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Diagnosing Field Crop Problems

Fact Sheet Number AF-16
Revision Date: 7/13/2012
Nancy F. Gregory, Plant Diagnostician

Introduction

Diagnosing crop problems is similar to being a detective. The investigator must examine and evaluate all clues, establish the facts, and synthesize them into a conclusion. A number of principles can be helpful in solving problems. These include the following: start by being prepared, bringing sampling tools, keeping an open mind, and investigating all possibilities. Gather a complete history. Be diplomatic. Collect environmental data and take adequate representative samples. Keep collected samples in good condition and solicit expert opinions when you’re not sure. Diagnose the problem and then help your client correct it. Keep complete records. Refer to the Agronomy Fact Sheet AF-15 entitled “Diagnosing Crop Problems for Ag Advisors” for detailed information on how to troubleshoot problems.

A troubleshooter must begin with an open mind and learn to investigate ALL the possibilities. Jumping to conclusions is the bane of successful troubleshooting. Don’t assume that the current problem is the same as the last similar one. Don’t let one of the interested parties lead you to erroneous conclusions. If you’re not sure about some aspect of the problem, bring in an expert to help. Difficulty in diagnosing problems can occur when more than one cause or stress is involved. Often, symptoms or damage become more severe when a combination of causal agents is involved.

Clues to the problem are either signs or symptoms. Symptoms are the external and internal reactions or alterations of a plant as a result of a disease or insect. A sign is either the pathogen or pest or its parts or products that are seen on an infected/infested host plant.

Symptoms

For field crops, the investigator should look at the pattern of symptoms in the field. Symptom patterns are useful in determining if the problem is man-made, biological, chemical, environmental, or is related to soil type. Patterns can be regular, circular, scattered, irregular, or involve whole fields or units. Clues often consist of a set of symptoms or signs observable on individual plants. By categorizing the symptoms and then using a symptomology key, the investigator can quickly define or narrow down to a number of possibilities. The following key contains only a partial listing of potential causes of the symptoms described. Always submit a sample to an expert for confirmation if you are unsure.

I. ENTIRE PLANT AFFECTED

  • 1. Entire plant showing sudden wilting and subsequent browning of foliage followed by plant death.
    • a. Lightning injury
    • b. Certain vascular diseases such as Fusarium or Verticillium wilt
    • c. Boring insects (on a small plant, wilting can be “sudden”)
    • d. Low temperatures
    • e. Chemical injury or exposure (herbicides, some insecticides and fungicides)
    • f. Too much or too little water
    • g. Fertilizer or salts injury
  • 2. Entire plant stripped of leaves or most leaves show shredding damage.
    • a. Hail
    • b. Violent wind, wind-driven sand, wind-driven rain
  • 3. Entire plant shows distorted growth, often leaves have difficulty emerging from the whorl.
    • a. Rapid growth syndrome due to rapid change in environmental conditions
    • b. Improper application or exposure to growth regulator type herbicides

II. WHOLE PLANT OR PARTS OF PLANT AFFECTED

  • 1. Whole plant or parts of plant yellow/brown or wilting
    • a. Certain diseases such as wilt, crown rot, or root rot
    • b. Root-feeding insects
    • c. Boring insects (look for evidence of frass and/or gum)
    • d. Physical, mechanical or animal damage
    • e. Chemical injury or exposure (look for damage on weeds or drift damage on nearby plants)
    • f. Poor soil conditions
  • 2. Whole Plant Stunted or Groups of Plants Stunted
    • a. Compaction
    • b. Nutrient imbalance
    • c. High or low pH
    • d. Root feeding insects
    • e. Nematodes (root knot or soybean cyst)
    • f. Canker diseases
    • g. Root and crown rot diseases

III. SCATTERED GROWING POINTS AFFECTED

  • a. Heat stress, too little water during hot periods
  • b. Excessive salts in soil due to fertilizer, poor quality irrigation water, or road salt (diagnose with electrical conductivity test)
  • c. Poor soil conditions due to:
    • i. low fertility
    • ii. compaction
    • iii. excessive water or poor drainage
  • d. Certain diseases such as late blight

