Cdg final t_dversion2


Published on

manual para el diagnostico de problemas y enfermedades en el maiz .Zea MAYS..

Published in: Technology
1 Comment
1 Like
No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Cdg final t_dversion2

  1. 1. Table of ContentsIntroduction ....................................... . . . . . .............................................................1Examining a field .................................................................................................. 2Tips for shipping to diagnostic laboratories ................................................................ 2-3Stage I: Scouting from emergence to knee-high ............................................................. 4Stage II: Scouting from knee-high to tasseling .............................................................. 11Stage III: Scouting from tasseling to maturity ................................................................ 16Disease symptoms .............................................................................................. 24Leaf diseases ................................................................................................ 24-26Smutting diseases ...................................................... . . . . ..................................... 26Virus and virus-like diseases ...................................................... . . . . ......................... 27Fungal systemic diseases .......................................... . . . ..................................... 27-28Stalk and root rot diseases ................................................................................. 28-29Ear and kernel rots ...................................................... . . .................................. 29-30Insect injury symptoms and management recommendations ....................... . . . . . . . ......... 31-39Herbicide injury symptoms ................................................................................. 40-42Herbicides that may be used or trigger symptomology in corn ....................................... 43-47Herbicides listed by active ingredient and mode of action ............................................ 48-51Nutrient deficiency symptoms ............................................................................. 52-53Index ............................................................................................................... 54
  2. 2. IntroductionThis manual is prepared and distributed by Monsanto Company to help farmers, seed dealers, and companypersonnel determine the likely causes of abnormal corn plant appearance. While the primary target is the NorthAmerican Region, the principles and many of the situations described in this publication extend to the regions ofthe world where corn is grown.Symptoms may be due to a single cause or the result of two or more interacting factors. Also, because of theirdifferent genetic backgrounds, different hybrids may not have identical symptoms in response to the samecause. Routine field examinations are crucial in spotting problems or potential problems. Some growers em-ploy professional crop scouts rather than perform this function themselves. Weekly examinations are generallysufficient.Once a problem has been identified, its extent and severity must be determined to decide whether correctiveaction is necessary. Many pest management recommendations include threshold levels when control mea-sures will return a profit. Consult Cooperative Extension Service and chemical company recommendations forcontrol measures.1 Corn Diagnostic Guide
  3. 3. 1 Diagnosing Field Problems in CornEXAMINING A FIELDCarry the appropriate tools to help effectively scout or monitor fields. Some basic tools to have onhand include:• Tape measure • Small plastic bags• Knife • Water source• Trowel (6 or 8 inch) • Paper towels• Magnifying glass • Calculator• Clipboard and record keeping materials • Field marking flagsA spade and a set of nesting pails may be useful if considerable digging is expected. Digital camerascan be helpful in getting a record of insects or disease symptoms.Be very careful when making area-to-area or field-to-field comparisons. Many factors can influenceappearance, including: soil type, slope, and drainage; previous crops; fertility practices; seedbedpreparation; date, depth, and rate of planting; pest control; seed lot; and hybrid.Look for positives, not just problems. Observe and note hybrid differences, as well as the effects offertilizer and cultural practices.The following diagnostic key separates plant growth into three primary stages of crop development: Stage I Emergence to knee-high Stage II Knee-high to tasseling Stage III Tasseling to maturityIn the field, onset of symptoms might occur earlier or later than indicated, and may be observed duringmore than one growth stage.TIPS FOR SHIPPING TODIAGNOSTIC LABORATORIESTrained, experienced agronomists, crop protection, Some of these are more formally conducted thanresearch, and sales personnel stand ready to assist others, and cost of the service varies. Also, there areyou in diagnosing field problems. Local seed dealers a few private laboratories that are equipped to provideare your first contact when questions arise. Positive such services. Your local Cooperative Extensiondiagnosis often requires identification or confirmation Service office can suggest companies and provideof causes by a diagnostic laboratory. Most states have contact information.some arrangement, usually through their CooperativeExtension Service office, for accessing expert When preparing plant or soil samples for a diagnosis,diagnosis. follow these instructions offered by the laboratories. 2
