Corn, Soybean, Wheat, and Alfalfa Field Guide

Field corn developmental stages — based on the Leaf Collar Method

The Horizontal Leaf Method

ESTIMATED % CORN GRAIN YIELD LOSS DUE TO DEFOLIATION
AT VARIOUS GROWTH STAGES¹

INSECT SCOUTING CALENDAR FOR CORN

CORN PESTS

SEEDCORN MAGGOT

Larva, Pupae & Damage

Identification & Incidence: Corn emergence failure due to feeding by small, yellowish white, legless fly larvae (maggots) on geminating seeds. Damage is likely to occur in fields having high organic matter (especially fields that have had green material, such as cover crops or weeds incorporated into the soil) and when cool and damp soil conditions delay emergence.

Sampling: Maggots may be detected by inspecting seed rows exhibiting lack of emergence. Assessment is based on examination of 3 feet of row at 3 or more locations.

Economic Threshold: No economic threshold exists for this insect.

SEED TREATMENT CORN¹

SOIL INSECTICIDES FOR CORN¹

POSTEMERGENCE INSECTICIDE TREATMENTS FOR CORN¹

WEED THRESHOLD INFORMATION FOR CORN

HERBICIDE INJURY DIAGNOSIS - CORN

Germination/Emergence

Poor germination and uneven emergence

• Not typical of herbicide injury, however, incorporating Prowl (not labeled for PPI in corn) or misapplication of any DNA herbicide can cause poor emergence. “Clubbed-roots” should be visible.

Leaf-out underground

• Axiom, Define, Dual II Magnum, Eradicane, Harness/Surpass, Lasso, Outlook, and premixes containing these herbicides may cause leaf-out under cold and wet conditions. Crusted or cloddy soils may cause this also.

Emergence To Tasseling

Buggy-whipping/twisting/leaf crinkling

• Characteristic of shoot inhibitors such as Axiom, Define, Dual II Magnum, Eradicane, Harness/Surpass, Lasso, Outlook, and premixes containing these herbicides. Usually occurs under cold and wet soil conditions and is often temporary. Dicamba, 2,4-D, or these two tank-mixed with a shoot inhibitor may cause similar symptoms but corn may be rolled tighter and lay more horizontal to soil surface. The longer the buggy-whipping is present, the greater the potential for yield loss. Stunted plants/interveinal yellowing and/or purpling of leaves.
• Misapplication or carryover of ALS-herbicides such as Backdraft, Canopy XL, Classic, Extreme, FirstRate, Pursuit, Raptor, Scepter, Squadron, and Synchrony STS. Check plant for a “bottlebrush” appearance on lateral roots. Fields affected by carryover may take on an uneven appearance with a series of stunted plants followed by healthy plants, intermittently. Persistence of Scepter, Pursuit, and premixes containing these herbicides will increase at soil pH of 5.5 or less and under drought conditions. Persistence of chlorimuron and premixes containing chlorimuron will increase with soil pH above 6.8.
• Soil applications of Hornet and Python may cause injury under cold and wet conditions.
• Application of Accent, Accent Gold, Basis, Basis Gold, Beacon, Celebrity Plus, Exceed, Hornet, Lightning on Clearfield Corn, Python, Spirit, or Steadfast in the same season as Counter (15G or CR), Phorate, or Thimet soil insecticides may cause the above symptoms including severe leaf margin crinkling and “onion-leafing.”Stunted plants/white leaves and/or shoots
• Misapplication of glyphosate at a low dose (usually at spike stage).
• Balance or Callisto may cause these symptoms. Severe injury with Balance may include severe stunting, leaves turning necrotic, buggy-whipping, and stand loss. Injury with Balance is most common under the following conditions: sandy soil types; shallow planting depth (less than 1.5 inches); cold and wet soil; spray overlaps; soil pH approaching 7.5; organic matter less than 1.5 %.

Stunted plants/short swollen and/or “clubbed roots”

• Carryover or misapplication of DNA herbicides such as Balan, Prowl, Sonalan, or Treflan.

