Can diseases be identified by symptoms alone?
I recently gave a talk on diagnosis at the Society of American Florists Pest Management Conference. I showed many images that are specific to particular diseases and tried to give listings of key differences between diseases and environmental or phytotoxicity problems. I also listed differences between fungal and bacterial leaf spots.
I don’t know how the audience felt, but by the end of the talk I was once again convinced that few diseases can be accurately identified by sight alone. These include powdery mildew, downy mildew (sometimes), rust, Anthracnose, Myrothecium, Botrytis, Sclerotinia and southern blight (Sclerotium rolfsii). Each of these fungi produces distinctive fruiting bodies. If they did not make these structures, you could not identify them with any degree of surety. This leaves a long list of diseases that one distinguishes by sight alone including nearly all (if not all) root diseases. You must have these diseases diagnosed by a lab to be able to choose the best control strategy. Failure to do this will inevitably lead to the use of the wrong fungicides or the wrong rates of the right fungicides.
Are there drawbacks to using diagnostic test kits?
A variety of diagnostic test kits for viruses, bacteria and fungi is available. I have used a few of the virus test kits with very good results. They are easy to use and generally accurate if reports I hear from growers and researchers are true. The only drawback here is that the virus you have might be a new one that has no test kit.
For bacteria, the kits have a different drawback. In the case of Xanthomonas campestris pathovars, there is a special test for each genus (or closely related plants). Sometimes the test kit identifies to X. campestris, but you cannot tell that it is the specific pathovar that attacks your crop. To further complicate matters, bacteria (even Xanthomonas) can live on the surface of plants they cannot attack, and not all of these bacteria are pathogens.
Finally, we tested some kits years ago for soil-borne fungi like Rhizoctonia. We paired the test kits with testing on the ornamental plant directly and found that while they were good at identifying to genus, they did not tell you if you had a pathogen. We have seen results on other fungi, such as Pythium, that were similar — not all Pythiums are pathogens. Many isolates of soil-borne fungi are saprophytes. In addition, the kits test positively for dead fungi. In both of these situations, you would come to the conclusion that you needed to treat with a soil fungicide when the test might simply be telling you that there is a Pythium present that might be a saprophyte (not a pathogen) and, even worse, might already be dead. Wouldn’t you rather know for sure that additional chemicals were needed and get a recommendation on which fungicides will work for your specific situation?