  4. 4. 1. Provide representative samples of each problem observed. Visit a problem area twice, on observed, as well as a healthy sample from dates about a week apart, to determine unaffected plants or plant parts. whether the problem is intensifying, spreading, or disappearing.2. If sending leaf tissue, place sections of leaves showing disease symptoms between pieces of • Crop symptoms as observed in the field, dry paper toweling or notebook paper. such as “plants were wilted” or “leaves3. If sending the whole plant, remove excess soil appeared spotted.” Describe the size of from the roots and wrap the roots in moist paper area affected: spots, strips, or the entire toweling. Put roots and towel in a plastic bag. Do field. not place the entire plant in a plastic bag. Wrap • Percentage of plants affected. stem and leaves with paper, foil, or cardboard. • Soil type (clay, sand, muck, etc.).4. Do not add water or crush specimens unnecessarily. • Topography around affected plants, such as high ground, low ground, or gently5. Use a sturdy envelope or box for shipping. sloping.6. Avoid shipping at times that are likely to result in • Fertility level (include a soil test report, the parcel lying in a post office or freight depot if available) and the amount, kind, and over a weekend or holiday. Overnight delivery, timing of fertilizer application. early in the week, is strongly recommended. • Pesticides applied (fungicides, herbicides,7. Information will need to be provided with the insecticides, application rate, and date). plant specimen. Often, laboratories have specific forms that are required when submitting a • Soil moisture situation at and since sample. Information requested on such forms planting. If the field was irrigated, often includes the following: indicate the amount and dates of water application. • Variety (hybrid number) of crop. • Unusual recent air temperatures or • Location where sample was taken humidity conditions. (county, township, and town).. • Previous cropping and tillage history. • Date of planting, date problem was first observed, and date sample was • Types of weeds in the field. collected. Indicate whether the problem is better or worse than when first3 Corn Diagnostic Guide
  5. 5. 2 Stage I: Scouting from Emergence to Knee-HighDetermine the extent and severity of any problem identified. Is the problem throughout the field orspotty and localized? Has emergence been completed or are there seeds sprouted, ready to emerge?Take accurate stand counts and determine percent of stand achieved.If replanting is necessary, take steps to prevent recurrence of the cause for poor emergence. Verify thatthe planter is operating properly and that fertilizer or pesticide issues have been corrected. number of plants established % stand achieved = X 100 number of seeds planted If the stand is uneven or if there are skips down the row, dig to find the planted seed and its distribution.GENERAL SYMPTOM POSSIBLE CAUSE REMARKS1. No seed • Planter Improper adjustment; row unit drive not engaged; worn parts; clogged spout; empty box or tank; wrong plates, disks, or drum; excess or wrong seed treatment. • Rodents or birds Digging and partly-eaten kernels.2. Normal seed • Unfavorable soil conditions Cold, dry soil. appearance; not • Poor seed-soil contact Inadequate press wheel pressure; improper swelled closing wheel adjustments; inadequate residue management; dry or cloddy soil.3. Normal seed • Unfavorable soil conditions Cold, wet soil. appearance; • Fertilizer or pesticide injury Phytotoxic pesticides or too much fertilizer swelled but not too close to the seed. sprouted4. Seed dead, rotted • Seed rots or seedling blights These are accentuated when soil conditions are unfavorable for germination and seedling growth. Many species of fungi and/or bacteria may be involved. Fungicide seed treatment protects the seed, not the seedling. 4
  6. 6. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS4. Seed dead, rotted • Fertilizer injury Fertilizer salts, nitrogen, and potassium draw (cont.) moisture and may leave seed in soil too dry to support growth. In-furrow applications are more likely to cause fertilizer injury than starter fertilizers placed at least 2 inches from the seed. Ammonia toxicity is caused when planting follows anhydrous or aqua ammonia application too closely or where application was too shallow. This can kill or stunt seedlings. Roots appear sheared off. Boron and some other micronutrients impair germination if they are too close to the seed. • Insecticide injury Some soil-applied organophosphate insecticides can impair germination if placed in furrow with the seed. Check the label and apply only as directed. • Dead seed planted • Unfavorable soil conditions Cold, dry, wet, or crusted soil.5. Seed hollowed out • Insects Seed corn beetle, seed corn maggot, or wireworm (see pp. 36-38).6. Sprout twisted or • Unfavorable soil conditions Crusted, cold, or cloddy soil. A cloddy leaves expanded surface can allow light to reach the sprout below ground and trigger leafing too soon. In the case of crusting, rotary hoeing may be beneficial. • Seed planted too deep • Mechanical injury to seed in handling or planting • Chemical injury Fertilizer (see p. 5); insecticides (see p. 5); or some herbicides such as acetanilides and dinitroanilines (see pp. 41-42).7. Slow, uneven • Planter Seed injury due to improper operation or emergence adjustment, including planting depth. • Unfavorable soil conditions Cold, dry, wet, or crusted soil. In the case of crusting, rotary hoeing may be beneficial. Properly banded fertilizer at planting may help seedlings overcome unfavorable soil conditions. • Seed planted too deep5 Corn Diagnostic Guide
