Brown Spots on Hazelnut Leaves: What Are They?

Spotty hazelnut leaves are a common sight from midsummer onward. The type of spot — its colour, shape, and whether the tissue falls out to leave a hole — points to different diseases with different management requirements. Most leaf spot problems on hazelnut are cosmetic rather than life-threatening, but severe infections weaken the tree and reduce nut quality over time.

Common Hazelnut Leaf Spot (Gnomonia coryli)

The most widespread leaf spot on hazelnut in the UK and Europe is caused by the fungus Gnomonia coryli. It produces irregular brown spots, often with a darker border, which appear from July onwards. As the season progresses the dead tissue may fall out leaving ragged holes in the leaf — a symptom sometimes called shot-hole. Heavy infections cause early leaf drop in August or September. The fungus overwinters in fallen leaves and reinfects the following spring during wet weather.

Bacterial Leaf Scorch

Xanthomonas bacterial infection produces angular brown spots bounded by leaf veins. The spots may have a yellow halo and tend to appear after wet, windy periods in spring when bacteria splash onto young leaves. Bacterial spots do not produce the tiny fruiting bodies visible in fungal infections and the spots often look more water-soaked when fresh. There is no chemical cure; removing affected material and improving air circulation reduces spread.

Management Steps

For fungal leaf spots the most effective management is sanitation. Rake up and dispose of fallen leaves in autumn — do not compost them as the fungal spores survive composting. This breaks the overwintering cycle and reduces the inoculum load the following spring. Light thinning of congested shoots in the canopy improves air movement and helps leaves dry faster after rain, which is the key factor in fungal infection: spores need standing water on the leaf surface to germinate. Copper-based fungicides can be applied as a protectant in spring if leaf spots were severe the previous year, but they must go on before infection occurs.

When to Worry

A few spots on lower leaves from August are normal and not worth treating. Concern is warranted when more than a third of the leaf area is affected across the whole canopy, when leaf drop occurs before August, or when the same tree suffers severe spotting for three or more consecutive years without improvement. Persistent severe disease on an older tree may indicate that the variety is particularly susceptible or that the site is fundamentally too damp.

Variety Choice and Resistance

Some hazelnut varieties are notably more resistant to leaf spot than others. If you are planning new plantings on a site with a history of leaf spot problems, choosing varieties with documented resistance saves a great deal of management effort in subsequent years.