White/bleached leaves

• Misapplication or carryover from Command or Command Extra. The plant will usually grow out of slight discoloration. However, if more than 75% of the plant tissue is white, the plant will probably die. Carryover is most likely on soils with a pH below 5.5.
• Postemergence application of Callisto. Increased injury when applied after a soil or postemergence application or tank-mixed with organophosphate insecticides or when mixed with methylated seed oil.Leaf burn on leaf margins of older leaves
• Photosynthetic inhibitors may cause yellowing (chlorosis) of leaf margins followed by death (necrosis) of the oldest leaves on the plant. Herbicides in this group include atrazine, Sencor, Lorox, Princep, and premixes containing these herbicides. Injury is more likely at high pH (above 7.2).

Speckled to burned leaves

• Postemergence applications of Aim, Buctril, Resource, Atrazine plus crop oil concentrate, or Callisto (only the first few days after application). Effects are usually temporary unless a large percentage of the plant is brown.

Yellow to translucent color on new leaves

• Misapplication of Assure II, Fusilade DX, Fusion, Poast, Poast Plus, or Select. The whorl will be easily pulled from the rest of the plant with a characteristic collapsed brown tip on the end near the growing point.

Speckled leaves due to drift

• Aim, Cobra, Flexstar/Reflex, Gramoxone Max, Resource, or Ultra Blazer drift. Usually only cosmetic injury. Bent or “goose-necked” stalks/fused brace-roots
• Postemergence applications of growth regulator type herbicides such as 2,4-D, dicamba, and premix products containing these herbicides. Plants may become brittle and prone to lodging. More severe during periods of high temperatures and rapid node elongation. Veinal chlorosis or browning/collapse at leaf arch/buggy-whipping
• Carryover of Flexstar or Reflex. Occurs with late applications of Flexstar/Reflex, dry soils, and early corn planting. Plants may die in areas of spray overlap, but most corn will survive injury.

Pollination

Poor pollination

• Late applications of Lightning on Clearfield corn.

Maturity

Ear-Pinching

• Postemergence broadcast applications of Accent, Accent Gold, Basis Gold, Celebrity Plus, or Steadfast after corn has reached 6 collars. This is characterized by the base of the ear having normal number of kernel rows and the upper part of the ear having about 50% fewer rows.

TIME OF DISEASE OCCURRENCE IN CORN

EXAMPLES OF CORN LEAF DAMAGE

PERCENTAGE OF LEAF DAMAGED

From: James, C. 1971. A manual of assessment keys for plant diseases. The American Phtopathological Society, 3340 Pilot Knob Rd., St. Paul, MN 55121

SEEDLING BLIGHTS

Description: Wilting and dying of young seedlings during and after emergence are the first signs of blight. A soft watery rot of the roots, mesocotyl, and crown are typical symptoms. Seeds with a white or pinkish weft of mold around them indicates seed rot. Various seedborne and soilborne fungi cause seedling blight. Not to be confused with insect injury.

Location: State wide, particularly in early planted fields or fields with compaction, wet soil, or very dry soil and reduced tillage fields when emergence has been delayed by cold soil. Cold, wet compacted fields have particular problems with seedling blight.

Time of attack: Blights occur when seedlings are put under stress or are subjected for extended periods to conditions limiting rapid growth of the young plant. Generally, April through early May.

Management:

• Fungicide seed treatment
• Improve drainage

NORTHERN CORN LEAF BLIGHT

Description: Lesions of northern corn leaf blight are large (1-6" long x 1/2-1" wide), cigar shaped and brown to tan in color. During periods of high humidity lesions may have grayish green centers due to formation of dark colored spores on dead tissue.

Location: NCLB can be found throughout Ohio.

Time of attack: Wet humid weather favors NCLB, especially during periods of heavy dew and fog. Symptoms may occur as early as silking, but are more prevalent during later stages of development.

Management:

• Resistant hybrids
• Crop rotation
• Till residues

SOUTHERN CORN LEAF BLIGHT

Description: Lesions of southern corn leaf blight are small (1/41" long x 1/4" wide) and tan in color. Lesions may be oval or have parallel sides. On some hybrids the lesions may be surrounded by yellow halos, but other hybrids may have reddishbrown borders.

Location: SCLB can be found throughout Ohio, but is more prevalent in the southern half of the state. Should only occur on hybrids that lack adequate resistance.

Time of attack: Wet humid weather favors SCLB, especially during periods of heavy dew and fog. Symptoms may occur as early as silking, but are more prevalent during later stages of development.