  7. 7. If plants are abnormal in appearance, try to identify one of the following specific symptoms.GENERAL SYMPTOM POSSIBLE CAUSE REMARKS1. Seedlings pulled • Bird or rodent damage Chemical repellents may help. or dug up, seed eaten2. Slow, uneven plant • Unfavorable growing Cold, dry, wet, or compacted soil. Properly growth conditions banded fertilizer at planting may help minimize the effects of some unfavorable growing conditions. • Low fertility If a nutrient deficiency due to inadequate amounts in the soil, is confirmed, consider sidedressing or foliar application, depending on the nutrients involved (see pp. 52-53). • Insects attacking roots Corn root aphid, corn rootworm, grape colaspis, webworm, white grub, or wireworm (see pp. 31-38). • Nematodes attacking roots Requires microscopic analysis. • Chemical injury Fertilizer (see p. 5); insecticides (see p. 5); herbicides such as Balance®; Command® or Scepter® carryover (see pp. 41-42); or liquid manure. • Non-uniform planting depth • Failure of secondary roots Dry, loose soil is not conducive to normal to develop (rootless corn root development. This condition is syndrome) accentuated by shallow planting and whipping by wind. Cultivation may help by throwing soil around the base of plants.3. Discolored leaves • Nutrient deficiency Magnesium, nitrogen, phosphorus, or sulfur (see pp. 52-53). Nitrogen deficiency results in a yellow discoloration of leaves. Phosphorus deficiency results in a purpling of leaves due to the accumulation of anthocyanins. If nutrient deficiency is confirmed, consider sidedressing or foliar application, depending on the nutrients involved. • Unfavorable soil conditions Waterlogged, cold, or compacted soil. These conditions can also affect nutrient uptake and translocation. 6
  8. 8. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS3. Discolored leaves • Insects attacking roots Most observed leaf discoloration is due to (cont.) nutrient deficiency induced by damaged or inadequate roots (see nutrient deficiency, above). • Nematodes attacking roots Requires microscopic analysis. • Chemical injury Fertilizer (see p. 5); insecticides (see p. 5); herbicides such as Balance®; Command® or Scepter® carryover (see pp. 41-42); liquid manure. • Wind damage Abrasion by sand or soil particles. Difficulty establishing secondary roots. • Frost or freeze Check growing point for damage. Seedlings often recover. Most pronounced in low-lying areas. • Cold (not freezing) Cool nights and warm days promote above- temperature stress ground plant growth at the expense of root development. This leads to increased demand by the above ground tissues for more nutrients than the roots can deliver. The result can be short-term deficiency symptoms until the root system becomes more developed. • Anhydrous burn • Mechanical injury • Hybrid differences Uneven Corn Plant Growth7 Corn Diagnostic Guide
  9. 9. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS4. Leaves rolled or • Drought puckered, may be • Insects attacking roots or See Section 6, Insect Injury Symptoms wilted stalks (p. 31-38), plus chinch bug, cutworm, Japanese beetle, stink bug, or webworm. • Nematodes attacking roots Requires microscopic analysis. • Mechanical root pruning5. Leaves rolled or • Herbicide injury Acetanilides, dinitroanilines, or phenoxys twisted together (see pp. 41-42). (“onion leaf” or • Temperature variation Alternating hot and cold weather, inducing “buggy whip”) very rapid growth spurts followed by little or no growth. • Nutrient imbalance Boron toxicity or calcium deficiency (see p. 52). • Hail damage Injury to the growing point at this growth stage can result in short-term wrapping of leaves.6. Shredded leaves • Wind damage or eaten plants • Hail damage • Insects Armyworm, common stalk borer, corn earworm, cutworm, European corn borer, grasshopper, slug, or webworm (see pp. 32-37). • Livestock or wild animal Look for tracks. grazing7. Leaves spotted, • Wind damage Abrasion by sand or soil particles. striped or dead • Low soil pH Beaded streaking of leaves, which turn reddish-purple and may die. • Nutrient deficiency Boron, copper, magnesium, potassium, sulfur, or zinc (see pp. 52-53). • Insects Flea beetle, leaf miner, or thrips (see pp. 35-37). • Disease Anthracnose, bacterial wilt, eyespot, Goss’s wilt, holcus spot, seedling blights (favored by cool, wet soil), virus or virus-like diseases (see pp. 24-27). • Fertilizer or herbicide injury Anhydrous burn; spray drift; foliar-applied herbicides such as Buctril®, Basagran®, Gramoxone®, or Blazer®; Classic®, Scepter®, or Reflex® carryover; premix acentanilides post-applied (see pp. 40-42). 8
  10. 10. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS7. Leaves spotted, • Sunscald or cold Cold nights followed by clear, bright, fast- striped or dead warming days. (cont.) • Hybrid differences8. Rows of holes • Insects Billbug, common stalk borer, corn borer, across leaves cutworm, or stink bug (see pp. 32-37).9. Plants wilt and • Insects Billbug, chinch bug, cutworm, stink bug, die suddenly white grub, or wireworm (see pp. 32-38). • Wind damage • Disease Seedling blights, bacterial wilt, or Goss’s wilt (see p. 26). • Herbicide injury Triazines or misapplications of glyphosate herbicides (see pp. 41-42). • Frost or freeze Check growing point for damage. Seedlings often recover. • Lightning Kills everything, usually in circular area. • Anhydrous burn • Flooded, water-logged soil “Buggywhip” Corn plant two days after frost9 Corn Diagnostic Guide
