Because a single symptom may result from different causes, it is often a good idea to collect as much information as possible about the field before attempting diagnosis. This includes:

soil map of field
cultivar planted
tillage practices
crops grown in the last 3 years
weather conditions while the plants have been growing
date of planting
pesticides used: date, rate and method of fertilizer application
For some problems, in-field tests are available to confirm diagnosis
- Portable pH meters
- Nitrate meter
- SPAD meter
- Soil penetrometer
- Plant tissue test
- GMO detection

Some growing seasons have considerable cool and wet conditions resulting in corn plants showing various shades of yellowing and interveinal leaf stripping. What may be the cause?

Cold temperatures. Not uncommon with early planting. Entire small plants can show lack of green color.mmon with early planting. Entire small plants can show lack of green color.lack of green color.mmon with early planting. Entire small plants can show lack of green color.
Wet soils. Corn roots need aerated soil for metabolic processes and nutrient uptake. Entire plants can show yellowing and many different symptoms, including phosphorus deficiency.
Slow soil organic matter mineralization. With cold temperatures, microbial conversion of organic nitrogen (N) compounds to inorganic N (ammonium and nitrate) is slow. If the corn plants are dependent on that source of plant available N, then plants could show N stress. Entire plants can show yellowing.
Sulfur (S) deficiency. This is related to item 3, that is, slow organic matter mineralization and lower supply of plant available sulfate-S (the form of S taken up by plants). Soil organic matter is the largest reserve of S in most soils, so slow mineralization can limit available S, especially in the upper soil profile.Classic S deficiency is the older leaves are green and the new leaves show yellowing and interveinal striping.
Continuous corn. In many springs, corn following corn tends to show more yellowing than corn following soybean, especially in reduced till and no-till. This is related to many factors, such as same crop allelopathy and less mineralization (for N and S).
Potassium deficiency. It typically begins to show on larger plants, about calf to knee high. Symptoms appear first on older leaves, with yellow to brown coloration on the leaf margins.
Corn hybrid. Some hybrids tend to show interveinal stripping more than other hybrids, and hybrids have different levels of greenness.

Equipment needed for "Trouble-Shooters Toolbox"
Plastic bags (gallon and garbage pail sizes)	2-gallon water jug	Gallon pail
Trowel	Mist bottle	Shovel
Linoleum knife	Flags and stakes	25' Tape measure
Paper towels	Multi-purpose tool	300' Tape
Marking pens	Dissecting kit	Soil probe
Penetrometer	Camera with date stamp	Notebook
Reference books	Hand lens	Rubber gloves

Troubleshooting Abnormal Corn Ears

The appearance of a corn ear can tell us much about a corn plant's development during the growing season. Ear size and numbers and distribution of kernels on the ear can indicate when the ear experienced stress and the severity of the stress. Ear number, kernel number and kernel weight are determined at six critical stages: at planting and emergence (VE-V4) when the potential number of ears in an acre is at a maximum; when the ear sets the maximum number of kernel rows (V5-V6); when the ear sets the maximum number of kernels along length of the ear (V15-VT); when the maximum number of ovules are pollinated to form developing embryos (R1-R2); when the maximum number of kernels is determined (R4-R5); and when the maximum kernel size is established (R5-R6).

Abnormal ear development has multiple causes – environmental stresses, pests, cultural practices. Combined with information on field history, knowledge of ear and kernel anomalies can be an effective diagnostic tool in troubleshooting corn production problems. A corn ear's response to a particular stress may be so unique that it can specify the stress condition involved. An ear abnormality may also be associated with more than one cause. Moreover, ears may often exhibit injury symptoms in response to more than one stress. Understanding how corn ears respond to stress can help determine the nature of the stress, when it occurred, and how it might be managed or avoided in the future.

See "Troubleshooting Abnormal Corn Ears" at http://u.osu.edu/mastercorn/

Compaction

Subsoiling is warranted only when well-defined, compacted soil layers are 4 inches or more deep hen well-defined, compacted soil layers are 4 inches or more deep deep hen well-defined, compacted soil layers are 4 inches or more deep. Compaction might suspected when a significant number of roots have grown sideways before finding a crack in the soil and growing down again. sideways before finding a crack in the soil and growing down again.