Management:

• Resistant hybrids
• Crop rotation
• Till residues

STEWART’S BACTERIAL LEAF BLIGHT

Description: Very long necrotic lesions with wavy margins. Some lesions extending nearly the length of the leaf on susceptible hybrids. The adult corn flea beetle carries the bacterium over winter and transmits it to corn when feeding on corn leaves. Small thin feeding scars are evidence of flea beetle injury. Sweet corn, popcorn, and inbreds may show seedling wilt when attacked early, but most hybrid dent corn is more resistant to this phase of the disease. All types of corn have the leaf blight phase.

Location: Stewart’s leaf blight occurs throughout Ohio.

Time of attack: Attack by adult flea beetles and transmission of the bacterium occurs soon after seedling emergence. Continued feeding by beetles throughout the summer increases the severity of the disease.

Management:

• Resistant hybrids
• Control flea beetle with insecticide foliar spray or seed treatment (see insect section on flea beetle)

ANTHRACNOSE LEAF BLIGHT

Description: Anthracnose lesions on leaves vary greatly in size and shape but are generally less than 1" long with dark tan centers, brown borders and yellowish to orange halos. Lesions generally appear first near the leaf tip and the midrib of the leaf, then coalesce to produce large dead areas and blotches. Black hair like structures emerging from fruiting bodies (acervuli) can be seen with a hand lens during periods of high humidity.

Location: Leaf blight phases of anthracnose can be found throughout the state. It is prevalent in continuous, reduced tillage corn fields.

Time of attack: The disease is favored by rainy weather any time from seedling emergence to maturity.

Management:

• Resistant hybrids
• Till residues
• Crop rotation

ANTHRACNOSE STALK ROT

Description: Anthracnose stalk rot is identified by the shiny black spots and/or streaks developing on the outer surface of the stalk. The entire stalk of susceptible hybrids may turn black.

Location: Stalk rot phases of anthracnose can be found throughout the state. It is prevalent in continuous, reduced tillage corn fields.

Time of attack: The disease is favored by rainy weather any time from seedling emergence to maturity. Anthracnose stalk rot can be detected starting around two weeks prior to physiological maturity.

Management:

• Resistant hybrids
• Till residues
• Crop rotation

CRAZY TOP

Description: Affected plants show excessive tillering, rolling of leaves, and/or proliferation of husks due to abnormal growth of tissues.

Location: Crazy top occurs anywhere in the state.

Time of attack: Saturated soil conditions or ponded water from excessive rainfall during early stages of plant growth favor infection. Plants become infected as seedlings and the fungus grows systemically in the plant causing abnormal development of plant tissues. Symptoms are most recognizable from mid whorl stage of development to maturity.

Management:

• Resistant hybrids
• Improve soil drainage

MAIZE DWARF MOSAIC

Description: Maize dwarf mosaic is a virus disease recognized as a mottling or mosaic of light and dark green areas in young leaves within the whorl. Later plants may develop leaves that are yellowed or reddened and plants may be stunted, but these later symptoms are not characteristic for diagnosis.

Location: Maize dwarf mosaic occurs in river bottom fields where johnsongrass occurs in southern Ohio.

Time of attack: The virus overwinter in johnsongrass. Aphids feeding on johnsongrass transmit the maize dwarf mosaic virus to young plants. Symptoms may develop on young plants, but diseased plants are easier to diagnose in the mid to late whorl stages of development than later in the season.

Management:

• Resistant hybrids
• Destroy johnsongrass

MAIZE CHLOROTIC DWARF

Description: The characteristic symptom of maize chlorotic dwarf is the presence of very fine chlorotic stripes adjacent to the secondary veins on leaves in the whorl stages of growth. Older plants become yellowed and reddened with severe stunting.

Location: Maize chlorotic dwarf occurs in river bottom fields where johnsongrass is a problem in southern Ohio.

Time of attack: The virus overwinter in johnsongrass. Leafhoppers feeding on johnsongrass transmit the maize dwarf virus to corn plants. Symptoms may develop on young plants, but diseased plants are easier to diagnose in the mid to late whorl stages of development than later in the season.

Management:

• Resistant hybrids
• Destroy johnsongrass

GRAY LEAF SPOT

Description: Lesions resemble elongate rectangles on leaf surfaces. Lesions have straight or parallel sides. Susceptible type lesions may be from 1/2-4" long and tan to gray in color with no borders. Some less susceptible hybrids have yellow halos surrounding small lesions (chlorotic lesion type).