  11. 11. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS10. Plants twisted or • Herbicide injury Especially 2,4-D followed by wind (see pp. broken off 41-42). • Insects Billbug, cutworm, lesser cornstalk borer, or stinkbug (see pp. 32, 36-37).11. Inhibited root • Nematode injury Requires microscopic analysis. development or • Insects Corn rootworm, grape colaspis, white grub, malformed roots or wireworm (see pp. 34-38). • Fertilizer injury • Herbicide injury Phenoxys, Banvel®, carryover dinitroanilines, and Scepter® or Classic® carryover (see pp. 41-42). • Soil conditions Planting when soils are too wet can cause sidewall compaction that can arrest or severely restrict corn root development. Look for flat-sided or abruptly arrested root systems. Wind Damage in Corn Field Hail Damage in Corn Field 10
  12. 12. 3 Stage II: Scouting from Knee-High to TasselingThis is the period of most rapid plant growth. Nutrient and moisture demands are high; deficiencies willlikely reduce crop yield potential. Problems must be evaluated for economic damage potential beforecontrol decisions can be made. Observe differences due to hybrids and management practices. If plants are abnormal in appearance, try to identify one of the following specific symptoms.GENERAL SYMPTOM POSSIBLE CAUSE REMARKS1. Uneven height • Emerged at different times Uneven planting depth; uneven soil (tall plants, short moisture. plants) • Varied nutrient or moisture availability under drought conditions • Low soil pH • Herbicide drift Use of burndown products adjacent to cropped area. • Nutrients concentrated at dry Nutrients are unavailable to the plant. soil surface • Fallow (idle land) syndrome Phosphorus is unavailable to the plant (see p. 53).2. Numerous tillers • Growing point injury Mechanical or insect damage. • Favorable early-season Optimum moisture, high soil fertility, or both. growing conditions • Low plant population • Adjacent to open spaces (population gaps) in the row • Hybrid differences • Disease Crazy top (see p. 27)3. Discolored or • Nutrient deficiency Nitrogen, phosphorus, or potassium (see dead leaves p. 53).11 Corn Diagnostic Guide
  13. 13. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS3. Discolored or dead • Fertilizer or herbicide injury Fertilizer or herbicide on foliage. This tends leaves (cont.) to be more pronounced at row ends or where overlap application was made. • High temperatures Noted as scalding or bleaching of top leaves first. • Sunscald or cold banding Cold nights followed by clear, bright, fast- warming days. • Insects Chinch bug, spider mite, or root-attacking insects, which can reduce nutrient uptake (see pp. 32-37). • Mechanical injury • Frost or freeze • Hybrid differences • Barren stalk Purpling or reddening due to anthocyanin expression in response to accumulation of sugars that cannot translocate to the initiating ear.4. Whorl leaves dead • Disease Bacterial stalk rot (see p. 28). (“deadheart”) • Insects Billbug, corn borer, or fall armyworm (see pp. 32-33). • Chemical injury More common following a late over-the-top application of certain herbicides. • Excessive heat Temperature exceeding 100°F, depending on moisture availability and genetics of the hybrid.5. Leaves spotted • Low soil pH Beaded streaking of leaves, which turn or striped reddish-purple and may die. • Nutrient deficiency Boron, iron, magnesium, manganese, nitrogen, or potassium (see p. 52-53). • Chemical injury Herbicides, additives’ reactions, or fertilizers; symptoms may be due to spray drift on foliage. • Disease Bacterial leaf blight, eyespot, Goss’s wilt, holcus spot, leaf blights, Physoderma brown spot, rust, sorghum downy mildew, virus or virus-like diseases (see pp. 24-28). 12
  14. 14. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS5. Leaves spotted or • Insects Cereal leaf beetle, corn rootworm beetle, striped (cont.) corn blotch leaf miner, flea beetle, spider mite, or thrips; root-damaging insects induce nutrient deficiency or drought symptoms (see pp. 32-37). • Wind damage Abrasion by sand or soil particles. • Hail damage • Genetic stripe Observed only on occasional plants. • Sunscald or cold banding Cold nights followed by clear, bright, fast- warming days.6. Leaves eaten or • Livestock or wild animals Look for tracks. shredded • Insects Armyworm, billbug, common stalk borer, corn borer, corn earworm, cutworm, grasshopper, Japanese beetle, leaf miner, slug, or wireworm (see pp. 32-38). • Hail damage Determine if the growing point survived. If it did not, plant will not produce a tassel. Despite this, ear fertilization may still occur. • Wind damage • Disease Sorghum downy mildew (see p. 28).7. Plants stunted, • Virus or virus-like diseases Corn lethal necrosis, corn stunt leaves close spiroplasma, maize chlorotic dwarf, or together with maize dwarf mosaic (see p. 27). mosaic mottle or streaks; leaves yellow or red8. Gray or black galls • Disease Common corn smut (see p. 26). growing on plants9. Stalks spindly, • Nutrient deficiency See pp. 52-53. unthrifty, yellow • Excess soil moisture • Excessive plant population • Root damage Insects, nematodes, chemical, or mechanical. • Stalk damage Insects, disease, or mechanical.10. Plants wilted or • Drought rolled • Root damage Insects, nematodes, chemical, or mechanical. • Stalk damage Mechanical or insects; chinch bug, common stalk borer, or corn borer (see pp. 32-33).13 Corn Diagnostic Guide
