By: Jeff Hagfors, Certified Arborist WI-0181A
As I drive around the “Badger” state, I am noticing many Colorado Spruce trees that look thin or transparent. In most cases the cause for this is Rhizosphaera needle cast. This is a fungal disease on the needles that flourishes during cool wet springs as the new needle growth is taking place. The cool wet spring will increase the duration and severity of the incubation period, causing the disease to be worse. We have seen the past 5 years with unusually cool wet springs. Rhizosphaera needle cast can also be found on
The first tell tale sign of a Spruce with Rhizosphaera needle cast is the loss of the innermost needles. These needles turn brown and with the use of a hand lens one can observe the black spherical spores erupting from the surface of the needle. This is an important diagnostic step because evergreens will normally shed needles at the end of the season even if they are not infected with Rhizosphaera. This is commonly referred as seasonal needle drop.
Spruce trees that are in the shade and in direct competition for sun light tend to have a greater propensity to become infected with Rhizosphaera needle cast. Proper plant spacing can help to avoid this issue.
Fortunately, trees with Rhizosphaera can be treated. Using a properly mixed blend of fungicides that are labeled and registered for controlling Rhizosphaera will curtail the formation and spread of the disease. The fungicides act as a barrier that prohibits the growth of the spores. In many cases this fungicide may need to be applied up to 3 times a year. Other tactics may also need to be employed such as fertilizing, Mycorrhizae injections, compost tea treatments and proper use of mulch. These treatments will aid in keeping the tree healthy and vigorous. Most importantly supplemental watering needs to be done when the soil is dry. One thorough watering is better than several shallow watering’s. The rule of thumb is 1 inch of water per week. When providing this important additional hydration, please avoid wetting the foliage where possible.
All Spruce trees are different. Your Wachtel Certified Arborist will help you to design a strategic plan custom made for you, your trees and your budget. If you think that your Spruce trees may have Rhizosphaera needle cast, give us a call we can help.