Diagnosing Crop Problems for Agricultural Advisors

AF- 15
Revised 7/14/2012
Nancy F Gregory, Plant Diagnostician

Troubleshooting is detective work. It is the art of examining a series of clues, establishing the facts, and arriving at a solution. To be successful, the troubleshooter must begin with an open mind and learn to investigate ALL the possibilities and not jump to conclusions. Don't assume that the current problem is the same as the last, regardless of how similar it is. Don't let anyone lead you to erroneous conclusions. If you're not sure about some aspect of the problem, call an expert(s) to help. Follow a set of clues to a logical conclusion. Often detective work involves eliminating possibilities until only one is left. The following are suggestions or guidelines, not an all-inclusive guide.

Be prepared
You will need a good set of references to help interpret the data gathered. References include reputable web sites, books, images, commercial fact sheets, compendia, identification keys, and experts. Experts include Cooperative Extension professionals, University faculty, seed, field, and other agribusiness personnel, technical representatives, or consultants.

Carry sampling tools with you
A number of items can be of use in troubleshooting. These include, but are not limited to, a sharp knife, soil probe, magnifying lens, various sizes of plastic bags, and plant or soil sample submission forms and a marker. Also helpful are a bucket, sweep net and shake cloth, small plastic jars for insects, trowel or small shovel, a measuring tape, a camera for documenting your findings or patterns or symptoms, a GPS, and a cooler to protect nematode and plant samples from heat damage. Obtain comparison samples with a margin between healthy and affected or good versus bad areas. Sampling can confirm or eliminate causes of the problem. Incorrect sampling techniques can either fail to confirm your diagnosis or lead you to the wrong diagnosis.

Go to the field with an open mind and investigate all possibilities
Review your reference materials such as field guides and Compendia before going to the field. Take the appropriate reference material with you, and don't be afraid to use the materials in front of clients, or to show them references.

Where to start
Follow a systematic procedure. Start by looking at the whole field(s) to determine if there is a pattern to the problem. Ask yourself the following questions: Has the problem occurred before? Is it spreading? Are both weeds and crops affected? (Answering these questions can indicate if the cause is biotic or abiotic.) Does the problem incidence or severity vary? Are affected plants on only one soil type or in a specific area of the field? Are there defined borders to the problem? From which direction do prevailing winds come? Are there any geographic features of importance? Are there border effects from woodlands or roadways? Are plants in or near ditches, woods, or in surrounding fields affected? Have nearby fields been sprayed? The answer to these last questions can help you determine if there was a drift problem or adverse environmental conditions. The percentage of plants affected and pattern can aid you in determining if a problem is man-made, physical, chemical, physiological, biological, environmental, or a combination of factors.

Gather a complete history and weather information
Ask the grower about cultural practices. Record previous crop history, lime application, fertilizer application, chemical application and equipment usage, tillage, source of seed, and any changes made in the field. Note when symptoms first appeared. The cropping history data will often lead you to the answer or indicate what patterns to look for and what laboratory tests are needed. Compare the last several soil tests with nutrient management plans. Investigate crop yield history and plant residues for clues. When did the grower plant and under what conditions? Check turn-arounds and start locations for planting and spraying? Sometimes questions can't be answered if the farm is new to the client or if the client keeps poor records. Encourage clients to start or improve record keeping. Not only is record keeping important from a legal standpoint, but it can help solve problems quickly and efficiently. Be diplomatic and respect the client. Ensure the client of confidentiality and importance of solving the problem in a cooperative manner that is best for all. Keep in mind the proliferation of new chemical names and terminology, when asking questions.

Find out the weather conditions at the time of critical events, such as emergence, flowering, seed fill, and spraying. What weather conditions has the field been subjected to in the last few weeks? Ask for rainfall data. Wind, snow, hail, and temperature are factors that can have a negative impact on crops, depending on timing and correlation with cultural operations. Last year's rainfall can impact herbicide carryover problems.

