So far this month of May has been very wet with more than sufficient rainfall and cooler than normal temperatures. All this is good for the trees that were planted last month but not so good for the developing fruit. It’s difficult to keep a protective covering on the trees and fruit when there is too much moisture. A common fungal disease that affects fruit during a cool, wet spring is Fruit Scab.
Fruit Scab is a very destructive fungal infection. The fungus (Venturia inaequalis) overwinters in fallen leaves and spreads during a cool, wet spring. Scab develops and presents itself as velvety, olive-green to black spots on the leaves of the apple tree. Leaves may curl, turn yellow and drop prematurely. On the actual fruit scab begins as small, circular spots that become brown or black. As the fruit continues to grow the skin breaks leaving rough, scabby lesions. Severe infections of scab can lead to deeply cracked and misshapen fruit.
We try to keep scab infestation in our orchard at a minimum by prevention and sanitation. We rake and destroy fallen leaves and dropped fruit so as to eliminate the primary overwintering source of the fungus. In the spring any fungi that has overwintered shoots spores into the air. These spores are carried by the wind to developing leaves, flowers, fruit or twigs. Spores need several hours of moisture on the plant’s surface to start a new infection. These infections grow into the spots that are seen on the leaf’s surface. This infection cycle can be repeated many times throughout the growing season. Warm, rainy weather in the spring and summer gives scab ideal conditions to grow and multiply.
Proper pruning of the fruit tree provides a certain degree of scab prevention as well. When pruning we try to cut branches so as to improve airflow and sunlight. This allows the leaves and fruit to dry faster and makes an inhospitable environment for the fungi to develop. Planting disease resistant varieties is also a good way to manage scab.
Timing is everything when it comes to the treatment of fruit scab. Treatment begins early and continues throughout the growing season. Fungicides or sulfur-based sprays are widely used to suppress the growth of the fungus. When using any spray it is necessary to coat the fruit and foliage thoroughly including the undersides of the leaves. The goal is to kill the spores on contact so there is no additional spread of the fungus. Remember, however, safety should always be first. Read the label directions of the product for exact methods of application, safety warnings and how long it is necessary to wait between the final spray and picking the fruit.
An orchardist works tirelessly trying to provide a product that is pleasing in appearance and safe to consume. At times Mother Nature creates many challenges and does not cooperate as well as the producer would like. It’s a tough job but for some of us, it becomes a life’s passion.