  15. 15. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS11. Top leaves tightly • Chemical injury 2,4-D and other phenoxy herbicides; Banvel® rolled (“buggy (see p. 41). whip” or “onion • Cold soil during seedling leafing”) stage • Temperature response or Alternating hot and cold periods, speeding rapid growth syndrome and slowing plant growth. • Nutrition Boron toxicity or calcium deficiency (see p. 52). • Mechanical injury • Hybrid differences12. Plants growing • Recovery after root damage Insects (usually, but not limited to corn up in a curved rootworm larvae), nematodes, herbicides “gooseneck” (especially phenoxys), soil compaction, mechanical injury, or wind. • Recovery after early season environmental conditions13. Curled or stubby • Wind and wet soils brace roots • Dry, hot soil surface • Wind action14. Plants twisted or • Mechanical injury growing off at 90˚ angle15. Plants leaning or • Wind damage Especially if soil is wet. Wind can cause broken off greensnap at lower nodes of rapidly growing stalk. • Livestock or wild animals Look for tracks. • Root damage Insects, nematodes, chemical, or mechanical. • Insects attacking stalk Common stalk borer or corn borer (see p. 32-33). • Chemical injury, especially if Especially Banvel® or phenoxy herbicides followed by wind (see pp. 39-41). • Soil compaction Inadequate root development. • Poor secondary root Rootless corn syndrome due to weather development in dry soil and aggravated by shallow planting. • Disease Bacterial or Pythium stalk rots (see pp. 28-29). • Hail damage 14
  16. 16. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS16. Plants wilt and • Lightning Kills everything, usually in a circular area. die suddenly • Drought Field areas with low water-holding capacity are first to show stress. • Disease Bacterial or Pythium stalk rots (see pp. 28-29). • Frost or freeze • Herbicide injury Misapplication of glyphosate, glufosinate, paraquat, or other non-selective herbicides (see pp. 41).17. Tassel feeding • Insects Armyworm, corn leaf aphid, or western (while in whorl) bean cutworm (see pp. 32-35).18. Tassels proliferated • Disease Crazy top (see p. 27). into green cluster of fingerlike branches, excessive tillering, leaves often thick and straplike19. Tassel proliferated, • Disease Head smut (see p. 26). with stringy black vascular bundles present “Greensnap” at lower nodes of rapidly growing corn stalk15 Corn Diagnostic Guide
  17. 17. 4 Stage III: Scouting from Tasseling to MaturityThis period, which commences with the critical pollination stage, includes grain fill, plant maturation,and death. It also includes the critical stage before harvest when standability and ear retention becomeconcerns. Field observations are essential to detect yield-depressing factors during this period. If LEAVES are affected, try to identify one of the following specific symptomsGENERAL SYMPTOM POSSIBLE CAUSE REMARKS1. Eaten • Livestock or wild animals Look for tracks. • Insects Armyworm, corn borer, corn rootworm beetles, grasshopper, or Japanese beetle (see pp. 32-36).2. Shredded • Hail damage • Wind damage • Disease Sorghum downy mildew (see p. 28).3. Dead “frosted” • Frost or freeze appearance • Drought • Disease Anthracnose, leaf blights, or stalk and root rots (see pp. 24-26; 28-29). • Insects Corn borer, corn leaf aphid, or spider mite (see pp. 31-33; 37).4. Spotted or dead • Disease Numerous leaf blights such as anthracnose, eyespot, Goss’s wilt, gray leaf spot, Helminthosporium leaf spot, northern corn leaf blight, Phaeosphaeria leaf spot, Physoderma brown spot, rust, or southern leaf blight (see p. 26). • Chemical injury Especially spray drift.5. Discolored • Insects Corn leaf aphid (see p. 31). sheath • Disease Purple sheath spot caused by saprophytic organisms existing on pollen trapped between the leaf sheath and stalk. 16
  18. 18. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS6. Red or purple • Mechanical injury color • Barren plant Anthocyanins develop in response to accumulation of sugars that cannot be translocated to filling grain. • Disease Corn rust, corn stunt spiroplasma, high plains virus, or maize chlorotic dwarf (see pp. 24-27). • Insects Corn borer (see p. 33). • Hybrid differences7. White striping • Disease Sorghum downy mildew (see p. 28). with white, downy growth on upper and lower leaf surfaces If SILKS are affected, try to identify one of the following specific symptomsGENERAL SYMPTOM POSSIBLE CAUSE REMARKS1. Eaten off • Insects Armyworm, corn earworm, corn rootworm beetle, grasshopper, Japanese beetle, or western bean cutworm (see pp. 32-35).2. None visible, or • Heat delayed several • Drought days after • Nutrient deficiency or Especially nitrogen or phosphorus. tasseling imbalance • Temperature variation Cold nights occurring just prior to silking. • Insects Corn leaf aphid, fall armyworm, or spider mite (see pp. 31-32; 37). • Excessive population for conditions3. Trapped, balled • Drought within the husk • Nutrient deficiency or See pp. 52-53. imbalance • Temperature variation Cold nights during early silking. • Hybrid differences17 Corn Diagnostic Guide
  19. 19. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS4. Red or green color • Hybrid differences Fresh silk color is genetically controlled. If TASSELS are affected, try to identify one of the following specific symptomsGENERAL SYMPTOM POSSIBLE CAUSE REMARKS1. Failed to emerge • Drought or heat stress • Nutrient deficiency Especially boron (see p. 52-53). • Insects Aphid stress, corn earworm, or fall armyworm (see pp. 31-32).2. A mass of leaves • Disease Crazy top, head smut, or sorghum downy mildew (see pp. 27-28).3. One or more small • Genotype by environment Most common on tillers. ears formed interaction4. Kernels develop in • Genotype by environment Most common on tillers. tassel interaction5. Broken off, stalk • Insects Corn borer (see p. 33). tunneled If STALKS are affected, try to identify one of the following specific symptomsGENERAL SYMPTOM POSSIBLE CAUSE REMARKS1. Red or purple • Mechanical injury to plant color • Barren stalk Anthocyanins develop in response to accumulation of sugars that cannot be translocated to filling grain. • Hybrid differences • Insects When corn borers tunnels into a stalk, the area above the point of entry may turn red or purple. 18
