Karl Foord, UMN Extension Educator
Dimitre Mollov - Director of Diagnostic Services, UMN Plant Disease Clinic
The challenge in diagnosis
You can see the challenge in diagnosis when someone hands you three leaves and asks what is wrong. Sometimes this can be easy with clearly diagnostic insect chewing or piercing damage or pathogens with characteristic necrotic lesions. However there are times when the evidence is asymptomatic, or confounded by more than one organism, or saprophytes who have followed wounds created by other means or organisms. This is why the information sheets submitted with the sample are so important; the more that is known about the specific situation the more information there is to work with in complicated situations.
The importance of information beyond the sample
A well supported sample has information about the plant's symptoms and what parts of the plant were affected. Is there a pattern observed such as the problems began at the top of the plant and worked their way down or started at the bottom and worked their way up. Were other plants in the area affected? When were the symptoms first observed? Other information about the site such as slope, or predominant compass direction of exposure is helpful. The soil type and drainage of the site as well as other chemical inputs to the situation are useful to the diagnostician.
As an example, a sample that had just been received was a three year old Fir tree sent in by a Christmas tree grower. The small tree was about two feet tall and the length of annual growth nodes had been decreasing for the last two years (Exhibit 1). In this case there was no evidence of the presence of an insect or disease pathogen. Given this one would have to expect abiotic factors. Also this stunted growth was not uniformly distributed throughout the field. It is possible that the tree is experiencing problems associated with a high pH soil. See the Climate and Site Requirements section of the publication entitled Choosing Landscape Evergreens. http://www.extension.umn.edu/distribution/horticulture/dg1430.html However, in this case a soil analysis had not been performed and the diagnosis could not be definitive. Dimitre recommends having a soil test done before sending in samples for pathological analysis. Having such basic information is a good base from which to continue diagnosis. But what if the sample does show other symptoms?
A systematic approach to diagnosis
The approach I learned from Jeff Hahn and Michelle Grabowski is to first look for insect damage which is typically at a macro level and can be viewed with the naked eye. The next step is to look for signs of fungal or bacterial pathogens. Dimitre begins this at a macro level with a dissecting scope (picture) and confirms identification with a microscope (picture). He will look for characteristic fungal structures such as spores or mycelia or evidence of the presence of bacteria from cell breakdown or lysis. Should these forms of identification not be present Dimitre has the ability to perform laboratory tests for viruses. Correct diagnosis of viral diseases normally requires laboratory tests because symptoms induced by viruses can also occur due to adverse environmental conditions. Common laboratory tests include identification of specific proteins of the virus by ELISA (enzyme-linked immunoasorbent assay) or DNA of the virus by PCR (polymerase chain reaction).
Some of these tests for viruses are now available to the commercial grower or passionate homeowner. One supplier is Biobest who make Flashkits for viral detection.
The lesson I took from my visit to Dimitre is as follows: Understand your limitations as a diagnostician and the environmental impacts of your decision. If you cannot identify the pathogen with certainty, avoid application of environmentally potentially harmful chemicals that may have little impact on the problem. To help in diagnosis you can use the diagnostics section of the extension website.
If you want more information about the clinic or to obtain sample submission instructions and forms, please go to the UMN Plant Disease Clinic website.