Caused by Venturia inaequalis, apple scab is a fungal disease of apple that can destroy a lot of crops. Apple scab can appear in all apple growing regions and it is active especially during spring, when the weather is moist and cool.
This fungus infection has a serious economic importance because it can destroy up to 70 percent of the harvest if the apple scab symptoms are not properly controlled. Still, there are some apple varieties such as Liberty, Prima, Redfree, Sir Prize, Janafree and Enterprise that can resist the apple scab fungus.
Unfortunately, this disease infects all tree areas, especially flowers, petioles, foliage and fruit. Symptoms can be seen also on the bud scale and young shoots of the tree, but the most serious harvest losses come from fruit infection. If the foliage is infected, defoliation of the tree occurs, influencing the entire health of the tree.
First apple scab symptoms appear in spring, when the new foliage undersides features velvety textured, olive green to brown-shaded lesions or spots. When leaves grow larger, lesions are present on both sides. The feathery-margin spots grow larger in size as the disease advances. The infection of the fruit manifests through a corklike texture of it. Among the advanced apple scab symptoms, the white fungal growth on lesions appear in early fall.
Apple scab fungus influences the proper growth of the tree. The distorted foliage shrivels and falls. This defoliation can also reduce the flower buds for the next year. Infected young fruits stop growing, even if the healthy ones develop normally. Due to this growth difference, there are irregular shapes and cracks of the tree. When the fruit is infected with this fungus in late season, there will be little, round-shaped, black lesions on the fruit while in storage.
There are some ways to prevent and treat the apple scab symptoms. Throw away all fallen leaves of the tree during fall, because the spores are still on the leaves and can affect the spring growth. You can use preventative sprays and fungicides such as chlorothalonil or propiconazole to apply on trees during bloom time and petal fall, but also two weeks after the petal have fallen.