Walnut - Plant Pests

Walnut, supplier of the shorenut. This deciduous tree can grow 10 to as tall as 40 meters.

Also known as:
English walnut
Common walnut
Madeira walnut

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recognize walnut — Photo: CC0 Public Domain

The leaves of the walnut are not poisonous; walnuts are widely eaten. Provided they dry properly, they last for six months.

Walnut (Juglans regia). Deciduous tree belonging to the walnut family (Juglandaceae). A walnut should be pruned as little as possible. If you must, then only in the summer or early fall. In winter – even in dormancy – the tree will bleed from the pruning wounds. In summer the juice flow goes to the leaves and the ripening fruits; bleeding is then minimal. Walnut is little susceptible to disease.

Do not force dry walnuts (oven, on the stove). Drying too quickly can affect the flavor. Some species are more sensitive to this than others. For example, Juglans regia ‘Broadview’ is very sensitive to this: the nuts become bitter due to forced drying.

Bugs

Galls on the leaf, photo: Rasbak – CC BY-SA 3.0

Galls (bumps) on the leaf; on the underside cavities with brown hairs. Between the hairs live felt mites: Walnut blister mite (Aceria erinea).

Common r, photo: Marcus Mitschack – CC0 Public Domain

Leaf is eaten by 3 cm brown beetles: Common Cockchafer (Melolontha melolontha). A swarm of cockchafers can eat a walnut completely bare.

recognize damage nuts from the walnut tree — Black, affected husks with bore holes, photo: Karel van de Pol

Bore holes can be seen in the ripening husks; the nuts in the husks show black spots. Usually the walnuts are also affected: they are covered with black, slimy spots. Damage by the Walnut Husk Fly (Rhagoletis completa).

recognize damage Walnut Husk Fly — Holes in the trunk of a willow by the Walnut Husk Fly, photo: Siga – CC0 Public Domain

Large, oval holes in the trunk. Damp floury powder in the area. Special detail: the borehole smells like vinegar because the caterpillar produces acetic acid to digest the wood: caterpillar of goat moth (Cossus cossus).

Fungi & diseases

recognize bacterial leaf blight — Black spots caused by Xanthomonas campestris ssp. Photo: Scot Nelson, CC0 Public Domain

Leaf shows black spots, falls early. Occurs especially during wet, cool summers: walnut blight (Xanthomonas campestris juglandis).

Walnut anthracnose, photo: Rasbak – CC BY-SA 3.0

Yellow, angular spots on leaves, discoloring to dark brown. Then a dark brown rim develops around the spots. The walnuts dry up and fall off prematurely. These can still be eaten, but do not taste as good: walnut anthracnose Ophiognomonia leptostyla.

Small, orange-red mushrooms appear on affected branches, which can be either dead or living branches: Coral spot (Nectria cinnabarina).

Growth remains stunted, at the base of the tree a delicate membrane of fungal threads forms behind the bark. Brown mushrooms follow in the fall: honey fungus (Armillaria mellea). Infestation by the honey fungus can cause the tree to die quickly.

Other

Birds and squirrels eat walnuts. Magpies and chews grab a nut and drop it from a great height onto a hard surface to get the nut open.

Frost damage, photo: Rasbak – CC BY-SA 3.0

Late night frosts (May, early June) can damage the foliage.

Vertical cracks in the bark of the trunk: Bark cracks.

recognize lichen nut tree — Lichens on the branches of a walnut, photo: PlantEnPlagen

Lichen on the branches.

After a period of drought, the nut tree loses many young nuts at the end of May and the beginning of June: nut drop due to drought. During periods of drought, water heavily once a week – between 50 and 100 liters depending on the size of the tree.

recognize fungi on walnuts — Black fungus on walnuts caused by Alternaria, Fusarium, or Aspergillus niger, photo: University of California (Themis Michailides)

Walnuts, stripped of their husks, appear to be covered with black fungus: Alternaria, Fusarium wilt, or Aspergillus niger. The nuts may also be covered with a drab layer of fungal down. Some of these fungi can produce toxins (mycotoxins).