Location: The disease can be found throughout the state at some level, but is most prevalent in east central, southern and western Ohio. Severe epidemics occur in continuous corn, reduced tillage fields in river bottoms or in locations with restricted air drainage. Lodging may result from excessive leaf blighting.

Time of attack: The disease is favored by heavy dew, fog or light rain. Periods of drying between these periods also is important. Lesions generally first appear near tasseling and disease spread occurs until maturity.

Management:

• Resistant hybrids
• Till residues
• Crop rotation
• Fungicide application on susceptible hybrids in chronic disease areas

NORTHERN LEAF SPOT

Description: Lesions of northern leaf spot are small (1/8-1/2" long), oval, and tan to brown in color with dark brown borders. The lesions are usually aligned with the veins of the leaf and occur in linear groups.

Location: Northern leaf spot is most prevalent in the northern third of Ohio counties, but can occur throughout the state in years with cooler weather. The disease is most prevalent on inbreds in seed production fields and some of the more susceptible hybrids. The disease is generally associated with continuous corn especially in reduced tillage.

Time of attack: Lesions are detected after tasseling to crop maturity.

Management:

• Resistant hybrids
• Crop rotation
• Till residues

COMMON RUST

Description: Small (1/8-1/4" long), reddish-brown, oblong pustules scattered over the surface of leaves.

Location: Can occur anywhere in the state, but frequently more common in northern Ohio.

Time of attack: Rust can be seen as early as late June in years with abnormally cool growing conditions. Disease continues to spread until plant maturity during periods of cool weather with light rain.

Management:

• Resistant hybrids

COMMON SMUT

Description: Grayish to white enlarged galls develop on ears, leaf surfaces and sometimes tassels. As galls mature, masses of black powdery spores are released after rupturing the galls’ outer covering.

Location: Common smut occurs throughout Ohio on field, sweet and popcorn.

Time of attack: Common smut spores overwinter in the soil and are spread by wind and splashing rain to plant surfaces. Insect, hail or mechanical injury increases the incidence of the disease. Smut can be detected from tasseling through harvest on ears and on leaves and other plant parts nearly anytime.

Management:

• Resistant hybrids
• Control insect pests

GIBBERELLA EAR AND STALK ROT

Description: Gibberella ear rot is identified by a pink to reddish mold that usually begins at the tip of the ear.

Description: Gibberella stalk rot is identified as a pink discoloration of the pith tissues in weakened or lodged stalks.

Location: Gibberella ear rot and stalk rot can occur anywhere in the state.

Time of attack: Average daily temperatures below 72° F and frequent rain during the first two weeks after silking favors Gibberella ear rot. Wet, warm temperatures in June and July followed by drought stress in August and September favor lodging caused by Gibberella stalk rot. Gibberella rots are associated with several mycotoxins that are toxic to livestock.

Management:

• Resistant hybrids
• Crop rotation
• Balanced fertility
• Till residues
• Control insect pests

DIPLODIA EAR ROT

Description: Ears with thick development of gray mold that develops from the base toward the tip is characteristic of Diplodia ear rot. The kernels appear stuck together with the thick layer of mold and the husks adhere to the ear.

Location: Diplodia ear rot is more common in the southern half of the state and appears to be associated with continuous corn and reduced tillage.

Time of attack: Warm, dry weather prior to silking followed by wet conditions following silking favors ear rot.

Management:

• Resistant hybrids
• Crop rotation
• Till residues

DIPLODIA STALK ROT

Description: Plants affected by Diplodia stalk rot have small, black fungal bodies (pycnidia) beneath the epidermis of the stalks usually near the nodes. Fungal bodies are not superficial and cannot easily be scraped off.

Location: Diplodia stalk rot is more common in the southern half of the state and appears to be associated with continuous corn and reduced tillage.

Time of attack: Wet, warm temperatures in June and July followed by drought stress in August and September favor stalk rot.

Management:

• Resistant hybrids
• Crop rotation
• Till residues

FUSARIUM KERNEL ROT

Description: Fusarium kernel rot has grayish discoloration of the caps of individual kernels or groups of kernels scattered over the ear.

