Issue 2, April 25, 2016
Spruce Needle Casts (Rhizosphaera and Stigmina)
Spring has barely sprung and the Plant Clinic has already received a number of samples this year. Most common have been spruce and pine samples with a variety of issues including the ever-present fungal needle casts (Rhizosphaera and Stigmina on spruce, Diplodia and Lophodermium on pine). Most evergreen samples have been diagnosed with environmental stress as a contributing factor. The last several years have been very stressful to plants, especially to evergreens with the historic wet spring last year, harsh winter the year before, and two years of drought a few years before that. We recommend taking an active role in maintaining plant vigor to reduce stress; many pest issues are found more frequently on stressed trees, and the trees are less able to repair any damage. Pruning out dead wood, lightly mulching the base of the tree, watering during periods of dryness during the growing season, and fertilizing appropriately are recommended practices for keeping evergreens healthy. Proper planting is also critical for a well-established tree or shrub.
Spruce tree infected with Rhizosphaera needle cast. Note the bare inner branches with tufts of green, un-affected needles on the tips. Photo credit: University of Illinois Plant Clinic.
There are two major fungal needle casts of spruce in Illinois: Rhizosphaera and Stigmina. Both diseases produce similar symptoms: last year's needles turn purple-brown and eventually drop while the new growth is asymptomatic, leading to bare branches with tufts of green needles at the tips. Both diseases are associated with stressed trees. Colorado blue spruce are the most commonly affected spruce trees in Illinois, while white and Norway spruce tend to be more resilient. Needles must be observed under magnification to distinguish between the two fungi. Management focuses on increasing tree vitality. Fungicides are recommended for managing Rhizosphaera, but have not been shown to be very effective against Stigmina. Infection occurs 12-18 months before symptoms appear, so fungicides must be applied in early spring to protect the new growth from infection. Chlorothalonil is labeled for use against Rhizosphaera and available for homeowners to purchase. Fungicides containing chlorothalonil, chlorothalonil + thiophanate-methyl, copper, and copper hydroxide are also labeled for use against Rhizosphaera, though a commercial applicator's license may be required to purchase or apply these chemicals. Fungicides should be applied in early spring when the needles are half-grown (as soon as the bud caps fall off) and again when the needles are fully elongated. Several years of repeated applications may be required to manage these needle casts. We also highly recommend reducing stress on the affected trees to make them less susceptible to infection and help them recover from damage. (Diane Plewa)