IV. LEAF SYMPTOMS (COLOR)

  • 1. Leaves with irregular brown blotches or “burned” areas
    • a. Certain diseases
    • b. Physiological- sunscald or lack of water reaching leaves
    • c. Spray damage
  • 2. Leaves with veins browned
    • a. Chemical damage
  • 3. Leaves with interveinal areas browned or off-color
    • a. Nutritional deficiency (nitrogen, manganese, magnesium, zinc)
    • b. Certain diseases
    • c. Leafhoppers (hopper burn)
    • d. Herbicide damage
    • e. Air pollution
  • 4. Leaves with green/yellow mottle or mosaic
    • a. Virus
    • b. Low night temperatures during leaf development
  • 5. Leaves stippled or off color (Stippling is numerous small discrete areas of discoloration and is often a symptom of insect feeding damage)
    • a. White to yellow stippling (leafhoppers or thrips)
    • b. Bronzed stippling (mites – associated with tiny reddish eggs and tiny cast white skins and sometimes fine webs)
    • c. Air pollution (also often bronzed)
  • 6. Leaves translucent
    • a. Leaf miner or skeletonizer insects
  • 7. Leaves unusually colored, sometimes attractively so.
    • a. Sport (cultivar or mutation)
    • b. Certain herbicides (pale yellow to white)
    • c. Virus

V. PHYSICAL DAMAGE TO LEAF

  • 1. Leaves skeletonized or fed on from only one surface
    • a. Beetles
    • b. Slugs
  • 2. Leaves chewed
    • a. Caterpillars – caterpillar or frass evident on plant or below
    • b. Sawfly larvae
    • c. Grasshoppers – ragged edges evident
    • d. Beetles (are usually skeletonizers except at early stage)
  • 3. Leaves with holes only
    • a. Weevils and other beetles
    • b. Small, young caterpillars
    • c. Mechanical damage such as hail, blown sand
  • 4. Leaves curled or distorted
    • a. Herbicide injury (ex. 2,4-D, dicamba)
    • b. Aphids
    • c. White flies
    • d. Viruses and certain fungi
    • e. Leaf rollers (insects)
    • f. Thrips
    • g. Leaf tiers (leaves tied together with strands of silk)

VI. ABNORMAL GROWTH OR GALLS

  • 1. Above ground portion of plant
    • a. Smut fungi
    • b. Insects (all parts of plant-abundant) such as mites, aphids, and midges
    • c. Phenoxy and benzoic herbicides
  • 2. Below ground portion of plant
    • a. Root knot nematode
    • b. Nitrogen-fixing bacteria nodules (normal, distinguish from look- alike injurious growths)
    • c. Herbicide injury to roots (causing shortened, thickened clubby roots and a swollen hypocotyl)

VII. PHYSICAL DAMAGE TO STEM

  • 1. Borers (frass or sawdust sometimes exuding from damaged area).
    • a. European corn borer
    • b. Dectes stem borer

VIII. DAMAGE TO PODS, EARS, OR HEADS

  • 1. Distortion of pod, ear, or head.
    • a. Insects such as maggots, earworm, beetles
    • b. Larvae of moth or butterfly
    • c. Stink bug
    • d. Nutrient deficiency such as calcium or boron
    • e. Certain diseases such as scab or smut
    • f. Pollination problem
    • g. Rapid change in soil Moisture
    • h. Virus
  • 2. Damage or holes in fruit
    • a. Insect
    • b. Mechanical damage, animal damage, or weather pitting

Acknowledgement: This fact sheet was developed using information originally developed by Derby Walker, Jr. and Dr. Dale Bray, revised in 1987 and 1998 by R. P. Mulrooney and Dr. R. W. Taylor.

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



Original Publication Date:

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.

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