Powdery Mildew in Hazelnuts: Symptoms, Control and Spraying

What Is Powdery Mildew?

Powdery mildew is a fungal disease that shows itself on hazelnut leaves as a white-grey powdery coating. The traditional powdery mildew agent is Phyllactinia guttata, a widely seen disease in hazelnuts. The disease spreads by wind and increases rapidly in warm, humid weather conditions.

The new powdery mildew (Erysiphe corylacearum) species that has begun to appear in Turkey in recent years, however, progresses far more aggressively and poses a serious threat to hazelnut growers.

If powdery mildew is left untreated, the photosynthetic capacity of the leaves drops, fruit development slows down and yield loss can reach 20-40%.

Symptoms

Leaf Symptoms

The most obvious signs of powdery mildew disease appear on the leaves:

• White powdery coating: White-grey powder forming on the leaf surface as if flour had been dusted over it
• Traditional powdery mildew: Generally starts on the lower leaf surface
• New powdery mildew: On both the upper and lower surfaces, with a denser white coating
• Leaf curling: In advanced stages the leaf margins curl upward
• Yellowing and drying: In severe infections the leaves turn yellow and drop prematurely
• Brown spots: As the fungal spores mature, the white powder turns brown

Early diagnosis is critical for successful control. Inspect your leaves regularly from April onward.

Fruit Symptoms

Severe powdery mildew infection also affects the fruit:

• Fruit development slows down, the nuts stay small
• White spots may appear on the husk
• The proportion of blank nuts increases
• Kernel quality declines (small, weak kernels)
• Premature fruit drop may occur in severe infections

Disease Cycle

The powdery mildew fungus overwinters on leaf debris and in tree bark as chasmothecia (closed spore structures). When air temperatures rise in spring, the spores are released and new infections begin.

Spring (April-May):

• The overwintered spores are spread by wind
• The first infections begin on young leaves
• Spread accelerates when the temperature is between 15-25°C

Summer (June-July):

• The disease reaches its peak
• Warm (20-30°C) and humid weather (60-80% relative humidity) increases the spread
• A wet leaf surface makes infection easier
• The disease is more severe in unpruned, crowded orchards

Autumn:

• The fungus forms its overwintering structures (chasmothecia)
• When the leaves fall, the infected debris drops to the ground
• The inoculum source for the new season is ready

Control Methods

Cultural Measures

Cultural measures that must be taken before and alongside chemical control:

• Pruning: Regular pruning must ensure ventilation of the tree canopy. Crowded branches create a favorable environment for the disease. See our pruning guide
• Orchard hygiene: Infected leaves that fall in autumn should be collected and removed from the orchard or burned
• Ventilation: The spacing between trees must be sufficient and the inter-row areas kept clean
• Balanced fertilization: Excessive nitrogen fertilization creates soft leaf tissue and increases susceptibility to powdery mildew

Chemical Control

The main product groups used in powdery mildew control:

Product Group	Example Active Ingredients	Mode of Action	Note
Sulfur products	Wettable sulfur, liquid sulfur	Preventative	Economical, widely used
Triazole group	Tebuconazole, Penconazole	Curative + preventative	Effective, resistance risk
Strobilurin group	Trifloxystrobin	Preventative + curative	Good efficacy
Boscalid	Boscalid	Curative	Effective against the new powdery mildew

Application: The lower and upper leaf surfaces must be thoroughly wetted. Spraying should be done early in the morning or in the evening hours.

Spray Timing

Correct timing determines the success of powdery mildew control:

1st spray (late April - early May):

• When the first symptoms appear, or as a preventative
• Sulfur-based products are the first choice

2nd spray (15-20 days later):

• Repeat application if the disease persists
• Rotate to a different active ingredient (to prevent resistance development)

3rd spray (if necessary, June):

• A third application if there is heavy infection
• Prefer systemic products if the new powdery mildew is suspected

For the detailed annual plan, see our spray schedule.

New Powdery Mildew (Erysiphe corylacearum)

Erysiphe corylacearum, which has been appearing in Turkey since 2013, is an invasive fungal species that poses a serious threat to hazelnut production. Of Asian origin, this species was detected first in Azerbaijan and Georgia, and subsequently in Turkey.

Differences from Traditional Powdery Mildew

Feature	Traditional Powdery Mildew	New Powdery Mildew
Pathogen	Phyllactinia guttata	Erysiphe corylacearum
Infection site	Lower leaf surface	Upper and lower leaf surface
Severity	Moderate	High - very high
Coating density	Thin powder	Thick white coating
Active period	May-July	June-October
Yield loss	10-20%	20-40%+

Controlling the New Powdery Mildew

• Early diagnosis is very important — monitor the leaves closely from June onward
• Prefer systemic products (the triazole group is more effective)
• The number of sprays can rise to 3-4
• Resistance management: Use a different active ingredient group at each spray
• Always follow the recommendations of your provincial agricultural directorate

Conclusion

Powdery mildew is an important problem that is widely seen in hazelnut orchards and causes serious yield losses when left uncontrolled. Particularly with the spread of the new powdery mildew species, more careful and timely intervention is required.

In summary:

• Inspect your leaves regularly from April onward
• Spray immediately at the first symptom, do not be late
• Cultural measures (pruning, hygiene) support chemical control
• Prevent resistance development by rotating different product groups
• Contact your provincial agricultural directorate if you suspect the new powdery mildew

For the annual spray program, review our spray schedule. For all stages of hazelnut care, see our hazelnut guide.

📚 Hazelnut Research Institute 📚 Giresun Provincial Agricultural Directorate