Apple scab can quickly endanger your apple tree. The fungal disease affects both the fruit and the foliage of the tree. We show you how to recognize apple scab and protect the tree from it.
Apple scab is a fungal disease that occurs worldwide and is particularly prevalent on apple trees. The so-called scab fungi cause the disease and multiply so rapidly, especially in summer, that they can even infect several apple trees. Along with powdery mildew, it is one of the most common pathogens in orchards. Apple scab can be dangerous not only for apple trees. The scab fungi occasionally infect sweet cherries and pear trees as well. By taking preventative measures, you can stop apple scab from attacking your fruit early on.
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Causes and symptoms of apple scab
The fungus “Venturia inaequalis” causes apple scab. If there is a lot of rain in spring and summer with relatively mild temperatures, the risk of spreading the fungal disease in your garden increases. The spores of the scab fungi often overwinter in fallen leaves. Then, in the spring, the wind carries the mature spores onto the leaves of your apple tree. If the foliage is wet for at least eleven hours, the scab fungi can infect the tree. The first symptoms of apple scab then usually appear on foliage and fruit in the early spring months:
- At the beginning, olive-green spots appear on the leaves.
- In the second stage, the spots on the leaves turn brown. Over time, the entire foliage thus dries up from the center.
- Infected leaves become wavy and bulbous and eventually fall off.
- Apples develop brown, torn sores on the skin. The tissue there looks dried and slightly sunken.
Apple scab mainly affects the foliage and apples of the apple tree. Fruits affected by the scab fungus are not dangerous to humans and therefore can still be eaten without any problems. However, it is no longer possible to store the apples for a long time because rot fungi penetrate through the cracks in the skin and cause the apples to spoil more quickly.
Due to apple scab, the shoots of your apple tree will not ripen properly, which means that only a few flower buds will form the following year after the disease. Therefore, you should take action at the first signs of the disease to save your apple tree from the worst effects.
By the way, the pear tree is also susceptible to the scab fungi that cause apple scab. The symptoms here are similar to those of the apple tree. In sweet cherries, usually only the fruits are affected.
Protect apple trees from apple scab as a precautionary measure
horf is one of the most common diseases of apple trees. Therefore, if you want to plant a new apple tree, you should prevent disease early. You can protect your crop from apple scab with the following tips:
Remove autumn leaves thoroughly: scab fungi prefer to overwinter in fallen leaves. Therefore, the best way to prevent apple scab is to thoroughly collect and remove the autumn leaves every year. Throw the old leaves on the compost and cover them with other waste. Leaves that are already infested should definitely be put in the organic waste garbage can, as the fungus could spread further in the compost pile.
Choose the right location: Be sure to plant your apple tree in as airy a location as possible. Air circulation will help the leaves dry faster after rain showers. Rainwater that remains on the leaves, on the other hand, increases the risk of the tree becoming infected with apple scab. It is also important that you always leave enough space between the individual fruit trees.
- Thin out the tree crown regularly: If the crown of your apple tree grows too dense, not enough air reaches the branches and leaves further in. This makes the foliage and apples there more susceptible to apple scab. Therefore, you should prune your apple tree regularly and, above all, thoroughly thin out the tree crown so that the leaves dry better.
- Field horsetail broth as a tonic: Another remedy you can use to prevent apple scab is field horsetail. From this plant you can make a silicic acid-containing broth that covers the leaves of your apple tree like a protective film. The silicic acid prevents the fungal spores from entering the leaf tissue.
- Prevent with net sulfur: Products containing sulfur have a similar preventive effect as the field horsetail broth. The so-called net sulfur kills the spores of the scab fungus on the leaf surface. When dissolved in water, the sulfur is easy to spread on leaves with a garden sprayer. While the solution poses no threat to most insects – but since net sulfur can harm beneficial ladybugs and predatory mites in certain concentrations, you should consider it only as a very last resort.
Prevent with resistant apple varieties
Some apple varieties are less likely to contract apple scab. If you want to buy a new apple tree, you should therefore preferably look for varieties with the designation “scab-resistant”. You should not do without natural plant protection even with these trees.
The following apple varieties are less frequently attacked by apple scab:
- Retina
- Rewena
- Berlepsch
- Boskoop
- Dülmener rose apple
- Oldenburg
- Topaz
- Rubinola
The Retina and Rewena apple trees belong to the so-called “Re” apple varieties. These varieties originate from the Institute for Fruit Breeding near Dresden and are often resistant not only to apple scab, but also to powdery mildew and fire blight, which is transmitted by bacteria.
How to fight apple scab: this is how you proceed
If you notice that your apple tree is infested with apple scab, you should not directly resort to chemical fungicides. These are often very aggressive and can cause lasting damage to your tree as well as to beneficial organisms. We are aware that in case of a severe infestation it is often impossible to avoid chemical treatment. Nevertheless, you should try to use pesticides and other chemicals in the garden only in case of emergency.
With the following means you can fight apple scab sustainably and environmentally friendly:
- Remove affected plant parts: As soon as you discover the first spots on foliage and apples of your tree, you should remove the diseased plant parts. Also collect leaves lying on the ground and throw them into the organic waste garbage can.
- Prune the tree crown: At the latest, when your apple tree has fallen ill with apple scab, it is time to thin out the tree crown. In this way, you ensure that the leaves dry better. However, this will only slow down the spread of the scab fungus.
If you still need to control apple scab with a fungicide, you should, if possible, only buy products without copper. Fungicides containing copper not only kill the fungus, but also many beneficial insects.
