New Powdery Mildew (Erysiphe corylacearum): The New Threat to Hazelnuts

What Is the New Powdery Mildew?

Erysiphe corylacearum is an invasive fungal species that causes a new type of powdery mildew disease in hazelnuts. Of Asian origin, this pathogen was originally known only on wild hazelnut species such as Asian hazel (Corylus heterophylla), Japanese hazel (C. sieboldiana) and American beaked hazel (C. cornuta). However, in the early 2010s, it jumped to cultivated hazelnut (Corylus avellana), triggering a global epidemic that threatens world hazelnut production.

The traditional powdery mildew agent Phyllactinia guttata had been a known and relatively harmless disease in hazelnut orchards for decades. The new powdery mildew, however, is an entirely different pathogen — far more aggressive and economically devastating.

Arrival in Turkey and Spread

First Detection (2013)

In the spring of 2013, previously unseen powdery mildew symptoms were noticed on hazelnut leaves, husks and shoots in the provinces of Giresun, Ordu and Trabzon. Subsequent examinations determined the agent to be the Asian-origin Erysiphe corylacearum, and this was announced to the scientific community as the first global report of the pathogen on cultivated hazelnut (Sezer & Dolar, 2017).

Rate of Spread

The disease spread at an incredible rate after its first detection:

Year	Country/Region	Source
2013	Turkey (Giresun, Ordu, Trabzon)	Sezer & Dolar, 2017
2014-2016	Spread to all Eastern and Western Black Sea regions	Ministry of Agriculture reports
2017	Official record in Turkey through scientific publication	Phytoparasitica
2018	Azerbaijan, Iran	Abasova et al.; Arzanlou et al.
2018-2019	Georgia	Meparishvili et al.
2019	Ukraine	Heluta et al.
2020	Italy, Switzerland, Austria	Multiple reports
2021	Germany, Hungary, Spain, Slovenia	EPPO reports
2023	Bulgaria, Czech Republic	Plant Disease

Differences from Traditional Powdery Mildew

There are important differences between Erysiphe corylacearum and the old type powdery mildew Phyllactinia guttata. Knowing these differences is critical for correct diagnosis and effective management:

Feature	Traditional Powdery Mildew (P. guttata)	New Powdery Mildew (E. corylacearum)
Origin	Native to Europe and Asia	Invasive from East Asia
Infection site	Generally lower leaf surface	Leaf both surfaces, shoots, husks
Symptom onset	Mid-summer - autumn (late season)	Spring - early season (April-May)
Severity	Mild-moderate	High - very high
Mycelium appearance	Thin, sparse powder	Thick, dense white coating
Conidium shape	Single-celled, rod/rhomboid	Single-celled, oval/ellipsoid/barrel-shaped
Chasmothecia	Round-flattened, with 3-15 appendages	Round, with dichotomously branched appendages
Yield impact	10-20%, limited	20-40+%, serious economic loss
Control required?	Generally little/none required	Absolutely necessary

Symptoms

Leaf Symptoms

Erysiphe corylacearum symptoms begin early in the growing season in spring:

1. Early stage: Round, white powdery spots 0.5-1.5 cm in diameter appear on the upper leaf surface. Pale-chlorotic areas are visible around the spots
2. Leaf lower surface: Spots changing from yellow to brown appear directly below the white spots on the upper surface
3. Progression: White powdery spots enlarge and merge; the leaf surface can become completely covered with a white coating as if dusted with flour
4. Late stage: Infected leaves turn purple-bronze, become dull and dry out
5. Chasmothecia: At the end of the season, brown-black spherical structures (winter spore structures) visible to the naked eye form on the spots
6. Defoliation: In severe infections, leaves dry and drop prematurely

Shoot Symptoms

On young shoots:

• Flour-dusted appearance on the shoot surface
• Dulling and browning as the disease progresses
• Shoot dieback in early-stage infections

Husk and Fruit Symptoms

• White fungal coating on husk surfaces
• Husks darken and become dull
• Fruit development slows or stops
• In early-stage infections, nuts remain small or are completely empty
• Premature fruit drop occurs in severe infections

Yield Loss and Economic Impact

The economic impact of Erysiphe corylacearum powdery mildew disease is far more severe than that of traditional Phyllactinia powdery mildew:

Yield Losses:

• 20-40% direct yield loss reported in untreated orchards
• Losses can be even higher in severe epidemic years
• Premature leaf drop also negatively affects the following year’s yield

Impact on Quality:

• Nut kernels remain small and weak
• Blank nut (empty shell) rate increases
• Internal kernel defect rate increases in long-term storage
• The proportion of product not meeting export quality standards rises

Economic Dimension:

• Turkey produces approximately 600,000-800,000 tons of hazelnuts annually, accounting for 65-70% of world production
• Powdery mildew disease control costs (spraying, labor) have increased noticeably per orchard
• Since the first epidemic in 2013, the disease has caused noticeable declines in yield per unit area, especially in the Eastern Black Sea region

Control Methods

Cultural Measures

Cultural measures must be applied to enhance the effectiveness of chemical control:

• Collection of infected leaves: In autumn, fallen leaves and diseased plant debris should be collected and removed from the orchard or burned. These are the source of winter spores (chasmothecia)
• Sucker cleaning: Infected suckers should be cut and destroyed
• Pruning: Ensure good air circulation and adequate light penetration through regular pruning. A crowded canopy structure increases disease severity
• Weed control: Control weeds to reduce humidity and improve air circulation in the orchard
• Balanced fertilization: Excessive nitrogen fertilization creates soft leaf tissue that increases powdery mildew susceptibility

Chemical Control Program

Scientific research conducted in hazelnut production areas of Giresun province (2015-2016) revealed the effectiveness of different spray programs. Six different plant protection products were tested in the research:

Active Ingredient	Mode of Action Group	Effect Type	Leaf Efficacy	Husk Efficacy
Sulfur SC 800 g/l	Inorganic	Preventative	85-96%	89-97%
Fluopyram + Tebuconazole	SDHI + Triazole	Systemic	Variable	Variable
Azoxystrobin + Difenoconazole	Strobilurin + Triazole	Preventative + Curative	Good	Good
Triadimenol 50%	Triazole	Curative	Effective	Effective
Myclobutanil 245 g/l	Triazole	Curative	Effective	Effective
Thiophanate methyl 70%	Benzimidazole	Systemic	Moderate	Moderate

Spray Timing

Correct timing is the most critical factor determining the success of powdery mildew management:

1st Spray — Husk formation period (mid-April):

• Must be applied as a preventative in orchards where the disease was observed the previous year
• Sulfur-based products are ideal for the first application
• Preventative application should be made even if no symptoms are visible

2nd Spray — 15-20 days later (early May):

• If the disease continues, repeat application with a different active ingredient group
• Triazole group (Tebuconazole, Myclobutanil) can be preferred for curative effect

3rd Spray — If necessary (late May - June):

• Third application if heavy infection is present
• Strobilurin + Triazole combination (Azoxystrobin + Difenoconazole) can be used

4th Spray — If necessary (mid-June - July):

• A fourth application may be needed in severe epidemic years
• Practice resistance management by using a different active ingredient group each time

For the detailed annual spray calendar, see our spray schedule.

Conclusion

Erysiphe corylacearum powdery mildew disease has been the most important new threat to Turkish hazelnut farming since 2013. This invasive fungus of Asian origin rapidly spread to all hazelnut production areas in a short time and has advanced worldwide. Far more aggressive than traditional Phyllactinia powdery mildew, this pathogen causes serious yield and quality losses if not addressed in a timely and correct manner.

For effective management:

• Regularly inspect leaves from April onward
• Perform preventative spraying at the husk formation period in orchards where the disease was observed the previous year
• Sulfur can be used as the first choice; practice rotation with triazole and strobilurin groups
• Do not neglect cultural measures (pruning, leaf collection, sucker cleaning)
• Follow the current control recommendations from your provincial agricultural directorate

For general powdery mildew information, see our Powdery mildew guide. For the annual spray program, see our spray schedule. For all aspects of hazelnut care, review our hazelnut guide.

📚 Sezer & Dolar (2017) - First Turkey Report 📚 Giresun Chemical Control Research 📚 PLOS ONE - Italy Research (2024) 📚 EPPO Reporting Service (2021) 📚 Hazelnut Research Institute