ISSUE 2 May 21, 2009
RHIZOSPHAERA NEEDLECAST ON SPRUCE: CORRECT DIAGNOSIS IS FIRST STEP IN TREATING THIS MANAGEABLE DISEASE
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 not yet begun to send out new growth. Cool spring temperatures and wet conditions can also favor development of needlecast disease on spruce. Initial chemical treatments for Rhizosphaera needlecast are coming up soon, but trees should not be sprayed just yet.
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 Rhizosphaera kalkhoffii with those of Stigmina lautii (Figure 3). Stigmina lautii is a fungus that superficially resembles Rhizosphaera kalkhoffii, though 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 needles’ 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(s) (B).
Some immature fruiting bodies appear smooth,
similar to pycnidia of Rhizosphaera kalkhoffii.
Photos by J. Knott.
Figure 4. Normal spruce needle with lines
of healthy white stomates.
We have to 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. Identifying the fungal fruiting bodies in the stomates is crucial for proper diagnosis and before fungicide treatments should be applied. 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", but this year tree development has been delayed. The second application should occur three to four weeks later. The third and fourth applications follow next year at the same times. A fifth and sixth application is 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 has 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 http://www.ag.ndsu.edu/pubs/plantsci/trees/pp1276w.htm. If a fungicide is used, be sure to read, understand, and follow the labeled instructions.
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.
EMERALD ASH BORER – ONE STEP CLOSER TO NORTH DAKOTA
About a week ago, on May 14, officials with the Minnesota Department of Agriculture announced that emerald ash borer (EAB) had been found in St. Paul. This destructive insect is from eastern Asia and was discovered in 2002 near Detroit, Michigan. It is killing all of the native ash trees (genus Fraxinus) in that region, and it has slowly been expanding its range. Green ash is the most common tree species in North Dakota shelterbelts and was the species of choice in urban areas to replace American elm trees lost to Dutch elm disease. EAB will have a huge impact in North Dakota.
Identification – EAB adults are a bright, metallic green, about 1/3- to 1/2-inch long (Figure 1). Larvae (Figure 2) grow to about 1- to 1-¼ inches in length. The larvae destroy the inner bark of ash trees, eventually killing them. Adults can be difficult to locate and, depending on the time of the year, even impossible to find. Locating larvae beneath the bark is possible only by scraping or peeling the bark away. This destructive sampling may not always be desired. Locating exit holes (Figure 3) may be a better way to locate EAB in an ash tree. However, EAB infestations usually begin near the top of the tree, so finding exit holes may initially require some climbing. It may be best to hire a professional arborist to inspect trees in which EAB infestation is suspected. The initial symptom of EAB infestation is a gradual dieback of the tree crown. However, this symptom can be found on many ash trees in North Dakota, so it cannot be used as the only characteristic for diagnosing an infestation of EAB.
Figure 1. Emerald ash borer adult. The
adults feed on the leaves of ash trees,
but the primary damage is done by the larvae.
Photo from USDA-APHIS.
Figure 2. EAB larvae, which grow to about
1- to 1-¼ inches in length, destroy the inner
bark of ash trees, eventually killing them.
Larvae can be found in ash saplings as
small as 1-inch in diameter. Photo from USDA-APHIS.
Figure 3. D-shaped exit hole, approximately
1/8-inch diameter, caused by emerald ash borer.
Many other insects attack ash trees in North Dakota;
however, their exit holes are distinctly different
from those of EAB. Photo from USDA-APHIS.
Treatment – Because EAB has not yet been identified in North Dakota, preventative insecticide treatments are not yet recommended. The chemical insecticide imidacloprid, sold under several trade names, has been used to treat trees for EAB though it is not 100% effective. Imidacloprid’s main advantage is that the homeowner can apply it as a simple soil drench around the tree. Injections of imidacloprid, applied by professionals, have been more effective than soil drenches. The chemical emamectin benzoate, sold under the trade name TreeÄge (pronounced triage), can also be injected into trees for control of EAB. Early test results with this chemical show nearly 100% effectiveness. However, treatments are expensive and must be applied every year.
One of the most-asked questions regarding EAB is, "When will it be found in North Dakota?" Indeed, that is the million-dollar question. The main method of dispersal for EAB is by people moving infested firewood. EAB has travelled hundreds of miles by this mechanism and this is likely how it will be introduced into North Dakota. If you have visitors from out-of-state, please make sure that they leave their firewood at home.
If you have questions about EAB or suspect that your trees are infested with EAB, please contact one of the following individuals:
Dave Nelson, ND Dept of Agriculture – 701 328-4765
Sarah Tunge, ND Forest Service – 701 228-3700
Joe Zeleznik, NDSU Extension Service – 701 231-8143