Diseases

Grafted and other walnut trees are susceptible to some viral and fungal diseases, some of which are common to many plants and some more specific.

Walnut Blight

Also known as ‘bacterial blight’, and formally as ‘Xanthomonas campestris pv. juglandis’ or ‘Pseudomonas juglandis’.

Causes small black spots on leaflets, leading to large withered areas. Can also cause dieback of shoots and damage to the fruits, notably blotches and holes. A large part of the crop is likely to be lost in a serious attack, especially when the male catkins are affected.

This is very much a disease of new growth and nuts, and infection occurs only in wet weather. The most damaging circumstances are wet, cool periods around flowering time.

The bacteria will survive overwinter in apparently healthy dormant buds, leading to new infection of young growth – varieties which leaf early are most at risk.

Infected parts should be cut out a good way back from visible damage and burnt. Bordeaux mixture spray may help. Defences against blight include keeping soil pH above 6, avoiding wetting the foliage with spray irrigation, guarding against excessive nitrogen feeding and pruning enough to give an open structure for aeration.

Walnut Leaf Blotch

Also known as ‘Walnut anthracnose’ and formally as ‘Gnomonia leptostyla’, ‘Marssonina juglandis’ or ‘Marssoniella juglandis’.

A widespread fungus causing brown blotches on young fruits and leaves, when severe will cause the nuts to abcise (fall prematurely) after turning black and considerable leaf fall.

Another disease which favours wet weather for infection, it usually appears in late May and early June and will overwinter on leaf litter.

Fallen leaves should be cleared and burnt or properly composted. Again, Bordeaux mixture may help a serious infection.

Honey Fungus

Honey fungus has many names, including ‘Shoestring fungus’, ‘Oak root fungus’ and ‘Mushroom root rot’. Officially it is ‘Armillaria mellea’.

This is a disease affecting many species of trees, and causes defoliation, dieback and death. It can enter at wounds and roots, and can spread through the ground to neighbouring plants. It rots roots, collar and butt and is most dangerous to already weakened trees.

The only defence is to remove diseased trees and their roots, and any nearby stumps. Some of the American Walnut species (J. hindsii, J. nigra and hybrids) are resistant to honey fungus so using them as rootstock is one option – however this increases the chance of attack by Blackline (see below).

Blackline

This disease, caused by a strain of the Cherry Leaf Roll virus (CLRV-W), affects only non-regia rootstocks.

Symptoms include a dark line between the stock and grafted tissue (‘scion’), poor growth, yellow drooping leaves and early leaf fall. As the disease progresses the tree suffers dieback and eventually death.

The virus is transmitted through the use of infected tissue for grafts, by pollen and seed – so it can and will spread rapidly. The spread through individual trees is slow so serious symptoms may not appear for several years after infection.

Crown Gall

Caused by a bacterium known as Agrobacterium tumefaciens which mainly enters through wounds, swellings appear on the roots and lower trunk. These are not usually damaging.

Downy Spot

Widespread but only slightly damaging leaf blotch, causing yellow blotches on the top of leaves and mildewy patches below.

Nectria

Both Nectria galligena (Nectria canker) and Nectria cinnabarina (Coral spot) can infect Walnuts. N galligena causes occasional cankers but N cinnabarina is more harmful, and can lead to wilting, rot and dieback. Any identified infection should be cut out and burned.

Botrytis

Botrytis cinerea or Common Grey Mould is only likely to affect plants in very wet or humid conditions. The fungus can affect harvested nuts if they are not dried correctly.