ISSUE 3 May 29, 2008
The cool weather this spring has slowed the growth and development of several tree species, including Colorado spruce. Here in Fargo, the spruce trees have begun to send out new growth, but they are definitely behind. Memorial Day is usually our rule-of-thumb date for beginning treatment for Rhizosphaera needlecast.
Rhizosphaera needlecast is a fungal disease that infects spruce trees, especially Colorado blue spruce. The classic symptoms of Rhizosphaera include death of the older needles, with healthy new growth towards the tips of the branches (Figure 1). The other key characteristic of Rhizosphaera is the line of small black dots (fungal fruiting bodies) along the length of the needles (Figure 2). It is easy to confuse the fruiting bodies of Rhizosphaera with those of Stigmina lautii (Figure 3). Stigmina is a fungus that is similar to Rhizosphaera, though it is still unknown if it is a pathogen or not. These fruiting bodies are noticeable with a 10X hand lens and are located in the needlesí stomates which are normally white (Figure 4).
Rhizosphaera can be controlled with fungicides containing chlorothalonil, but it takes a total of four applications over two years. The first application should occur when the new needles are half elongated; usually that is approximately Memorial Day, but this year it should occur in the next week or two. The second application should occur three weeks later. The third and fourth applications follow next year at the same times. For further information on Rhizosphaera, see the NDSU Extension service publication PP-1276, "Spruce Diseases in North Dakota."
Rhizosphaera needs wet conditions to thrive and is more common in the northern and eastern parts of our state. Although I have seen Rhizosphaera as far west as Minot, I doubt that it would be seen 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.
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 have defined margins, unlike
those of Stigmina lautii (Figure 3). Not all
of the stomates show this sign.
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 tendrils sticking out from
the central fruiting body(s) (B). Some non-sporulating
fruiting bodies appear smooth, similar to pycnidia of
Rhizosphaera. Photos by Justin Knott.
Figure 4. Normal spruce needle with lines of white stomates.