Which of the following methods is NOT a field sampling technique for pathogens?

Visual estimation of pest presence is not classified as a field sampling technique for pathogens. This method relies on the observer's judgment regarding the presence and abundance of pests based on visual cues rather than systematic collection and analysis of samples. Field sampling for pathogens generally involves more structured and quantitative approaches, such as collecting soil or tissue samples, which can then be tested in a laboratory setting for specific pathogens.

In contrast, the other methods mentioned represent established sampling techniques. Random soil sampling helps ensure a representative sample of the soil is obtained across a given area, addressing spatial variability. Tissue sampling based on symptom progression involves selecting samples from plants that exhibit specific symptomatic changes, facilitating targeted pathogen identification. The "Z" pattern sampling method for soil involves a systematic approach to collecting soil samples in a specific geometric pattern to encompass a larger area effectively. Each of these techniques provides a more reliable and objective means of assessing pathogen presence compared to simple visual estimation.