Apple scab is one of the most popular fungal diseases of apple and it is caused by a fungus called Venturia inaequalis. This fungus is active especially during moist, cool spring weather, so an effective control for the apple scab symptoms is required in order to prevent possible damages.
All apple growing areas can be affected by the apple scab. This disease also has an economic importance in the mid-Atlantic regions of the US because it can compromise and destroy crops up to 70 percent if it is not treated correctly and in time.
This dangerous disease can affect all the parts of an apple tree, from foliage and flowers, to petioles and fruit. Apple scab symptoms can also be noticed on the young shoots or the bud scale of a tree. The biggest crop loss comes from damage of the fruit, while affected foliage leads to defoliation of tree.
First signs of infections are the existence of velvety textured, olive green to brown-shaded lesions or spots on the new undersides of the foliage in spring. These lesions can be seen on both sides of the leaves as leaves grow. The spots caused by apple scab symptoms feature feathery margins and grow larges as the disease evolutes. As for the lesions on the fruit, they have a corklike texture. In early fall, the later stages of infections come with white fungal growth on lesions.
There are many damages produced by the apple scab, all of them impeding the growth of foliage and fruit. The curled and distorted foliage starts shivering and eventually falls. The defoliation of the tree diminishes flower buds for the next year. Young fruits cannot develop on infected areas, while the healthy fruit develop normally. The effect of this disproportion is deformed shapes and cracks. The fruit affected in late season features tiny, round, black lesions while in storage.
But what can you do in order to prevent the appearance of apple scab symptoms? Start by clearing all fallen leaves under the tree during fall, because spores remain on leaves and may affect the new spring growth. During bloom and petal fall, apply preventive sprays on trees. Repeat this procedure two weeks after petal fall, too. Chlorothalonil or propiconazole are among the recommended fungicides to use for apple scab prevention.