Have you had a confirmed diagnosis of nematode issues? They are generally not visible without magnification or a microscope, and we don't usually see plant problems arise due to nematode damage. Many nematodes are "free-living" and not parasites of plants, as they eat insects or other organisms in the soil.

Standard soil testing cannot screen for pathogens; it's used only to measure nutrient content, acidity level (pH), and organic matter level (and in some cases, also lead or heavy metal content). The linked page has information about labs that run these basic tests, if you're interested. A plant diagnostic lab would need to know what pathogen is suspected in order to look for it and try to culture it to isolate which genus/species is present. Different testing methods and materials are used for different fungi and bacteria. There are a large number of common fungi in the environment which can cause plant infections and symptoms, but only a handful of them damage plants enough to warrant management or intervention. If you can share images of plant symptoms, either from prior years or from this year if they appear later this season, we can try to narrow-down what pathogen, if any, is responsible. Sometimes mites or an insect pest is the actual cause of symptoms mistaken for infection.

The identity of plants that have developed symptoms is also very important, as a pest or disease problem may be managed differently depending on what plant species we're dealing with. If you don't know what the plant type is, we can also usually determine this from photos (once it has flowers or leaves, if it's deciduous).

Miri