Tree Talk: Trees and leaves

Aaron Kiesz
City of Aberdeen, Parks, Recreation and Forestry Department

It is not uncommon to start seeing various leaf diseases on trees and plants this time of year. Many of these diseases can reoccur annually, and some depend on what kind of weather we are having in the spring and summer. Apple scab is a common ailment that affects apple and crabapple trees. It appears on leaves as dull, brown, irregular spots and can change to light green velvety spots. These leaves will normally start to fall off the tree in various degrees. The best way to manage this disease is to plant varieties that are resistant to apple scab. There are treatments available, but they are very timely and aren’t always successful. Fireblight is another apple disease which quickly turns leaves brown/black, and they do not drop to the ground. The bark in this area will look shriveled and also turn brownish/black. There is no effective treatment for this disease other than pruning out the infected branches.

Marssonina leaf spot is showing up on cottonwoods and poplars and is driving a lot of phone calls. This is a common leaf disease that shows up on poplars and turns the leaves black and causes them to fall to the ground. There are no effective treatments for this problem, but it shouldn’t affect the overall health of the tree even if it causes significant defoliation.

Another disease we are seeing on several different kinds of plants including lilacs is powdery mildew. This fungal disease will have leaves appear with a white, powdery substance on them. The disease is worse in summers where we experience hot and humid weather, which we have seen plenty of this summer! The best way to manage powdery mildew is to prune planting beds to open them up for more air flow which will decrease the night humidity. If watering plants at night, don’t get the leaves wet.

Many of the leaf disorders we see this time of year are common and don’t require treatment. If you have questions, or want more information, please feel free to call or email me at or 605-626-7015.