Jim Kerns has written this post on accurate disease diagnosis at Turf Diseases. You'll want to read the whole post – it is really good, and useful not only in understanding disease diagnosis but also in understanding diseases so they can be avoided or controlled more effectively. Here are a couple excerpts:
Golf course superintendents are forced to manage their plants on the brink of death to ensure firm, fast playing conditions. When managed on the precipice of death, many times turf falls off the edge and succumbs to decline. The golf course superintendent then must determine if a disease or some other factor is killing their turf. In this situation it is always easiest to assume a disease is causing the problem, however many times a biotic entity is not the cause of decline. Over the past 8 years, our Turfgrass Disease Diagnostic Lab has received approximately 4,000 samples. Of those samples 30 to 50% do not have the appropriate characteristics of a plant disease.
That's interesting. Almost half the samples submitted to the diagnostic lab do not have appropriate characteristics of a disease.
And when sending a sample to a lab for diagnosis, are photos useful, and if so, just what type of photo should be taken?
Photos are an excellent addition to any submission. They clearly show what stand symptoms are present and the pattern of development. We suggest only taking pictures from a distance, we typically find that close-up images are not useful.
This is something quite interesting too, that pathogens are in almost every sample, even in healthy turf.
Turfgrass pathogens are ubiquitous in nature. Turfgrass systems produce a lot of carbon and are excellent environments for microbes. In many cases we see multiple plant pathogens in a given sample. Yet we do not report all the organisms we find. For example, in almost every sample we receive significant amounts of dark, ectotrophic fungi are characteristic of take-all patch or summer patch are present on roots. Again coming back to the diagnosis triangle, diagnosing a disease is more than simply finding a plant pathogen. A disease is an abnormality in structure or function of plant due to the continuous irritation of plant pathogen, therefore it is important to realize that pathogen does not equal disease. In my humble opinion, receiving a report listing plant pathogens is not an accurate disease diagnosis.
In many cases, especially with warm-season turf during times of active growth, one sees very few diseases even though pathogens are still present.
If you have a turfgrass disease problem, this list of turf diagnostic labs from PACE Turf will help you find a suitable place to send a sample.