Diagnosing plant problems can be challenging. A site visit can provide the information necessary for a complete and accurate diagnosis. However, once on-site, it is important to know how to proceed. The following guidelines are intended to assist in the process of gathering pertinent information and determining a possible cause. Often abiotic conditions such as environment, mechanical damage, or living organisms like insects or wildlife may be to blame. Should the field site diagnosis be inconclusive and samples need to be submitted to the UK Plant Diagnostic Laboratories, the information gathered here can provide valuable supplementary information.