.

Cladosporium fulvum (Leaf Mold)

It infects older leaves under continuous humidity (>88% RH), especially in poorly ventilated areas. Ventilation is the primary control tool. Genetic resistance is crucial—avoid susceptible varieties. Fungicides show poor efficacy; cultural control is more reliable.

Powdery Mildew

Develops on middle-aged leaves during warmer midday hours with alternating wet-and-dry periods. Resistant varieties significantly reduce pressure.

3. Physiological Disorders: Micro- and Macro-Cracking

This season revealed a critical consequence of rainy conditions: excessive irrigation combined with saturated soil triggers fruit cracking. Both micro-cracking (fine surface fissures) and macro-cracking (large radial cracks) result from water management decisions, not climate alone.

3.1 Root Causes

• High N, low Ca++/K+: Creates weak fruit skin and excessive internal pressure
• Fast water fluctuations: Soil water varying >6–12% per day creates pressure swings
• Early morning over-irrigation: excessive uptake without root resistance increases fruit turgor
• Temperature swings: slow activation <15°C followed by rapid heat creates stress
• Humidity condensation on fruit: wet fruits plus air heating increase pressure stress

3.2 Prevention Strategy

• Maintain moderate substrate water content—avoid saturation during rain
• Multiple short irrigations instead of one large dose
• Smooth transitions: Target ±3–4% water variation per day, not ±6–12%
• Balanced nutrition: Adequate Ca++ and K+ relative to N reduces fruit fragility
• Optimize greenhouse temperature and humidity: avoid extreme morning-to-afternoon swings

4. Real-Time Climate Monitoring: The Dew Point Strategy

Visual crop inspection alone is insufficient. The difference between air temperature and dew point (ΔT) is the most reliable disease predictor:

• ΔT < 1°C = Condensation forming on leaves and stems
• If condensation >3 consecutive hours = Critical disease risk window
• September–March in Agadir = High-risk period (RH often >90% at night)

4.2 Actionable Sensor Thresholds

Growers with continuous sensor data can adjust ventilation, irrigation, and sprays proactively—before infection occurs. This predictive approach reduces fungicide use while maintaining crop health.

5. Fungicide Strategy and Resistance Management

Critical lesson: Fungicides are most effective when applied preventively, not curatively. Once symptoms appear, curative options are limited and often ineffective.

5.1 Preventive vs. Curative Activity

Preventive (Protectants): Block spore germination before infection. Most effective before disease develops. Residual activity 3–7 days depending on rainfall.

Curative: Limited options—generally 2–3 days post-infection. Metalaxyl-M offers longest window (5 days) for late blight.

5.2 Resistance Management: Rotation of Modes of Action

Excessive use of the same active ingredient rapidly selects resistant populations. For Morocco ONSSA products:

• Cytotropic/translaminar/systemic fungicides: Maximum 3 treatments per season
• Contact fungicides: Lower resistance risk but shorter residual (3–5 days in rain)
• Copper products: Useful protectants but lose efficacy quickly in rainy conditions

6. Irrigation Strategy and Substrate Water Management

Disease management cannot be separated from irrigation management. Many growers continued normal schedules during the rainy period, resulting in saturated substrates, excessive canopy humidity, and increased disease and cracking.

6.1 The P1–P2–P3 Water Management Framework

• P1 (sunrise): Small irrigation to establish substrate baseline
• P2 (mid-morning): Second small irrigation after uptake; allow run-off balance
• P3 (late afternoon): Final irrigation reduced 6–12% under rainy conditions

Key principle: During rainy periods, decrease water content target by 6–12%, avoid early morning over-irrigation, and use multiple smaller applications. This prevents soil moisture fluctuations that trigger fruit cracking while maintaining canopy dryness.

7. Practical Recommendations for Future Seasons

7.1 Install Climate Monitoring Sensors

Deploy sensors measuring temperature, RH, and dew point. Establish thresholds (ΔT <1°C for >3 hours = high risk) and base fungicide timing on predicted conditions, not visual inspection.

7.2 Preventive Fungicide Programs

Begin applications before disease develops. Use weather forecasts and sensor data to predict critical windows and spray proactively. Fewer applications, longer product efficacy.

7.3 Rotate Fungicide Modes of Action

Never exceed resistance-management limits (3 applications/year of systemic products). Alternate chemical families to preserve high-value products like Metalaxyl-M.

7.4 Adjust Irrigation for Rainy Conditions

Reduce irrigation 6–12% from normal during wet periods. Use multiple small applications. Monitor substrate water content to maintain moderate conditions—avoid saturation and stress—reducing both disease and physiological disorder risk.

7.5 Select Appropriate Varieties

Choose varieties with documented resistance to Cladosporium. Genetic resistance reduces fungicide dependency and provides insurance against high-pressure seasons.

Conclusion

Climate variability is creating new challenges even in traditionally dry regions. Future success requires:

• Accurate external weather forecasting
• Continuous greenhouse climate monitoring via sensors
• Preventive crop protection with active resistance management
• Precise irrigation and substrate management