Issue 1, April 16, 2010

Boxwood Problems Remain

Calls about boxwood problems started for me about two years ago. I continue to receive calls concerning boxwoods, and the problem is the same. Plants show areas of bronze foliage that eventually brown and die. Sometimes the bark on the stem falls off, especially near the soil line. Often the tissue is black under that loose bark. Roots in general appear healthy but we often find some black roots interspersed when we have a root system to inspect. Some plants die.

The major disease problems of boxwood include Phytophthora root and crown rot (Phytophthora cinnamomi and P. parasitica), Verticillium wilt (though actually rare), Root Knot and Lesion nematodes (Meloidogyne and Pratylenchus spp), Volutella leaf and stem blight (Volutella buxi), Macrophoma leaf spot (Macrophoma sp), and a condition called boxwood decline.

There are some insect and mite problems of boxwood to consider. Boxwood leafminer, boxwood psyllids, and spider mites may be found on boxwood in Illinois. None of these pests cause the symptoms described here.

When a complete boxwood sample is received, such as the one in the image, our lab has tested for Phytophthora (with ELISA), Verticillium (via cultures), nematodes (via soil extraction), Volutella and Macrophoma (incubation). The only pathogen we have found is Volutella. Since the literature mostly refers to this pathogen as a secondary invader, it does not appear to be the cause of the symptoms listed above.

At a recent meeting with North Central diagnosticians in the NPDN network, others reported seeing the problem as well. No one had a pathogen to blame.

Boxwood decline is a situation that has been associated with English boxwoods. The decline has been reported since the 1970s, and appears to be caused by a complex of stress factors. Reports of boxwood decline refer to it as a slow decline, over several years. The boxwood situations that I have seen have been very quick, often over one season. Still, there may be some connection. A fungus called Paecilomyces buxi has been associated with the English boxwood decline. Clinic staff will look more closely at boxwood roots for this fungus in 2010.

It appears that the main problem with many boxwoods is related to environmental stress and not a specific pathogen. This injury is much more severe than winter burn or scorching. Entire stems have died. In some cases plants have died or injury is severe enough to warrant plant removal. Some plants show problems in wet areas, some in areas with little care, others in high maintenance areas. I do not know exactly what is happening. I have spoken with many horticulture and plant pathology specialists, growers, and retail folks about this problem. They all know it exists and would all like a quick fix. As far as I can determine there is no quick fix. Fungicides to control Volutella would not be expected to give long term control. If your client is demanding long term control, I would be careful not to promise anything. In such a case your best option might be to remove the plant. Investigate tops and roots to determine whether the problem started below ground or above ground. If this is a root rot situation, do not replant in the same area. Improve site stress where practical before replanting. If using boxwood, choose a boxwood species and cultivar that has been proven to be hardy in your area for many years.

Follow this link for an article on boxwood diseases by a plant pathologist at Virginia Tech. The author addresses both Phytophthora root rot and English boxwood decline in that article. Images are included.

Another link to read is this Clemson fact sheet that addresses both disease and insect/mite problems of boxwood.--Nancy Pataky

Nancy Pataky

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