Apple Tree Disease: Scab

Learn about venturia inaequalis, or apple tree scab a fungal infection which can severely damage your apple harvest.

What is it?

Scab in apple trees is a plant disease which is caused by venturia inaequalis. Venturia inaequalis is a fungus and in addition to peach and nectarine trees, venturia inaequalis scab also attacks pears. Damage from apple tree scab can be severe, affecting the fruit harvest, as well as the apple tree itself. Deformations, cracks and entire tree death are common in apple trees infected with venturia inaequalis fungal scab.

What does it look like?

The first signs of fungal scab in apple trees usually presents as small olive to brownish and velvety feeling spots on the fruit. Typically these small spots are and olive green to a brownish or tannish color and the spots of scab often appear about half way through the apple fruits growing cycle. Apples usually have spots which begin as a small olive colored cluster on the fruit near the steam and spread outward in an ever growing circular pattern of small olive green spots which turn brownish the more velvety they feel. As the venturia inaequalis fungal scab infection gets more severe and intense, the small olive spots will turn brownish. Often as the olive or brown spots continue to erupt and spread the apples size will become dwarfed, and the apples will have deformations in their growth pattern, or crack open and split open entirely. Leaves of the apple trees may also be infected with venturia inaequalis scab. This venturia inaequalis damage will show up as small holes in the leaves, or brownish spots and pitting on the apple fruits, leaves and twigs. Often many of the twigs die off. In severe cases of venturia inaequalis fungal infection, the entire apple tree can die if the fungal scab is not treated.

How does it manifest?

Fungal spores which have over wintered on apple plant debris such as fallen leaves and twigs, emerge. Venturia inaequalis fungal spores often over winter on fallen fruit which is known as a mummy, or mummies if there are more than one apple fruit. Fungal spores are also splashed from the cankerous scab lesions on the apple tree bark. Water splashes venturia inaequalis fungal spores from these over wintered portions of infected apple trees. Wind can also spread venturia inaequalis fungal spores. Those fungal spores which land on uninfected portions of healthy apple fruit trees, then begin to infect the healthy areas rapidly. As venturia inaequalis scab infection continues on previously uninfected leaves, twigs and apples the entire tree may quickly become reinfected and show signs of scab damage due to the spread of fungal spores which over wintered and spread each spring. This is why it is especially important to treat signs of venturia inaequalis scab as soon as they emerge and are evident as apple tree damage and not let the damage continue on to the following year's growing season.

What can you do about it?

Once the characteristic olive or brownish velvety spots appear, you have no choice but to ride out the rest of the season. Apples which have spots but have not cracked open or split can be eaten if peeled. Although these apples may look unsightly, they are still edible. Those appleswhich have cracks or splits in their skins should be discarded because they have been open to additional fungal or bacterial infections. At the end of the harvest be sure to clear up all fallen fruits and plant debris so that the venturia inaequalis fungus cannot over winter on them as mummies. The following year as soon as the petals have fallen from the apple trees, you will want to spray the apple trees with captan containing fungicide. You may need to re-spray at ten day intervals if venturia inaequalis scab is particularly troublesome in your region. You can continue applying captan to your apple trees up until about 30 days before the apple harvest.

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