Location: Fusarium ear rot occurs state wide.

Time of attack: Wet, warm weather following silking and damage to kernels by insects, hail or other mechanical means favors disease development. This kernel rot is associated with the mycotoxin Fumonisin, which is toxic to livestock.

Management:

• Resistant hybrids
• Crop rotation
• Till residues

RELATIVE EFFICACY OF SEED TREATMENTS FOR CONTROL OF CERTAIN DISEASES OF CORN IN OHIO

FOLIAR FUNGICIDES FOR CONTROL OF GRAY LEAF SPOT IN CORN

NUTRIENT DEFICIENCY SYMPTOMS IN CORN

Nitrogen

• stunted plants, yellowing (chlorosis) of lower leaves first
• chlorosis begins at leaf tip and progresses along midrib, creating a “V”

Phosphorus

• in seedlings: slow growth, purpling or reddening of leaves especially on leaf margin affects lower leaves first
• stunted overall growth, poor root development
• twisted ears, irregular kernel rows, imperfectly developed ear tips

Potassium

• chlorosis on lower leaves first, begins at the margin and progresses inward
• weak stalks
• poor drought tolerance

Calcium

• failure of new leaves to emerge and unfold
• tips of leaves colorless and covered with a sticky gelatinous substance which causes them to adhere to one another
• deficiencies are rare in Ohio

Magnesium

• interveinal chlorosis of the leaf, veins remain green
• affects lower leaves first
• reddening of leaf margins where severe deficiencies exist
• deficiencies are rare in Ohio

Sulfur

• uniformly chlorotic plant
• stunted thin-stemmed and spindly
• similar to nitrogen deficiency symptoms but more pronounced on younger leaves

Manganese

• shows up in young leaves first as an interveinal chlorosis
• severe cases exhibit elongated white streaks, the center of which may turn brown and fall out

Iron

• young leaves develop interveinal chlorosis, veins remain green in early stages
• progresses rapidly and may eventually turn the entire leaf white
• deficiencies are rare in Ohio

Boron

• irregularly shaped white spots between veins which could develop into stripes with a waxy, raised appearance
• discoloration found on youngest leaves first, starting at the base of the leaf and progressing toward the tip
• may prevent tassel development

Copper

• youngest leaves become yellow and stunted, eventually turning pale while the old leaves die back
• dead leaf tissue may appear along the tips and leaf edges in a pattern similar to potassium deficiency
• deficiencies are rare in Ohio

Zinc

• light green to white streaks on either side of the midrib that may develop into a broad band of bleached tissue most evident on lower leaves
• leaf midribs and margins remain green
• sometimes stalk and leaf edges appear to be tinted red or brown

Molybdenum

• wilted leaves, youngest leaves may twist
• deficiencies are rare in Ohio

NITROGEN RECOMMENDATIONS^a FOR CORN BASED ON YIELD POTENTIAL & PREVIOUS CROP.

ESTIMATING NITROGEN LOSSES

The following tool can be used to assess nitrogen loss and the need for sidedress nitrogen when soil samples are not collected. Soil temperature, nitrogen form, length of soil saturation, and organic matter all play critical roles in determining microbial activity and resultant denitrification of fall and/or spring applied nitrogen. As an aid to help make sidedressing decisions, University of Minnesota scientists have developed a simple question and answer point system. We have adapted that point system to Ohio and propose its use for Ohio’s corn crop (see below).

1. What N source was utilized?
   1. Anhydrous ammonia with nitrification inhibitor 1 pt
   2. Anhydrous ammonia 2 pt
   3. Other fertilizer banded 3 pt
   4. Other fertilizer broadcast 4 pt


2. When was the N applied?
   1. After April 20 2 pt
   2. Before April 20 5 pt


3. How much N has been applied?
   1. > 200 lbs/A 1 pt
   2. 150-200 lbs/A 2 pt
   3. 100-150 lbs/A 3 pt
   4. < 150 lbs/A 4 pt


4. What has been the predominant soil moisture status in the field this spring?
1. Normal 1 pt
2. Wet 2 pt
3. Excessively wet (saturated—standing water) 4 pt


5. What is the crop’s condition?
   1. Green plants > 12" tall 1 pt
   2. Green plants < 12" tall 2 pt
   3. Chlorotic plants < 12" tall 3 pt
   4. Chlorotic plants > 12" tall 5 pt

Total the score and use the following guidelines:

Less than 13 Additional fertilizer not recommended 13-16 Evaluate again in 4-7 days 17 or greater Add an additional 40-70 lbs N/A

The “re-evaluation” option is only viable until you no longer can sidedress. While a total score of 17–18 may merit 40 lb N/acre, a score of more than 18 may require higher rates. Research conducted in Illinois has found that 50 lb N/acre was satisfactory for a wide range of conditions. Keep in mind that good judgment is still important when using various methods to estimate N needs. Also, each field needs to be evaluated individually.