  20. 20. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS2. Barren (no ear • Drought enlargement) • Heat • Nutrient deficiency or See pp. 52-53. imbalance • Insects Corn leaf aphid or spider mite (see pp. 31, 37). • Silks eaten off prior to Corn rootworm beetle and Japanese pollination beetle (see pp. 34-36). • Disease Head smut, mildews, virus or virus-like disease (see pp. 26-27). • Excess population for conditions • Tillers Seldom produce a fully-developed ear. • Timing of silk or pollen shed Environmental stress. disrupted • Mechanical injury3. Broken below ear • Wind Plants just prior to tasseling are especially vulnerable to greensnap by wind. • Disease Stalk and root rots (see pp. 28-29). • Nutrient imbalance Excess nitrogen, insufficient potassium (see p. 53). • Insects Southwestern corn borer (see p. 33). • Weather stress Drought, heat, or other conditions limiting photosynthesis. • Mechanical injury Machinery; livestock or wild animals. • Excessive population for conditions • Delayed harvest4. Broken above ear • Wind • Insects European corn borer or fall armyworm (see pp. 32-33). • Mechanical injury Machinery; livestock or wild animals. • Delayed harvest5. Multiple ears at • Hybrid differences one node • Mechanical injury19 Corn Diagnostic Guide
  21. 21. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS5. Multiple ears at • Cold temperature when ears one node (cont.) formed • Disease Mildews, virus or virus-like disease (see pp. 27-28). • Insects Results from severe silk clipping by insects such as corn rootworm beetle and Japanese beetle (see pp. 34, 36).6. Leaning, but • Wind and wet soil not broken (root • Poor root development Drought, fertilizer placement, soil lodged) compaction, low soil pH, or poorly drained soil. • Insects Corn rootworm and other root feeders (see pp. 34-35). • Nematode activity • Disease Stalk and root rots (see pp. 28-29). • Nutrient deficiency Especially potassium (see p. 53). • Herbicide injury Especially 2,4-D and other phenoxys (see p. 41). • Mechanical injury Machinery, livestock or wild animals • Delayed harvest • Hybrid differences7. Premature death • Disease Leaf blights or stalk and root rots (see pp. 24-26, 28-29). • Insects Corn borer (see p. 34). • Frost or freeze • Severe drought • Lightning Kills everything, usually in a circular area.8. Black mold • Saprophytic Buildup on dead stalk tissue under warm, humid weather conditions. Timely harvest will deter. If EARS are affected, try to identify one of the following specific symptomsGENERAL SYMPTOM POSSIBLE CAUSE REMARKS1. Numerous ear • Disease Crazy top, sorghum downy mildew, virus or shoots, leafy and virus-like diseases (see pp. 27-28). barren 20
  22. 22. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS2. Soft, glistening • Disease Common corn smut (see p. 26). smut galls, black and powdery when mature3. Poorly filled tips • Hybrid differences • Nutrient deficiency Especially nitrogen or potassium (see p. 53). • Interaction between population, hybrid, and environment • Insects Silks clipped before pollination. Corn borer, corn earworm, corn rootworm beetle, fall armyworm, or Japanese beetle (see pp. 33- 36). • Disease Foliar disease that reduce photosynthesizing area (see pp. 24-26). • Drought Silks at ear tips were not present when pollen was available • Unusually favorable growing Cob elongates, but rarely fills up tip kernel. conditions after pollination • Suboptimum light, carbon Reduced photosynthesis; tip kernels abort. dioxide, or temperature during kernel fill4. Small malformed, • Nutrient deficiency Especially nitrogen and phosphorus (see p. 48). light weight • Drought or heat stress • Plant damage Mechanical or disease. • Excessive population for conditions • Second or third ear on stalk, or on a tiller • Insect-caused plant stress Numerous ear-feeding insects (see pp. 32-38). • Disease Leaf blights, stalk and ear rots, virus or virus- like diseases (see pp. 24-30). • Soil compaction5. Very short husk, • Weather Usually observed after drought or heat has remains tight at stopped husk growth, but later favorable maturity; ear tip conditions permit more normal ear size to exposed develop. • Hybrid differences21 Corn Diagnostic Guide
  23. 23. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS6. Barren (little to no • Disease Head smut, virus or virus-like diseases (see grain) pp. 26-27). • Pollination failure Drought, heat, or other stress interfered with silk/pollen shed timing; insect (clipped silks or caused tassel to abort); chemical injury (especially by growth regulators like 2,4-D and Banvel®). • Pesticide injury Following drift or misapplications.7. Pinched ear • Chilling injury during early ear Also called beer can ear. Ear is reduced syndrome development to 3-4 inches. Well filled kernels are on bottom third of ear, cob tissue on middle third, and undeveloped tissue on top third.8. Dropped • Drought Weakened shank. • Nutrient deficiency Weakened shank. • Hybrid differences Ears usually drop free of husk. • Insect damage to shank Husk usually remains on the dropped ear; European corn borer (see p. 33).9. Scattered kernel • Lack of adequate viable pollen set when silks are receptive • Silks eaten off before Insect or animals. pollination • Heat or drought at pollination • Nutrient deficiency • Herbicide injury Phenoxy herbicides or too-late applications of glyphosate (see p. 41).10. Kernel feeding • Insects Corn borer, corn earworm, fall armyworm, picnic beetle, or western bean cutworm (see pp. 33-35). • Birds, animals11. Rotten (spots or • Disease Ear and kernel rots or Helminthosporium entire ear) leaf spot (see pp. 29-30; 25). • Insects Corn borer, corn earworm, fall armyworm, picnic beetle, or western bean cutworm (see pp. 33-35). Followed by weathering or disease. • Birds Followed by weathering or disease. • Hail or other mechanical injury Followed by weathering or disease. 22
