We hear this question a bunch. It is a tough one to answer but the best way to describe it is with the disease triangle. All diseases are a fungus that is affecting the growth of your tree or shrub. The Disease Triangle model lays out the three variables needed for a fungus to thrive. The first variable is the host; what type of diseases go after it? The next piece is the pathogen; is that disease itself present in that plants area? Finally, what are the environmental conditions in your area? Some fungi like cool, others like it warm.
Fungicides work better as preventatives instead of curatives. Which is why if your lawn, plant, or tree has a history of a disease it is a good idea to treat it when all the conditions of the triangle are present before your see damage rather than wait until you see it. Repetitive infection of a lawn, plant, or tree can cause death.
Get a Closer Look at Common Diseases
The following pictures are examples of diseases and fungi that could appear this time of year. You will notice most of these diseases have the same effects but different hosts. You may also notice, all of these fungi thrive in cool, damp weather.
Needle Casts effect pine and spruce trees. When looking at the needles on infected trees you will notice the tips are green, then there will be a yellow band, and the rest of the needle will be brown.
Cedar Apple Rust is a unique disease that actually takes two forms on two different hosts. During the summer it attacks apples and crab apples. It’s the cause of the bruising we find on our apples and other fruits. It also causes black spots on the trees leaves which eventually turn yellow and fall off.
During the summer, hot dry winds will blow spores to the Cedar Trees and form the brown balls that you can occasionally find at the end of their branches. These balls will slowly open and push out their telial horns when rain falls.This is where the snot like substance pictured above comes from and is how the spores are transferred back to fruit trees.
Apple Scab is very similar to Cedar Apple Scab in appearance on crab apples and apples but it overwinters on fallen leaves instead of the cedar tree. The difference is, Apple Scab creates a olive green like blemish while Cedar Apple Rust forms more of an rust orange blemish.
The disease Anthracnose is another one that causes brown spots that follow the major leaves veins. Again the leaves will eventually turn brown and fall off. Anthracnose can also continue back into the tree branches and cause it to die back. It mainly effects Maples, Ashes, and Sycamore. Weather is a major factor in the severity of this disease, a cool wet spring will make this more severe.
Just like most other diseases and disease, Shot Hole Fungus, creates black spots on the leaves of rose plants. This disease is commonly mistaken for bugs eating the leaves because over time it creates holes in the leaves; then they turn yellow and fall off.
The Long Term Effects
As you may have noticed, most of these disease eventually end in their hosts leaves falling off. Trees leaves fall off every year in the fall, so what’s the big deal? Well, when trees lose their leaves early they’re missing out on crucial time where they should be conducting photosynthesis and storing nutrients. After years of early defoliation these trees and plants will be weakened. Their natural immunity won’t work as well making them more susceptible to insect infestations which will eventually lead to death. To help prevent this from happening check out tree and shrub disease preventative and treatment programs.