Strategies for effective diagnostic systems appropriate to plant pathology in less developed countries are presented. The development of diagnostic facilities (adapting current methods and developing new systems) must allow for the prevailing conditions, especially lack of trained personnel and finance, poor communications and unreliable utilities. Appropriate systems for each category of pathogen are discussed in this context. New molecular and serological technologies have great potential, but mainly where there is a clearly denned need, for example with routine work on specific pathogens. The first priority is a general diagnostic capacity to deal with new or unknown cases. New information technology will be an essential resource in making identifications and in consequent decision-making. To gain the maximum benefit from improved technology, diagnosticians should also be trained to take appropriate decisions during identification work. Because resources are generally scarce, identification should proceed only as far as is necessary in each case, providing a timely diagnostic report fitting the needs of the users. A computer program has been developed to assist training in decision-making and resource use in diagnosis.