The walnut tree not only delights us with the tasty, versatile and healthy walnut, the imposing tree also enriches gardens, garden plots and orchards with its abundance and grandeur. So that both the visual enrichment and the yield are permanently secured, every hobby gardener should be able to recognize and combat diseases and pests at an early stage. Here you will find a brief summary of everything you need to know about the most common pests and diseases of walnuts.
Diseases
Although the number of diseases that are theoretically possible in the walnut tree is enormous, the diseases that actually appear frequently can be limited to a handful of pathogens.
bacterial burn
cause
- Xanthomonas juglandis bacterium
appearance
- black, water-soaked round spots on leaves, young shoots and fruits
- partly with a yellow border
- in the further course the spots flow into one another to form extensive, mostly sunken black areas
transmission
- usually hibernation or dormancy of the latent pathogen in dormant buds
- Eruption with rising temperatures and high humidity
- subsequently infestation of young tissue, older tissue, on the other hand, is usually less susceptible
Follow
- death of young shoots
- premature leaf and fruit drop
remedy
- effective control is not possible because effective pesticides are not available and infested areas are difficult to identify and ultimately remove
- disease that is particularly common in wet years
anthracnose
(also Marssonina disease)
Marssonina leaf spot disease
cause
- Mushroom Gnomonia leptostyla (formerly also called Marssonina juglandis)
appearance
- in the initial stage very similar to bacterial blight
- small black, mostly angular spots on leaves and fruits
- on the underside of the leaf in the area of the spots often rings of dark, brown to black dots - fruiting bodies of the pathogen
transmission
- Transport of fungal spores through the air after the fungi have overwintered in frost-hard capsules in dead walnut leaves
- Release in wet weather by throwing off the spores and releasing into the air
danger: In contrast to bacterial blight, old tissue on the walnut tree is far more susceptible than, for example, young leaf tissue!
Follow
- premature shedding of heavily infected leaves
- Infections of the leaf veins and shoots are very rare
remedy
- Fighting is currently not possible, since the use of fungicides is currently not permitted
tip: The removal and destruction of the dry leaves under the walnut tree in autumn effectively prevents an infestation in spring and prevents an intensive infestation from the start!
shell fragility
cause
not known
appearance
- incomplete, thin shell of the walnut, so-called "paper nut"
- partially perforated shell, especially at the nut tip
transmission
- not known
- Occurs more frequently in cool, rainy years with little sunshine
Follow
- Damaged nuts either fall off or are at least more susceptible to parasites and other pathogens
- if the shell has holes, the fruit usually spoils due to mold or rot
remedy
- not known, as the cause has not yet been clarified
Although there are other clinical pictures, they occur to such a small extent that they almost have to be regarded as exotic. A detailed consideration from the point of view of the hobby gardener does not make much sense, since the exemplary selection would be just as arbitrary, the occurrence of precisely these selected diseases on the home tree. Examples of these unspecific and rather rare diseases are:
- Powdery mildew - recognizable by a whitish mealy coating on the leaves
- Stem and bark cancer - recognizable by changes and growths in the bark
- Wood rot - recognizable by changes in the wood structure up to the softening of entire trunk segments, as well as by the fungi that cause it, e.g. honey fungus or sulfur polypore
- Diseases of Phytophthora - lead to degeneration of the roots, recognizable by yellowed leaves, low growth and drying out young shoots
pests
In addition to diseases, pests also threaten the walnut tree and the walnut itself. However, the most common pests can be easily identified and controlled with appropriate countermeasures:
aphid
Look
- sucking pest with an elongated, transparent to greenish body
- Body length a few millimeters
damage
- Accumulation of aphids can usually be seen on the underside of the leaves
- there biting the plant veins for nourishment via the plant sap
- actual damage only occurs when it occurs enormously, then initially curling up and later drying out of affected leaves
remedy
- with moderate infestation no immediate pressure to act
- only cut off and remove infested branches ideally in the case of intensive infestation
- Fight severe damage from chemical or "natural" insecticides, for example by spraying with potash soap or rapeseed oil
tip: An intensive aphid infestation can be effectively prevented, particularly by the colonization of the natural predators of the aphid. These include parasitic wasps, ladybugs and lacewings.
bluesie caterpillars
Look
- yellowish white body
- covered with pinpoint warts
- Body length up to 6 centimeters
damage
- Death of shoots and whole branches by piercing the bark and creating feeding passages in the wood
remedy
- effective control hardly possible
- Cut off and remove affected shoots
- However, if the trunk is infested, felling of the tree is usually unavoidable
walnut fruit fly
Look
- yellow, elongated body with black tip on abdomen
- Wings transparent with black horizontal stripes
- Maggots yellowish white
- but mostly recognizable by the damage caused
damage
- Damage to the walnut by penetrating the pericarp and duct seizure
- initially formation of black dots on the still green nutshell
- later watering down of the peel and formation of an ever softer consistency
remedy
- After infestation, collect infested nuts and dispose of them
Tip: In order to avoid an infestation and thus a reduction in the yield of the walnut from the outset, the flies can be kept away with yellow boards that are hung in the tree. Even more effective, but also more time-consuming, is to tie down the entire tree with a fly net.
leaf damage
Regardless of a single cause, numerous caterpillars and beetle species lead to leaf feeding. However, as long as this only occurs sporadically, there is usually no danger to the tree or the hoped-for yield. However, if certain caterpillar species occur massively in individual years, a tree can be severely affected as a result. In these cases, it is then worthwhile to identify the infested species at an early stage and then determine suitable control methods.
As with the symptoms, there are numerous other parasites that do not specifically attack the walnut tree, but appear here with very low frequency, as they do on many other tree species. A detailed naming and explanation should therefore be dispensed with. Examples of these parasites include:
- Willow borer - damage to the branches and trunk by gang boring
- Gall mites - Damage to the leaves by so-called "galls", sometimes complete destruction of affected leaves
- Butterfly caterpillars - again and again appearing invasively, then unspecifically eating off entire leaf stocks of the trees
willow borer