It happens every year, almost like clockwork. (I say "almost" because not every tree leaf disease shows up every year.) Another good point to make right way is common leaf diseases are rarely fatal to a tree.
Some of our common tree leaf diseases are: Anthracnose, often seen on sycamores; and Apple Scab and Cedar Apple Rust are very likely on fruiting apple trees and the ornamental flowering crabapples. A more recent leaf disease, Tar Spot on maples, has made its presence known in the last three to five years. And, while rose growers are very familiar with Black Spot on rose foliage, others may be used to Powdery Mildew on Lilacs and Phlox.
What they all have in common is weather. Leaf diseases are not as common and the infections not as bad when we have drier, warmer springs. The fungal spores that infect the leaves cannot travel in the air and still be viable in those hotter, drier temperatures. This was certainly not the case for the spring of 2018. There is a natural decline of infections as our normal spring weather starts to warm and the rains are less often. You easily can see this on flowering crabapples once the infection period ends; you see the new growth at the ends of the branches looking good, while the first leaves of the season are spotted or have fallen from the tree.
If you have an established landscape with disease-prone ornamentals, you will have to manage those leaf diseases appropriately. As you have the chance to update and replace plants, just like we do for vegetables, choose a naturally disease-resistant replacement. There are disease resistant plants for every leaf disease listed here. One point to remember, these plants are only resistant to leaf disease, not immune. In a normal season, they can appear quite free of disease. In a year where disease pressure is high, you are likely to see some spotting. Resistance can mean the plants do not allow infection in the first place. If infection occurs, the plant is able to confine that infection and does not allow it to enlarge.
There may be key plants in the landscape that make a focal point in the yard or maybe there is a tree next to the patio that you do not want leaves raining down from all summer. Fungal diseases must be treated in a timely manner and can only be prevented. There is no catching up if you miss those first sprays. Knowing that fungal spores are out there at the time of bud break is key. It will not be comfortable at that time of year, so you will need to "bundle up" a bit. Sprays are directed at the buds as they swell and begin to lose the over wintering bud scales to show the very first green tips of the new growth emerging.
A large shade tree can have spot diseases, but it is not one to treat due to the size. Many times, it is the lower limbs that exhibit the leaf diseases while the upper canopy is fine. One strategy is to spray as high as is possible with the equipment you have.
So, take note for 2019 if you want to incorporate disease prevention measures in the future.