ISSUE 3 May 27, 2010
RHIZOSPHAERA NEEDLECAST ON SPRUCE: CORRECT DIAGNOSIS IS FIRST STEP IN PROPER TREATMENT
With the recent warm up, many trees that had delayed growth are now taking off. That includes Colorado blue spruce trees. While the precise timing of growth and development varies slightly from one tree to the next, many spruce trees have needles that are at the stage that they may need protection from Rhizosphaera needlecast.
Rhizosphaera needlecast is a fungal disease that infects spruce trees, especially Colorado blue spruce. The classic symptoms of Rhizosphaera needlecast include brownish purple discoloration and eventual death of the older needles, with current year needles showing no symptoms (Figure 1). The other key characteristic of Rhizosphaera needlecast is the tiny rows of small black dots (fungal fruiting bodies) along the length of the needles (Figure 2). It is easy to confuse the fruiting bodies of the Rhizosphaera needlecast fungus with those of Stigmina lautii (Figure 3). While the fruiting bodies of the two fungi resemble each other superficially, it is still unknown if Stigmina lautii is a pathogen or not. The fruiting bodies of both fungi are noticeable with a 10X hand lens and are located in the needlesí stomates, which are normally white (Figure 4).
Figure 1. Spruce tree severely infected with
Rhizosphaera needlecast near Walhalla, N.D.
Notice that most of the older needles are gone,
with only current-year needles remaining.
Figure 2. Spruce needle with lines of black dots,
indicating the fruiting bodies of Rhizosphaera needlecast.
Note that the fruiting bodies are smooth and round with
defined margins, unlike those of Stigmina lautii (Figure 3).
Not all of the stomates in this sample contain fruiting bodies.
Figure 3. Stigmina lautii fruiting bodies on spruce
needles. Fruiting bodies may occur in most of the
stomates (A), resulting in black rows replacing the
white rows of wax, or the fruiting bodies may be more
scattered (B). The spores appear as tiny, hair-like
projections sticking out from the central fruiting body (B).
Some immature fruiting bodies appear smooth, similar to
Rhizosphaera kalkhoffii. Photos by Justin Knott.
Figure 4. Normal spruce needle with lines
of healthy white stomates.
We must emphasize the importance of proper diagnosis of needlecast disease before treatment is initiated. Other pests and environmental problems can cause browning and death of older needles. These other causes can be easily confused with needlecast disease. Before the decision is made to apply fungicide you must accurately identify the fungal fruiting bodies. The fruiting bodies can be seen with a good hand lens or occasionally by eye.
Rhizosphaera needlecast can be controlled with fungicides containing chlorothalonil, but it takes a total of four applications over two consecutive years. The first application should occur when the new needles are half elongated. We usually say "around Memorial Day", though tree development may be delayed or accelerated in specific parts of the state. The second application should occur three to four weeks later. The third and fourth applications follow next year at the same stages of needle development. A fifth and sixth application are sometimes recommended during the third consecutive year. Application timing is critical. Spraying too early or too late will miss the stages when the plant can be protected from infection by the fungus. Unfortunately, many people treat trees only when it is convenient for them, rather than when the plant can be protected. A lot of time and money have been wasted by applying pesticides at the wrong time. For further information on Rhizosphaera needlecast, see the NDSU Extension Service publication PP-1276, "Spruce Diseases in North Dakota" at www.ext.nodak.edu/extpubs/plantsci/trees/pp1276w.htm If a fungicide is used, be sure to read, understand, and follow the label instructions.
The best way to reduce the likelihood of needlecast infection is to promote air circulation around the trees by not planting trees too close together and by minimizing vegetation growing in close proximity to the tree. Rhizosphaera needlecast needs wet conditions to thrive and is more common in the north and east parts of our state. Although we have seen Rhizosphaera needlecast as far west as Minot, we expect it to occur rarely, if at all, west of U.S. Highway 83. It is most common east of U.S. Highway 52 and is especially prominent in the Devils Lake basin and the Red River Valley.
NDFS Forest Health Specialist