The month of May brought cooler temperatures than Colorado landscapes are used to in spring. Additionally, we received twice the average amount of moisture in the month of May, and humidity levels barely dropped below 60% all month. These three factors of temperatures in the 50’s, excess moisture, and consistently high humidity have come together to create conditions that will promote several fungal and bacterial pathogens that will affect our landscapes all summer long. It is important to be aware of these diseases, their symptoms, and their potential damage to our plants.
Some of the most common fungal and bacterial pathogens that may affect our trees:
| Disease | Host(s) | Symptoms |
| Powdery Mildew | Lilacs, Cranberry bushes, Roses, Privets and turf. | White thin covering on leaf surface, yellowing leaves,and early leaf drop. |
| Fungal Leaf spot (various species) |
Aspens, Cottonwoods, Roses, Maples, Ash, Oak, and many others. |
Round or oval spots are often brown and/or black with dark margins. |
| Bacterial Leaf spot (various species) |
Ash, Oak, Prunus species, fruits veggies, Aspens and Cottonwoods. | Round or oval spots often dark in color with yellow or orange margins. |
| Fire blight | Crabapples, Apples, Pears, Hawthorns and Cotoneasters. | Twig or branch die back with characteristic hook on tips, bacterial oozing from infected twigs and discolored wood tissue. |
These diseases often infect tissue in spring, but symptoms do not manifest themselves until mid-summer. Once infections have occurred it is important to clean up diseased leaf and twig tissue to help prevent further infections. Some diseases can be prevented with fungicide/bactericide applications in the spring.
Tip of the Month:
The snow and cold temperatures of Mother’s Day weekend damaged several trees. 
The newly emerging leaf tissue of many Locust, Ash, Redbuds, and Maples was damaged by the rapid temperature drop. It will take time and energy for these trees to reproduce new leaf tissue. Be sure to appropriately fertilize your stressed trees to encourage them to reestablish their leaf canopy.