You Are Here: Management Updates 2005 Archive June 10, 2005

Date: June 10, 2005
Category: Diseases, Nematodes
Subject: Discrepancies in Diagnoses from Various Labs

Discrepancies in diagnoses from various labs
From time to time you will note that different diagnostic labs will provide you with conflicting information regarding the cause of a turf disease or nematode population. There are several reasons why this may occur and suggestions are given below. It is extremely helpful to the diagnosticians if you provide them with accurate case-history information such as overall field symptoms, fungicides used within the previous two weeks, when symptoms first occurred, and past and prevailing environmental conditions. One other thing to keep in mind, if you received two different answers, than you can be fairly certain that in either case, detection of two different pathogens occurred. You may have to use your own judgment as to which diagnosis is more important. Your own experience will be most helpful in this instance. Don't hesitate to give us a call to discuss the results.

Fungal Disease Discrepancies:            

1. The specimen had more than one disease, and different labs zeroed in on one but not the other. An example showed up recently; a bentgrass specimen was received that had take-all in the roots but anthracnose was also present on the foliage. Take-all may have predisposed the bentgrass to anthracnose although both diseases should be treated. One of the two disease may have been much more prevalent than the other (depending on the specimen) resulting in the diagnosticians coming to different conclusions.

2. Two diseases are present but one is on the way out, and the other is coming in. A diagnostician may have ignored one disease and focused in on the other.

3. The plugs incubated during transit, or the diagnostician incubated the plugs to try to force the pathogen to develop to facilitate the diagnosis. In this case, a pathogen may have been forced into activity that was of little or minor importance. This occurs because most of the pathogens are present in the turf all year. When incubated they can be forced into activity and cause the diagnostician to come to a wrong, or less accurate diagnosis.

4. Non-infectious agents (physical injury, environment, agricultural chemicals, etc.) may be the primary cause but secondary pathogens of various kinds may be present. Depending on who is doing the analysis, various conclusions may be reached.

Nematode Population Discrepancies:

1. Nematode populations are not well distributed in soils. This is especially true for ring nematodes which can be in dense populations in a single soil core while absent in another core. If you send cup-cutter plugs to two different labs, you can expect to see significant differences in the two plugs.

2. Composite samples (15 to 25 soil cores mixed together) will give the most accurate average population but even in this case, if a single lab runs the same sample twice, the numbers will not be exactly the same. This occurs because it is difficult to thoroughly mix the soil; and, only about  75% of the nematodes can be extracted during the assay. The exact numbers of nematodes are not important. If the numbers were very high, then the nematodes probably contributed to root dysfunction, if they were low, they were probably not a factor.

3. In addition, nematode populations are highly variable from green to green and year to year

Submitted by: Dr. Robert Wick
in cooperation with Dr. Nathaniel Mitkowski, University of Rhode Island

Links followed by the symbol are external links that open in a new window.