  24. 24. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS12. Kernels eaten • Birds from ear tips • Wild animals or rodents • Insects Corn earworm, fall armyworm, picnic beetle, or western bean cutworm (see pp. 33-35).13. Tunnels in ear, • Insects Corn borer, corn earworm, or fall armyworm cob, shank, or (see pp. 32-33). stalk If KERNELS are affected, try to identify one of the following specific symptomsGENERAL SYMPTOM POSSIBLE CAUSE REMARKS1. Broken seedcoat, • Genetic and environment Popped kernels usually occur under high “popped” interaction temperatures. appearance2. Horizontally cut or • Silk-cut A genetic and environment interaction. Not split seedcoat visible until kernels are shelled from the cob.3. Pink or red • Kernel red streak Caused by toxin secreted during feeding of streaking or wheat curl mite. More pronounced toward lengthwise stripes, ear tip. No detrimental effects are known. especially running over crown4. Sprouted • High rainfall and warm Usually accompanied by mold. especially at base temperature while ear of ear remained erect on stalk5. White streaking, • Disease Associated with Fusarium ear rot. also known as “starburst” Barren corn Pinched ear syndrome23 Corn Diagnostic Guide
  25. 25. 5 Disease SymptomsThis section will help you identify the common corn diseases based on plant symptoms and conditions. Itis not uncommon to have symptoms of several different diseases present at the same time. Symptoms ofdifferent diseases may appear similar, particularly during early stages of disease development. Laboratoryculturing and microscopic examination may be required to make a positive identification.Growth stages during which symptoms generally appear are listed following the disease name and aredescribed as follows: Stage I Emergence to knee-high Stage II Knee-high to tasseling Stage III Tasseling to maturityLEAF DISEASES The more common leaf blight phase appears after tasseling. Leaves are streaked with gray-green toAnthracnose (Stage I, III) yellow-green lesions, each distinguished by the Oval to spindle-shaped presence of a flea beetle feeding scar toward the water-soaked lesions on base of the streak. Streaks are long and irregular, youngest leaves turn tan turning tan as the tissue dies. to brown with yellow to reddish brown borders. Flea beetles (small, oval, black insects) are the Heavily infected leaves primary carrier. Incidence of the disease is relative wither and die. Top-leaf to beetle population. Sweet corn tends to be more die-back may occur 4 sensitive than field corn to this disease. weeks to 6 weeks after Common Corn Rust (Stage II, III) pollination, leaving the Cinnamon-brown,lower stalk green. The organism thrives in warm, powdery, circular-to-humid weather. The stalk-rot phase of the organism elongated pustules (blister-is of greater concern than the leaf blight phase. like growths) can occurBacterial Wilt and Bacterial Leaf Blight on any aboveground plant(Stewart’s Wilt, Stewart’s Disease) (Stage I, III) tissue, but especially on both surfaces of the Young plants exhibit long, leaves. In contrast, green-gray, water-soaked pustules of southern corn lesions with wavy margins, rust occur primarily on accompanied by stunting the upper leaf surface. Pustules rupture leaf surface and wilting which lead to and rusty “powder” can be rubbed off with fingers. plant death. Cavities may Pustules become dark brown to black late in the form in stalk near the soil growing season. The organism thrives in moderate line. Bacterial masses ooze from cut end of infected to cool temperatures and high humidity. stalks or leaves. 24
  26. 26. Eyespot (Stage II, III) Helminthosporium Leaf Spot Small (less than ¼ inch), (Northern Leaf Spot) (Stage III) circular, translucent lesions Numerous races of the surrounded by a yellow to organism have been purple margin, give a halo identified. Symptoms vary effect. Lesions occur on by race. Lesions tend to leaves (most commonly as be oblong to blocky, and plants approach maturity), tan to brown in color. This sheath and husk. The leaf spot may also cause a disease is favored by cool, black, charred-appearing moist weather. ear rot. It prefers moderate temperaturesGoss’s Wilt and high humidity.(Leaf Freckles and Wilt, Nebraska BacterialWilt and Leaf Freckles) (Stage I, III) Holcus Spot (Stage I) Young plants wilt and This organism causes die. Vascular bundles are small, circular to oblong, discolored. More common water-soaked lesions later-season infections toward tips of lower produce dull gray-green leaves. Later, lesions to orange lesions forming become creamy white to water-soaked streaks tan, then light brown with with irregular margins on reddish margins. Holcus leaves. Within developing spot is very similar in lesions, small, irregular appearance to paraquatshaped water-soaked “freckles” appear. Bacterial damage. Rainstorms accompanied by wind splashdroplets may ooze from the leaf surface early in overwintering bacteria from the residue onto youngthe morning. Plant injury, such as from hail or wind plant leaves. This organism does not cause seriousdamage, enhances infection. loss.Gray Leaf Spot (Stage III) Northern Corn Leaf Blight (Stage II, III) Gray to tan, rectangular Long (up to 6 inches), lesions on leaf, sheath or elliptical, gray-green husk tissue. Spots are lesions that become tan- opaque and long (up to brown identify infections 2 inches). Lower leaves caused by this organism. are affected first, usually Infection spreads up the not until after silking. plant starting on lower The organism thrives leaves. It is favored by high in extended periods of humidity and moderate warm, overcast days and temperatures. Numeroushigh humidity. It has become more prevalent with physiologic races have been described. Host specificincreased use of reduced tillage and continuous races of the organism may also attack sorghum.corn.25 Corn Diagnostic Guide
