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How Rainffall and Fungal Growth Impact Spruce and Boxwoods

In recent weeks, Purdue has shared that they’ve received several samples of spruce and boxwood that have been showing some serious issues worth noting. Although it has been cold and relatively early for significant in-season disease development, we have had enough rainfall to encourage fungal growth. While it might be easy to assume that issues showing up in your plant material would be from a current disease/infection we’ll share some other thoughts about what could be going wrong below.

Understanding Tree Health

Purdue acknowledged that since the beginning of the year, they had received spruce samples showing needle thinning, browning, and loss in the lower canopy. Normally we might assume that this could be a needle-cast fungus, Rhizosphaera, which causes trees’ needles to turn brown and shed. However, this fungal infection primarily affects Colorado Blue Spruce, and Purdue mentioned these samples are primarily from other spruce species. It’s also important to remember that it takes time for symptoms to develop in evergreen conifers, whether due to disease or abiotic factors. The majority of the branches sent in for sampling lacked any discoloration within, suggesting that there was no infection and that the limbs were still alive. Last year, we experienced drought conditions leading to below-average precipitation, which could cause winter injury or burn and needle browning, especially near branch tips (exposed areas). To help prevent this it’s critical to water your trees not only during periods of hot and dry weather as this will help mitigate drought stress, but also may still be necessary in the fall to avoid needle desiccation. Each tree is unique, and even trees of the same species on the same property can have different reactions to stress due to factors like the overall health of the tree when it was planted, the amount of love and care it received, and the site conditions (soil, light, general water levels).

Spoting Early Signs of Damage

Purdue also mentioned that they have also been receiving boxwood samples with yellow ring spots on the leaves. This is a symptom of active feeding by the boxwood leafminer, which can be quite striking. The maggot feeds within the leaves and causes the leaf to bubble out where these blotch-like leaf mines are located. This year, there have been anecdotal reports of seeing early leafminer activity, which could be associated with the milder winter we have had in the Midwest. We typically see leafminer activity starting later in the year, and the damage often goes undetected by homeowners until much later in the season.