Diagnosing the Causes(s) of Unhealthy Plants

Diagnosing the cause(s) of plant problems can be done by asking a series of pertinent questions and investigating the answers:

Questions About the Plant

• What are the genus, species, and cultivar of the plant in question?
• Is this particular plant suited to the production area?
• Is the cultivar resistant or especially susceptible to diseases or other problems?
• Is the plant sensitive to certain environmental factors (e.g., salinity, excess or deficient soil moisture)?
• How does the plant normally appear when grown under various conditions (full sun, shadehouse, greenhouse, coastal versus inland production locations, winter versus summer) or at different stages of growth and development?
• What is the normal growth rate?
• What are the characteristics, appearance, and growth habits of a healthy plant, and could these features be confused with an unhealthy plant?

Questions About the Symptoms

• What are the symptoms of the affected plant?
• What plant parts are affected?
• Are symptoms restricted to external plant surfaces or are there also internal symptoms (vascular streaking, discolored crown tissue)?
• Are symptoms present only on exposed plant surfaces or also on protected covered tissues such as unexpanded inner leaves or unopened flowers?
• What is the distribution of the symptoms on any one particular plant (do symptoms occur on one side of the plant, only on older or newer leaves, on secondary roots but not on primary roots, etc.)?
• How did the symptoms first start?
• How do early symptoms differ from more advanced symptoms?
• How long have the symptoms been present?
• What is the extent (distribution and severity) of symptoms in the population of plants of a given species or cultivar?

Growth Stage

• What is the growth stage of the affected plant (seedling, cutting, newly transplanted, mature flowering or fruiting plant, senescent plant)?
• Is a particular growth stage associated with the problem?
• What was the condition of the plant when first placed in the production area?
• How does the growth rate of the affected plant compare to that of a healthy plant?

Symptoms on Other Plants

• Are symptoms restricted to one species or one cultivar of plant?
• Do the same symptoms occur on one type of plant that is located in different growing areas?
• Alternatively are there different plantings of the same plant type that are symptomless?
• What other plant species and cultivars appear affected?
• Do adjacent plantings, weeds, or nearby landscape plants exhibit similar symptoms?
• If other plants are affected, do they belong to a common group or family of plants?

Patterns

• How are the symptoms distributed within the specific production area(s) of concern?
• Are there patterns (repeating numbers of plants or plant rows) to the symptoms or do symptoms occur randomly throughout the planting?
• Are symptomatic plants found in clustered groups?
• Do the symptoms occur in lines, streaks, or circles?
• Are symptomatic plants found mostly along the edges of the planting?
• Are affected plants next to buildings, ditches, roads, weedy areas, or other crops or production areas?
• Are symptoms associated with subsets of plants within the planting, indicating an association with plants from certain transplant trays, different sources of plant material, or other production factors?
• Are symptoms associated with physical features at the nursery, such as low or high spots of the field, places where water does not drain well, presence or absence of underlying gravel, or particular soil types?
• Do such areas become flooded after rains or receive irrigation runoff?
• Are affected areas under sprinklers or watered by drip irrigation?

Timing

• What temporal factors are important?
• When did the symptoms first occur?
• Are there various stages of symptoms indicating new infections versus older ones?
• Have the features or severity of the symptoms changed over time?
• Do symptoms appear to have developed gradually over a period of time or rapidly and all at once?

Biotic or Abiotic

• Do symptoms resemble those caused by biotic agents, such as pathogens, nematodes, arthropods, or vertebrate pests?
• Alternatively, are symptoms more suggestive of physiological or abiotic factors, such as nutritional problems, physiological disorders, genetic mutations (chimeras), chemical damage, or environmental extremes?
• Do the symptoms provide evidence that more than one factor or pathogen is involved?

Signs

• A sign is the visible presence of a causal agent or factor. Are such signs present?
• Examples of signs are bacterial ooze, chemical residue, fungal growth (e.g., mycelia or spores), and insect bodies or excrement (frass).
• Are there multiple signs indicating that more than one factor may be involved?
• What is the distribution of signs on the affected plant (present on all or only on certain plant parts)?
• Are signs present on all symptomatic plants and do they appear to be associated with the symptoms?
• Are signs present on adjacent plantings or nonsymptomatic plants?

Timing and Context

• What time of year did the problem occur?
• If the problem occurred before, did it take place during the same time of year?
• What are the current and past weather conditions?
• Have there been any unusual weather patterns or developments recently or in the past few weeks or months?
• Have there been any conditions that would hinder plant growth or favor arthropod or pathogen development?
• Is there evidence of abiotic stress factors (e.g., mineral deficiencies and toxicities, pollution, temperature extremes, salt buildup, water stress or excess, wind or other mechanical damage, etc.)?
• What is the general location of the nursery (coastal versus inland, next to other crops or nurseries, next to roads, etc.)?

Biotic Factors

• Which arthropod pests, nematodes, pathogens, snails and slugs, and vertebrate pests are known to occur on the host or cause feeding damage or other injury to the host?
• Which biotic agents occur in the geographic area of concern?
• Compile a list of common pests that occur in the area and may be the cause(s) of the current problems. For sources of this information see COMMON SIGNS AND SYMPTOMS ON PLANTS DAMAGED BY PEST INSECTS, MITES, SLUGS, AND SNAILS (ARTHROPODS) AND THE PROBABLE CAUSES, the "Crop Tables" at the back of Integrated Pest Management for Floriculture and Nurseries and the similar information online: Flowers; Fruit Trees, Nuts, Berries, and Grapevines; Trees and Shrubs; and Vegetables and Melons.

Source: Container Nursery Production and Business Management Manual. See this publication for more detailed help with problem diagnosis, such as example answers and problem-solving steps for the questions above.