Diagnostic Symptoms
Purple color in corn is NOT diagnostic. Purpling is a symptom and can be due to a genetic response to cold soil temperature; a soil phosphorus (P) deficiency from inadequate fertilization or from P fixation by very low or very high pH; virus infection; or a response to restricted root growth due to compaction, environmental stress, or chemical or insect damage. To diagnose the problem, correctly identify diagnostic plant symptoms or signs of pathogens. Symptoms may include yellowing, spotting, necrosis, distortion, stunting, wilt, etc. Symptoms might include root pruning characteristic of some herbicide damage and/or insect damage, low soil test P levels, evidence of compaction plus restricted root growth, or soil temperatures below 60 F or excessive water levels. If more than one cultivar or crop is in a field, are both showing the same symptoms or damage? Note the stage of growth when symptoms first became evident. Many diseases occur only at certain growth stages. Stress may pre-dispose plants to insects or pathogens.

Field Patterns
Field patterns are useful in determining if the problem is man-made, biological, chemical, environmental, or related to soil type. Patterns can be regular, circular, scattered, irregular, or involve topographical areas in fields or whole fields.


  • 1. Same row or rows across the field
    • a. Planter unit, insecticide applicator, or fertilizer unit may be set up differently, not turned on, out of materials, have mixed materials, or be experiencing mechanical problems
    • b. Nozzle problems on the sprayer - may be different-sized, worn-out, or plugged
  • 2. Abnormal growth occurs across the field in regular pattern at about the same width. Measure the widths of normal and affected areas to determine which piece of equipment was not operating properly, or whether operator  overlapped too much or drove too wide.
    • a. Spreader truck applying manure, lime, or fertilizer
    • b. Sprayers (ground or aerial)
    • c. Band applications of fertilizer or chemicals to a previous crop
    • d. Alternating strips of crop residue from the harvest of the previous crop
  • 3. A strip the width of the sprayer, with a decreasing amount of injury, followed by several acres without injury. In a large field, there may be more than one strip. The other strips begin with a new load of spray material.
    • a. Agitator on the sprayer doesn't work or is not turned on
    • b. Material left in lines from the previous spraying is toxic to this crop
    • c. Materials did not mix (compatibility problem, bad batch, or not mixed in the proper order)
    • d. Carryover problem from last year's spray application
  • 4. A streak along an edge of the field, ditch, or road
    • a. Low pH
    • b. Compaction from equipment
    • c. Double application of lime, fertilizer, or chemicals
    • d. Salt damage from road salt, manure, or flooding
    • e. Animal feeding (birds, deer, rabbits, etc.) and some insects (spider mites and grasshoppers)
  • 5. Regular-shaped sections of field are affected or appear normal
    • a. Sections had different crops or varieties the previous year
    • b. The sections received different treatments, either by design or accident such as lime or fertilizer, chemicals, different source materials, or different applicator equipment.
    • c. The sections were worked or had the treatments applied at different times because of rain or equipment breakdowns
    • d. Sections of field had been flooded or irrigated with pond water or saltwater
  • 6. Circular spots
    • a. Lightning strike--Almost a perfect circle. Often dark from electrical burn, plants may appear limp or water soaked. Crop plants and weeds present will be affected.
    • b. Nematodes--Somewhat rounded or oblong areas often elongated in the direction of tillage. Often with more injury in the center than on the outside edge
    • c. Pesticide spill
    • d. Some soil-borne fungal diseases
    • e. Some insect problems--Early infestations of sorghum by sorghum green bugs show bright reddish-purple circular spots
    • f. Stump burn area
    • g. Old orchard site
  • 7. Scattered Plant or Groups of Plants
    • a. Insects
    • b. Diseases
    • c. Animal feeding
    • d. Fertility



  • 1. Weather-related injury
    • a. Cool temperatures can intensify chemical damage, reduce nutrient uptake, increase insect and disease damage, prevent pollination, and cause physical plant damage due to slow plant growth
    • b. High temperatures can enhance crop injury from chemicals, blossom and fruit drop, and physical plant stress
    • c. Wind can desiccate plants, cause sandblast injury, bury small seedlings, and injure fruit, resulting in wound avenues for pathogens. Wind-blown rain is important in the spread of several bacterial diseases
    • d. Hail can cause physical damage which opens avenues for pathogens
    • e. Frost can damage or kill plants. It may kill the buds and flowers without damaging the rest of the plant
    • f. Excessive rain can result in flood damage, nutrient leaching, nitrogen volatilization, soil compaction, and increased disease pressure
    • g. Thunderstorms, temperature inversions, and smog can result in near-surface ozone concentrations that cause leaf bronzing and yield loss
  • 2. Man-made injury
    • a. Using the wrong chemical, improper rate, omitting one product in a tank mix, or application of the product at the wrong time
    • b. Failure to follow the pesticide label or pre-plant instructions for a crop, and planting in a field before it was safe to do so
    • c. Soil compaction from working land when it's too wet
    • d. Over application of liming materials; failure to lime
    • e. Failure to inoculate legumes
    • f. Planting too deep, early, or late in the season
    • g. Chemical drift
    • h. Improper nutrient management

