Diagnosis is the art and science of identifying the cause of the plant problem under investigation. Problem causes can be
- abiotic (cultural and environmental) including chemical injury, inappropriate temperatures or soil moisture, mechanical injury, nutrient deficiency or excess, and salt damage
- biotic (living organisms, pests) including insects, nematodes, mites, mollusks, plant pathogens, and weeds
- combinations of causes including both abiotic and biotic (e.g., drought stress and spider mite damage) or a pest insect and plant pathogen it vectors (glassy-winged sharpshooter and Xylella fastidiosa)
To diagnose the cause(s) of unhealthy plants
- Ask and answer appropriate questions to define the problem and obtain information relevant to the situation under investigation. See DIAGNOSING THE CAUSE(S) OF UNHEALTHY PLANTS for a detailed example of this.
- Thoroughly examine the plants, production areas, and plant care practices.
- Use appropriate field diagnostic (quick-test) kits and properly collected samples for laboratory analyses of specific, suspected causes of abiotic and biotic maladies.
- Compile the information and consult additional references and resources including local University of California Cooperative Extension (UCCE) Advisors and their publications.
- Make an informed diagnosis.
For example, to diagnose whether an insect is the cause of damage, it is important to
- Learn the pest's life cycle.
- Be able to identify each of development stages of the pest.
- Know how that pest feeds on the crop or otherwise damages plants.
- Learn which pest life stages are susceptible to the available management methods.
Note that the most apparent life stage (e.g., flying adults captured in sticky traps, or adult female pests) can differ from the stage(s) that feed on and damage plants and are susceptible to control actions (commonly the immatures, larvae or nymphs). To help you diagnose pest problems, consult COMMON SIGNS AND SYMPTOMS ON PLANTS DAMAGED BY PEST INSECTS, MITES, SLUGS, AND SNAILS (ARTHROPODS) AND THE PROBABLE CAUSES.
See DIAGNOSING THE CAUSE(S) OF UNHEALTHY PLANTS for a detailed example.