Weather conditions have been a bit abnormal this summer. We can’t be certain there is a direct connection, but woody plant disease problems have also been a bit strange. At this writing, the Plant Clinic has processed 628 woody plant samples. The samples received at the Plant Clinic are certainly not a true survey of problems in the state. In fact, many problems are diagnosed on site and never reach our doors. As an example, we rarely see a case of apple scab although that disease was severe this year on crabapples. Still, the trends we see seem to reflect many concerns we hear about by telephone, e-mail, other newsletters, and “coffee talk.” The following comments refer to the woody plant disease problems at the Plant Clinic in 2004.
The wet weather early in the season produced some problems with root rots, including Pythium and Phytophthora root rots. Azaleas and rhododendrons infected with Phytophthora root rot were apparent early in the season. This early-season moisture and humidity also revealed two cases of downy mildew on rose in retail and wholesale locations. Rhizospheara needle cast of spruce infects in wet weather and is manifested 12 to 18 months later. We saw 11 cases of this disease at the Plant Clinic in 2004.
I would have expected the cases of Verticillium wilt to be few this summer. That disease tends to be worse on trees under stress; and 2004 did not appear to be a stressful year for trees. Regardless, the Plant Clinic staff isolated the Verticillium fungus from ash, lilac, magnolia, maple, redbud, and sumac this year. Watch viburnums for Verticillium wilt, too. Although we have not seen a positive case on this host at the clinic in 2004, we have had reports of growth problems that seem typical of Verticillium wilt infection on viburnum.
Cytospora is a fungus that is known as a stress pathogen. Stress can be caused by environmental extremes, as well as by factors other than environment. Cytospora canker was found in Illinois this year on aspen, cherry, fir, pine, and spruce. White pine decline is another stress situation, and 10 cases were recorded at the Plant Clinic this summer.
A few diseases that are usually common at the Plant Clinic were down in numbers in 2004. We saw chlorosis only on birch, lilac, and oak. Sphaeropsis blight of pine was found on only seven samples. Pinewood nematode was confirmed on only one pine.
The fungal wilt diseases were plentiful in 2004. This is unfortunate because these diseases tend to kill the host. In addition to Verticillium wilt described above, we found 21 cases of Dutch elm disease and 14 cases of oak wilt. Hopefully, early detection was useful in saving nearby trees.
Bacterial leaf scorch is a disease that we have discussed with concern over the past 3 years. We currently have 27 samples (mostly oaks) in process for BLS testing at Agdia, Inc., in Indiana. Watch for test results in the next issue of this newsletter. We will also keep you posted on sudden oak death reports and findings as information unfolds.