Hazelnut Diseases

On leaves, three patterns are worth separating carefully. Small round or angular spots roughly 1-3 mm across, water-soaked and yellowish-green at first and darkening to reddish-brown with a pale halo around them, are the signature of bacterial blight. A white, flour-like coating that smears when you rub it is powdery mildew; since 2013 the species responsible for most of it in Turkey has been Erysiphe corylacearum, covered in the new powdery mildew guide, while the older, traditional powdery mildew has its own guide in the Turkish edition of this site. Yellow rings, oak-leaf shaped blotches and yellow banding along the veins - patterns in the leaf tissue itself, with nothing sitting on the surface - point instead to mosaic virus.

On wood, look at the age of the tissue. Long cankers on one- and two-year-old shoots, with cracked bark and leaves that dry out but stay attached to the branch instead of falling, again point to bacterial blight, which is at its most dangerous on young trees because a canker can girdle the trunk outright. Sunken wounds with raised edges and nested rings on the trunk and main branches, developing slowly over two or three years, are typical of branch canker (Nectria galligena) - a problem that becomes markedly more common in higher, cooler, wetter orchards. The branch canker guide covers both the wound work and the cultivar question.

When a whole bush goes into decline, dig. Yellowing, undersized leaves and dieback that worsen year on year, often on one side of the plant first, combined with black shoelace-like strands on the roots and in the surrounding soil, indicate Armillaria root rot; a white felt of mycelium on the roots points to Rosellinia instead. Both are in the root rot guide. Root diseases are the most commonly misread problem in the orchard, because from above the picture looks exactly like drought stress or a nutrient shortage.

The single most important route is planting material. Bacterial blight moves into new areas mainly on infected seedlings, and mosaic virus spreads almost entirely through vegetative propagation - layering, cuttings and grafting material taken from bushes that were already carrying it. Buy from a reliable nursery, ask for health documentation, and never take propagation material from a bush showing symptoms, however good its yield looks. A single infected seedling can compromise an entire new planting.

The second route is the one you carry yourself. Shears and saws move bacteria and fungal spores from bush to bush in seconds. Disinfect the cutting surfaces at every change of tree - the bacterial blight guide gives 10% household bleach or 70% ethyl alcohol, with a soak of at least twenty seconds. Rain splash, wind-driven rain and wounds left by hail and frost do the rest, which is why a season with heavy hail or a late spring frost is usually followed by a year with more infection than usual.

The third is what you leave lying around. Fallen leaves and prunings are inoculum, not rubbish: the blight bacterium survives on fallen leaves for months, and powdery mildew overwinters in fruiting bodies on the leaf litter. Collect diseased material and destroy it rather than piling it at the orchard edge. Cut cankered wood well below the visible margin - the blight guide recommends 60-100 cm below the infected area - and treat balanced nutrition as part of hygiene, because excess nitrogen produces soft tissue that is far easier to infect. That is one reason the fertilization guides belong beside this category rather than in a separate world.

Not every disease can be sprayed at all. Nothing on the market cures a virus, so mosaic management is entirely a matter of clean planting material, tool hygiene and removing infected bushes. Copper preparations used against bacterial blight are protectants: they stop infection from starting and do very little once it has begun, which is why the guides put so much weight on three timing points - after harvest, at leaf fall and at bud swell - and comparatively little on the rate in the tank.

Where a fungicide is genuinely needed, the second question after 'which product' is 'which one did I use last time'. Powdery mildew can require several applications in a bad season, and repeating the same active ingredient through all of them is the fastest way to select for a population it no longer touches. The new powdery mildew guide sets out the practical version of this: sulfur as the first choice, with triazole and strobilurin group products rotated rather than repeated. Rotation means alternating between chemical groups, not simply switching to a different brand name that contains the same molecule.

This site deliberately publishes no product names or doses. Licensed preparations, permitted rates and pre-harvest intervals change by country, by region and by season, and a product that is legal this year may not be next. Take that list from your provincial or district directorate of agriculture, or from an agricultural engineer who has actually seen the orchard - never from a neighbour's leftover bag.

Some situations do not repay another week of reading. Ask for help when the symptoms genuinely do not match anything described here, when two candidates fit equally well, or when a treatment that should have worked plainly has not. A misdiagnosis is not just a wasted spray; it also costs you the season in which the real problem could have been contained.

Young orchards deserve a lower threshold for asking. Bacterial blight is at its most destructive in bushes between one and eight years old, where trunk girdling kills the plant outright, and a mistake in those years is one you live with for the life of the orchard. The same applies to a patch of neighbouring bushes failing together: that pattern usually means a soil-borne root disease spreading through the ground, and confirming it means digging and examining roots, not looking at leaves.

There is also a reporting duty worth taking seriously. New pathogens do arrive - Erysiphe corylacearum was first recorded in Turkey in 2013 and spread through the growing regions within a few years - and official surveys depend on growers reporting what they see. Finally, before you uproot anything, get a second opinion. Removing a mature bush is irreversible and expensive, and it is the one decision that deserves a qualified pair of eyes.