QUICK TIP: Rake up all apple scab infected leaves as they drop. Throw the infected leaves away or burn them. Do not use apple scab infected leaves in compost as the fungal spores can survive winter and will increase the spore count around your yard the following spring!

Apple scab disease is caused by a fungus (Venturia ineaqualis) that survives winter on previously infected leaves and twigs. In spring, the spores are released and may be blown by the wind or splashed by rain onto tender emerging leaves. Signs of apple scab infection include yellow/brown lesions on the leaf surface and leaves dropping prematurely. The leaf drop is usually seen mid-August, but in the case of a severe infection, new leaves will fall as early as May. In order for the apple scab fungus to infect your tree, three factors must occur simultaneously. The “disease triangle” for apple scab includes: wet, cool weather; available spores; available leaf surface. Because we live in Minnesota, these three conditions occur nearly every spring, around late April/early May.

Apple scab affects trees in the genus Malus which includes ornamental crabapples and eating apples. It is a species-specific fungus and will not be transmitted to your other landscape trees or shrubs. Apple scab is not considered to be a serious threat to a healthy crabapple or apple tree; however, repeated annual infections of apple scab will progressively weaken the tree and leave it susceptible to attack by secondary disease or insect problems. Apple scab can also disfigure fruit from your eating apple tree.*

CONTROL:

Your tree should be treated to prevent apple scab if it’s:

a) an important tree to keep healthy

b) it’s aesthetically unbearable

c) it’s already stressed by construction/root zone disturbance or previous years’ infections of apple scab

d) you don’t want to spend your summer cleaning up fallen leaves.

Prevention is the best control. Two fungicide applications spaced 2-3 weeks apart as the leaves are emerging in spring will achieve about 80% control for that season. Pruning in the winter following a severe infection can help to remove any twigs killed by the apple scab fungus. Removing excessive sucker growth from inside the canopy as well as proper structural pruning will improve the air circulation and appearance of your tree. Because the apple scab fungus is transmitted by water, don’t allow your sprinkler system to hit the leaves. Water only in the early morning hours so the foliage can dry by nightfall. If possible, install a soaker hose around the critical root zone. A soaker hose will use less water as well as irrigate only the targeted plants.

Plant growth regulators have shown promise in treating apple scab fungus. Growth regulators such as Cambistat™ work inside the tree and last 3-5 seasons. Cambistat™ benefits your tree by promoting more root growth during times of stress, such as a heavy infection of apple scab. Cambistat™ may be the correct solution for your landscape. Your arborist will evaluate the situation and make recommendations which work for you.

*Majestic Tree Care is licensed to treat ornamental trees not agricultural crops.