Take adequate representative samples

Comparison samples are a necessity to diagnose problems properly. Use field patterns, crop symptoms, cropping history, and cropping practices to determine what kind of samples you need and where to collect them. Label all samples and make notes about the samples on sample submission forms. Digital images may be e-mailed in advance of sample shipment to a lab. For comparison samples, collect a sample with a margin of healthy and affected or normal and affected plants, or soil from both good and bad areas. If nematodes are suspected, take soil samples in border areas, where nematode populations are highest. The center of severe nematode infestation areas supports few nematodes because the plant root system has already been destroyed. If there are varying degrees of injury, take plants that represent all stages. Desiccated samples or rotted plant samples are worthless because secondary organisms may have colonized the sample and masked the original problem. Use a penetrometer or a soil probe to get an indication of possible soil compaction problems. Subjectively compare the amount of pressure it takes to push the soil probe to a one-foot depth in the problem area and along a field edge where no tillage or parking has occurred. Dig some plants and observe the roots for restricted or abnormal growth. Look for shallow root systems or roots that take an abrupt lateral turn. For suspected salt damage problems, pull your soil sample from the plant rooting zone. For a possible temporary nutrient deficiency problem, take a root zone sample and a normal depth sample for comparison.

Keep the samples in good condition

Package samples dry in zip-locking plastic bags. Do not add water. Soft-bodied insects may be kept in small plastic containers to avoid crushing. Nematode and disease samples must be kept at room temperature or cooler to keep nematodes, insects, and other pathogens alive during transport to the laboratory. This also decreases development of secondary organisms which can confuse diagnosis.


Diagnose the problem and then help your client correct it

No one can know everything so use your expert contacts to examine samples, fill in the details and confirm your diagnosis. All parties should work cooperatively to solve grower problems. When trouble-shooting, a quick and accurate response is important. However, if laboratory tests are needed to confirm a diagnosis, wait for results before recommending corrective action. Throughout the investigative process, eliminate the minor factors to discover the primary cause. For example, although phosphorus deficiency symptoms may indicate a need for phosphorus fertilization, the deficiency can be caused by root pruning insects, poor drainage, low pH, low temperature, or chemical damage. To correct the deficiency effectively, determine which factor(s) is responsible before taking action. Applying the wrong treatment will cost your client money and may still not solve the problem. Many problems disappear with a change in environmental conditions such as sunshine, warm temperatures, and/or a little rain. There will be times when it is not economically feasible to correct the problem, or it may be too late in the season to correct the problem economically. In these cases, use the opportunity to teach your client how to prevent the problem in the future. New problems do develop occasionally, and may be preliminarily misidentified. For example, Dectes stem borer was mistaken for soybean lodging due to a lack of potassium fertilizer. Soybean cyst nematode infestations were confused with potassium deficiency problems. Soybean severe stunt disease was confused with 2,4-D injury. New problems can often look very similar to old problems. Pay closer attention to the clues that don't fit the old pattern. Keep an open mind! Remember, new problems may require basic research and a longer time to diagnose effectively.

Keep Good Records!

Use a camera to take pictures. A visual record of affected plants, normal plants, and field patterns will be very useful. Keep written records of all the information you collect, all expert diagnoses and recommendations, special sampling you do, and other pertinent information. This documentation may be needed in court and will be invaluable in backing up your conclusions as well as refreshing your memory on the situation.
Acknowledgement: This fact sheet was original developed and revised in 1998 by Derby Walker, Jr. and Dr. Richard Taylor.


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Cooperative Extension Education in Agriculture and Home Economics, University of Delaware, Delaware State University and the United States Department of Agriculture cooperating. Distributed in furtherance of Acts of Congress of May 8 and June 30, 1914. It is the policy of the Delaware Cooperative Extension System that no person shall be subjected to discrimination on the grounds of race, color, sex, disability, age, or national origin.

Disclaimer: Reference to commercial products or trade names does not imply endorsement by University of Delaware Cooperative Extension or bias against those not mentioned.