Walnut black spot (Ophiognomonia leptostyla) and how to protect your tree effectively

Black walnut

Are you waiting for a great walnut harvest but looking at your tree made you sad? Well, you probably saw black nuts and drying leaves with black-brown spots. If so, you are dealing with a disease called walnut black spot (Ophiognomonia leptostyla). This disease is widespread not only in Europe, but also in North America. What is the life cycle of the fungus, how to fight it and which sprays work best? You should find out from the following lines.

What are the main causes of walnut blackening?

The fungus that causes this disease can survive throughout the winter in leaves that have fallen down. At the beginning of spring, after new leaves appear, the fungus infects them and later reaches the fruits. If the weather is wet enough, the fungus spreads very quickly. Even healthy trees can get infected – by the wind. After the tree has been infected, brown spots with borders starts to appear on leaves. At first they are yellow-brown, but gradually turn brown and finally, the entire leaf turns brown. It deforms and falls off. Spots appear not only on the leaves, but they also appear on petioles and fruits. Spots appearing on the fruits are black and gray and slightly indented.


Photo: Pixabay

There are several ways to fight back

First of all, you must get rid of all fallen leaves. Try fertilizing your trees using a fertilizer with higher content of nitrogen and you will be able to slow down the spread of the disease. Treatment is most efficient when applied before and after flowering. If you have young trees then you should apply chemical sprays. Recommended sprays include for example Kuprikol 50 or Cuprocaffaro or Flowbrix.

How to spray mature trees

Spraying large mature trees is impossible. That is why you should spray smaller trees – while you can. If you need to spray a large tree then use a gas-powered garden sprayer. But keep in mind that the most important thing is to keep removing the affected leaves.

Preview photo: Pixabay

Radek Štěpán

Gardening is my hobby, I have a lot of experience and I am happy to share it.


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