Keep an eye on your spruces during our rainy spring

As of May 23, 2017 the state of Minnesota is nearly four inches above its normal amount of rain. At Altura in Winona County, nearly five inches of rain fell on May 15, setting a record for the day. Another record was set on May 16 when 1.40 inches of rain fell in Duluth.

The rain, in combination with cooler temperatures in recent weeks, provides suitable conditions for needle cast diseases to infect spruce. Needle cast diseases can be spotted by the discoloration of older needles on spruce, while the current year’s foliage is not affected. Infected trees appear barren close to the stem, especially on lower branches. The pathogens which cause needle casts overwinter on living and decaying needles.

Rhizosphaera needle cast on an infected spruce. (photo: Michelle Grabowski)

The Rhizosphaera needle cast causes significant damage to spruces and is commonly found across Minnesota. The appearance of small black dots along the length of needles can confirm Rhizosphaera. Colorado blue spruce is most susceptible, followed by white spruce, Black Hills spruce, and to a lesser extent, Norway spruce.

Management of Rhizosphaera can be accomplished by applying a fungicide with the active ingredient Chlorothalonil. However, according to Extension Pathologist Michelle Grabowski, timing is critical when applying fungicides. A first spray should occur when new needles are half the length of mature needles, followed by a second spray three to four weeks later after needles have developed. Two years of treatment are required for spruces to recover.

Additional management techniques include pruning lower branches to increase air circulation beneath trees. Avoid planting spruce at close spacings and near older trees that show signs of infection.

Submitting a foliage sample to the University of Minnesota’s Plant Disease Clinic can confirm the presence of Rhizosphaera. For more on Rhizosphaera, see Michelle Grabowski and Cynthia Ash Kanner’s post on identifying and managing Rhizosphaera.

Matt works on issues related to forest ecosystem health. He's based in St. Paul.

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  1. We are located in Winona, and our spruce trees have been infected. We are having them sprayed and will trim the lower branches. My question is whether the affected branches will recover or if they will need to be cut off, leaving gaps in the tree.

    1. Amy,
      A general rule of thumb is that if needles are infected for 4 years in a row, the entire branch will die. If you’re within four years, the branches will likely recover.