At the field look for:

Patterns of the symptoms across the field - Does damage appear to follow a planter, sprayer, tillage, fertilizer, or spray drift pattern?
Look closely at the symptoms on the vegetation
Check to see if the root system is abnormal or injured

Fertilizer Nutrients

Mobile nutrients known to cause deficiencies in corn include nitrogen, phosphorus, potassium, sulfur, and magnesium. Below is a rundown of what these nutrient deficiencies typically look like:

Nitrogen (N) deficiency makes older leaves (the bottom portion of the corn plant) turn pale or yellowish-green. The deficiency then creates a V-shape, starting at the tip of the leaf. If the problem continues, the deficiency works its way up the plant from older to newer leaves. Nitrogen deficient corn stalks tend to be thin and spindly.

Phosphorus (P) deficiency causes a distinct dark green with reddish to purplish leaf margins, typically starting from the tip. The deficiency is observed in the older leaves. Stunted growth is also typical. At early development stages, cool weather can lead to poor P uptake and purple leaves due to slow root development. Since P is fairly immobile in the soil, any soil condition that limits root growth (cool temperature, wet or very dry conditions, compaction) can induce the deficiency. Phosphorus deficiency symptoms normally disappear by the time the plant is knee-high.

Potassium (K) deficiency leads to yellowing and necrosis (death) of the edge of older leaves. When the problem persists, this deficiency will continue to move up from older to newer leaves, while the top leaves may look completely green. Potassium is critical to strong stalk development, and K deficiency can cause lodging of the crop later in the season.

Sulfur (S) deficiency causes general yellowing of the foliage. Sulfur deficiency is often confused with N deficiency. Since S is not easily translocated, deficiency tends to be more visible in the newer leaves. It is not uncommon to see interveinal chlorosis of the newer leaves. The symptoms are typically seen in soils with low organic matter, low pH, and cold, wet conditions that reduce mineralization of S.

Magnesium (Mg) deficiency appears as yellow or white streaking between veins of the lower leaves. The leaves eventually become reddish-purple, and the edge and tip die if the deficiency is severe. Acidic and sandy soils are most likely to be deficient in Mg.

Stress Situations

Winter-Kill
Chemical Injury
Frost
Hail
Drought

Special Crop Problems

Bloat
Nitrate poisoning
Prussic Acid
Toxicities
Mycotoxins

Limits of Mycotoxins (Fumonsin, DON and T2) on a dry matter basis. Action levels established by FDA (BLS lowest level of detection (LLOD) = 0.2 PPM and level of quantification = 1.0 PPM)

Fumonisin:

Equine / Rabbits = Corn, 5 ppm (not to exceed 20% of diet)
Swine = Corn, 20 ppm (not to exceed 50% of diet)
Breeding Ruminants/ Breeding Poultry (Includes lactating dairy cattle and hens laying eggs for human consumption) = Corn, 30 ppm (not to exceed 50% of diet)
Ruminants >3 months of age being fed for slaughter = Corn, 60 ppm (not to exceed 50% of diet)
Poultry for slaughter = Corn, 100 ppm (not to exceed 50% of diet)
Other species and pets = Corn, 10 ppm (not to exceed 50% of diet)

DON (Vomitoxin) FDA advisory (LLOD = 0.1 PPM and level of quantification = 0.25 PPM)

Swine and pet foods Corn, 4 ppm (not to exceed 10% of diet)
Cattle Corn, 4 ppm (not to exceed 50% of diet)

T2 toxin No established limits from FDA. These are levels of concern (Level of quantification = 25.00 PPB)

Cattle (Includes lactating dairy cattle) = 500 ppb in total ration
Breeding swine = 300 ppb in total ration
Young swine = 100 ppb in total ration
Older feeder swine = 300 ppb in total ration

Aflatoxin LLOD = 2.0 PPB and level of quantification = 5.0 PPB

Wisconsin Source for In-Season Information and Troubleshooting

Corn Growth Problems

General categories

Poor emergence or reduced plant population

soil crusting
insect damage
disease
dry soils
soils saturated with water for several days
salt injury from fertilizer
ammonia damage
rodents
herbicide damage
poor quality seed
planting too deep