PRE-SIDEDRESS NITRATE TEST (PSNT) FOR MANURED-FIELDS

Producers that have applied manure to their fields should consider the PSNT as a nitrogen management tool for corn production. This test estimates the level of nitrate-nitrogen in a field and is adapted to a sidedress program. Samples should be collected 5 to 10 days prior to time of sidedress. The optimum time for sidedress is between the V4 to V6 growth stage. The depth of core samples should be 12 inches and removed from 15 to 20 sites across a uniform area no larger than 20 acres. Samples should be dried, or at least, not sent to laboratories in plastic bags.

Lab selection should be based on accuracy and timeliness (receiving samples and reporting results). If the results are in excess of 30 ppm, adequate nitrogen should be available for this year’s corn crop. If it is less than 15 ppm, the normal nitrogen rate should be applied. Between 15 and 30 ppm, other factors should be considered before reduction of the normal nitrogen rate.

Research from Ohio State University has shown that in most situations, normal nitrogen rates are required at values less than 30 ppm. The test only measures nitrate-nitrogen. If samples are collected too soon, ammonium nitrogen may have not converted to the nitrate form. Some labs include the ammonium value. This value may be used to confirm the conversion (or lack of conversion) of ammonium-nitrogen to nitrate-nitrogen. However, only use the nitrate value for management decisions. Test results may not be useful if heavy rains or several days of soil saturation occur between date of sampling and reporting of results. In summary, the pre-sidedress nitrogen test has been a useful management tool for manured-fields, and occasionally useful for fields with cover crops. In some cases, it may assist in estimating nitrogen losses from early preplant nitrogen.

POTASH (K₂O) RECOMMENDATIONS FOR CORN AT VARIOUS YIELD POTENTIALS, CEC'S & SOIL TEST LEVELS.

POTASH (K₂O) RECOMMENDATIONS FOR CORN SILAGE AT VARIOUS YIELD POTENTIALS, CEC'S & SOIL TEST LEVELS

PHOSPHATE (P₂O₅) RECOMMENDATIONS FOR CORN

PHOSPHATE (P₂O₅) RECOMMENDATIONS FOR CORN SILAGE

NUTRIENT SUFFICIENCY RANGES FOR CORN
(ear leaf sampled at initial silking)

INFORMATION REQUIRED TO MAKE A CORN REPLANTING DECISION

The following information must be gathered while you are in the field inspecting the damage. Additional information, such as original planting date and possible replant cost, will be required to determine if replanting is justified.

After-Damage Plant Population

ESTIMATING CORN YIELDS PRIOR TO HARVEST

There are several techniques for estimating corn grain yield prior to harvest. This version was developed by the Ag. Engineering Department at the University of Illinois and is the one most commonly used. A numerical constant for kernel weight is figured into the equation in order to calculate grain yield. Since weight per kernel will vary depending on hybrid and environment, the yield equation should only be used to estimate relative grain yield. For example, yield will be overestimated in a year with poor grain fill conditions, while it will be under-estimated in a year with good grain fill conditions.

• Step 1. Count the number of harvestable ears per 1/1000th acre (see page 220).
• Step 2. Count the number of kernel rows per ear on every fifth ear. Calculate the average.
• Step 3. Count the number of kernels per row on each of the same ears, but do not count kernels on either the butt or tip that are less than half size. Calculate the average.
• Step 4. Yield (bushels per acre) equals:
  [(ear #) x (avg. row #) x (kernel #)] ÷ 90

EXPECTED GRAIN YIELD DUE TO VARIOUS PLANTING DATES AND POPULATION RATES

GROWING DEGREE DAY ACCUMULATION NORMALS FROM APRIL 1 - OHIO