Walnuts are susceptible to several diseases and pests, and even though most of them do not threaten the life of plants, they may however ruin a sizable portion of the edible nuts and the visual appearance of the plant
Black walnut trees can be infected by diseases specific to this plant, the most common are:
Also known as bacterial blight, it is caused by Pseudomonas juglandis or Xanthomonas campestris pv. juglandis. It usually causes black spots on leaves, holes and blotches on fruit, and possibly dieback of shoots. Fruits are usually damaged extensively, especially when the male catkins are affected
Walnut Blight affects mainly young growth and nuts, and is the most damaging when it occurs during cool and wet periods, around flowering time (so it is most dangerous to species which leaf early). The bacteria will overwinter in seemingly healthy buds, launching a new attack when new growth is at hand.
Treatment consists of cutting out damaged parts (cutting too much is better than cutting too little) and burning them. Bordeaux mixture may help too. Prevention may be attained by using a soil with a pH above 6, avoiding excessive nitrogen, excessive wetness and allowing for good aeration by pruning
This disease also goes by the name of Walnut anthracnose, its scientific names being Gnomonia leptostyla, Marssonina juglandis or Marssoniella juglandis.
This widespread fungus, it causes nuts to turn black and then fall, as well as brown blotches on leaves and fruits. It also causes a great number of leaves to fall.
This disease spreads well in wet weather, usually during May and early June and will overwinter on leaf litter. The only treatment consists of burning fallen leaves and maybe using a Bordeaux mixture
Other diseases are common to several plants, including walnut trees, and the most common are:
Also called Shoestring fungus, Mushroom root rot and Oak root fungus, this disease is caused by Armillaria mellea
This is a fairly dangerous disease: it enters through roots and wounds, affecting and rotting roots, butt and collar. It can also spread through the ground to neighboring trees. Eventually, the plant may suffer defoliation, dieback and death
It should be noted that American Walnuts (in particular J. hindsii, J. nigra and hybrids) are resistant to Armillaria mellea and using them as rootstock may be useful in preventing the disease (even though they suffer increased probabilities of contracting Blackline). Diseased plants should be removed carefully to prevent spreading of the disease
This is a dangerous disease caused by a virus (a strain of the Cherry Leaf Roll virus - CLRV-W), and it only affects non-regia rootstocks.
Plants affected by blackline suffer yellow drooping leaves, stunted growth and early leaf fall. The name of the disease comes from the typical dark line between the stock and grafted tissue. The tree may suffer dieback and death
The virus spreads through pollen, seed, and infected grafts, so it tends to spread fast
These pests generally do not cause the death of walnut trees, but they may damage them and reduce their visual impact or fruit yield
These pests are not encountered frequently, but it might be useful for you to know them
The following pests are (usually) only found in North America