Chlorosis: yellowing of plants across the entire leaf surface or between the veins

nitrogen deficiency
potassium deficiency
sulfur deficiency
zinc deficiency
manganese deficiency
iron deficiency
cold soils
cold air temperature
herbicide injury
diseases
insects

Necrosis: spots or streaks of dead tissue

fertilizer spray injury
herbicide spray injury
disease
wind damage
hail damage
insects
nitrogen deficiency
potassium deficiency

Holes in leaf and stalk tissue, or plant tissue consumed

insect feeding
hail or wind damage

Buggy whip or onion leaf: failure of leaf tissue to unfurl from the whorl

calcium deficiency
biuret injury
boron toxicity
herbicide damage
disease

Purple leaves: indicates accumulation of sugars in the tissue. Most often sugars accumulate due to problems with roots

phosphorous deficiency
zinc deficiency
magnesium deficiency
cold soils
dry soils
excessively wet soils
insect damage
mechanical root pruning
herbicide damage
disease

Lodging

strong wind
insect or herbicide damage to roots
high plant populations
nutrient deficiencies
insect damage to stalks

Injury to ears

disease
insects

Weather related problems

soil crusting
poor seedling color
frost injury
dry weather
heat daflooding

Herbicide injury symptoms

Diagnosing Herbicide Injured Corn

Acetamide and thiocarbamate injurybicide injury symptoms
- Acetamide and thiocarbamate injury
- Carfentrazone injury
- Bentazon, bromoxynil, paraquat, and pyridate injury
- Chlorimuron, chlorosulfuron, cloransulam, flumetsulam, halosulfuron, imazamox, imazaquin, imazethapyr, nicosulfuron, primisulfuron, prosulfuron, and rimsulfuron+thifensulfuron injury
- Clethodim, diclofop, fenoxoprop, fluazifop, quizalofop, and sethoxydim injury
- Clomazone injury
- Dinitroaniline injury
- Glyphosate and glufosinate injury
- Triazine injury
- 2,4-D, clopyralid and dicamba injury
Diseases
- Soil decay and seedling blight
- Root rots
- Nematodes
- Leaf diseases
  - Northern leaf blight
  - Southern leaf blight
  - Helmintosporium leaf spot
  - Anthracnose leaf blight
  - Gray leaf spot
  - Eyespot
  - Common rust
  - Southern rust
  - Stewart's disease
  - Goss' Bacterial wilt
  - Holcus leaf spot
- Common smut
- Head smut
- Crazy Top
- Maize dwarf mosaic
- Stalk rots
  - Antrhacnose stalk rot
  - Diplodia stalk rot
  - Fusarium stalk rot
  - Gibberella stalk rot
  - Charcoal rot
- Ear and kernel rots
  - Fusarium ear and kernel rot
  - Gibberella ear rot
  - Diplosia ear rot
  - Aspergillus ear and kernel rot
Nutrient deficiency and toxicity symptoms
- Nitrogen deficiency
  - Symptoms of nitrogen deficiency appear first as a light green coloring of the plant.
  - As the deficiency becomes more severe, lower leaves turn yellow and may 'fire'.
  - The yellowing starts at the midrib of the leaf with the leaf edge remaining green.
- Phosphorous deficiency
- Potassium deficiency
- Calcium deficiency
- Magnesium deficiency
- Sulfur deficiency
- Boron deficiency
- Chloride deficiency
- Copper deficiency
- Iron deficiency
- Manganese deficiency
- Zinc deficiency
- Salt effect (fertilizer burn)
- Salt injury (foliar burn)
- Anhydrous ammonia toxicity (preplant)
- Anhydrous ammonia "leaf burn"
- Biuret damage
- Livestock waste
- Air pollution injury
Insect Injury

Early Season Injury: Planting to Early Whorl
- Injury to planted seeds
  - Seedcorn maggots
  - Seedcorn beetles
  - Wireworms
- Injury to roots
  - Grape colapsis
  - White grubs
  - Garden symphylans
Key References

Uneven Emergence in Corn North Central Region 344

Scouting Corn--A Guide for Wisconsin Corn Production UWEX Bulletin A3547

Assessing Hail Damage to Corn National Corn Handbook - 1

Utilizing Drought-Damaged Corn National Corn Handbook - 58

Troubleshooting

Techniques for Diagnosing Field Crop Problems