  27. 27. Phaeosphaeria Leaf Spot (Stage III) rust pustules attack upper and lower leaf surfaces Lesions are initially small, and readily break through the epidermis.) This round to oval, pale green organism is favored by warm, humid weather. or yellow, and scattered on leaves of mature plants. SMUTTING DISEASES Spots become light Common Corn Smut tan with reddish-brown (Boil Smut, Blister Smut) (Stage I, II, III) margins and may coalesce Local infection of any into irregularly shaped plant part, even below lesions. This disease is the soil surface, occurs most prevalent in areas of through plant wounds orhigh rainfall and moderate temperatures. thin-walled cells of activelyPhysoderma Brown Spot (Stage I, III) growing tissue. Resulting Small yellow spots appear first at the base of the leaf. These spots become brown and combine to form chocolate-brown to galls are first silver-white, reddish irregular blotches, then become gray to sometimes as bands black powdery masses of infection across leaf of smut spores that are blades. Sheath, husk, released when the galls tassel, stalk, and leaves break open. Galls onmay exhibit symptoms late in the season. Infected leaves seldom develop beyond pea-size and tend tostalks may break at a node. This organism is favored harden and dry without rupturing. High fertility andby warm, wet weather. plant injury favor the disease. Common smut is notSouthern Corn Leaf Blight (Stage II, III) toxic to animals. The most common race, Head Smut (Stage III) named “O,” produces small, Seedling infection results elongated (up to 1 inch in systemic development long), parallel-sided lesions of the disease as the plant that are tan with brownish grows and develops. borders. This blight primarily Tassels of affected plants attacks leaves. It is favored may multiply as a mass of by high humidity and warm leaves or be replaced by a temperatures. black, smutty mass which ruptures to release spores,Southern Corn Rust (Stage II, III) leaving black, thread-like Small, circular, orange- vascular strands. Ears may be completely replaced to-light-red pustules by similar, often triangular-shaped, smut masses. (blister-like swelling) occur Occasionally, part of an ear escapes and produces on leaves, especially a few kernels. Hot, dry soil at seedling stage favors the upper surface, and infection. One type of head smut also attacks sheaths. Pustules rarely sorghum. break the leaf surface. (In contrast, common corn 26
  28. 28. VIRUS AND VIRUS-LIKE DISEASES Maize Chlorotic Dwarf VirusCorn Lethal Necrosis (CLN) (Stage II, III) (MCDV) (Stage II, III) Younger leaves are yellow Mosaic patterns appear on and finely striped. There leaves and husk. Leaves is a general yellowing die from the margins or reddening of leaves inward and plants may and plants are stunted. die prematurely. The Most affected plants are organism generally starts barren. MCDV is spread by at the tassel and works leafhoppers from infected downward. Barrenness Johnsongrass and other or sharply reduced grain host species. MCDV is production is common. detected more frequently and is more damaging thanCLN is caused by synergistic interaction whenplants become infected by maize chlorotic mottle maize dwarf mosaic.virus and either maize dwarf mosaic virus or wheat Maize Dwarf Mosaic Virusstreak mosaic virus. Presently, the disease has been (MDMV) (Stage II, III)identified only in parts of Kansas and Nebraska. Mottling of younger leaves progresses into narrow,Corn Stunt Spiroplasma (CSS) (Stage II, III) light-green to yellowish Corn stunt is caused streaks along leaf veins. by a spiroplasma, not a Leaves, sheaths and husks virus. Margins of whorl may show symptoms. leaf turn yellow followed Plants are stunted and by reddening of older ear size and seed set are leaves and yellow striping reduced. Sometimes, which runs the length of multiple tillers or ear shoots leaves. Plants are stunted, develop. MDMV is most prevalent in areas where UGA1235014 have multiple tillers and Johnsongrass grows and serves as a winter host for produce numerous small the virus. It is spread by aphids.ear shoots. Root systems are reduced. Corn stuntsymptom development and epidemiological patterns FUNGAL SYSTEMIC DISEASESare similar to those of virus diseases. Crazy Top (Stage II, III)High Plains Virus (HPV) (Stage I, II, III) Plants have excessive Symptoms begin as small tillering with rolling or yellowish flecks which twisting of newer leaves. often appear as lines The tassel becomes a running parallel to leaf mass of leaves; ears, if veins. Infected seedlings present, often do likewise. turn bright yellow and Leaves are usually narrow, quickly die. Plants may thick and appear strap- be stunted, older leaves like. Occasionally over- 536657 may become red and sized plants will develop. then necrotic, and ear and Infection occurs when young plants are underkernel size may be reduced. Considerable symptom flooded soil conditions; therefore, it is more frequentvariation exists among cultivars. It is spread by the in low-lying areas. This is one of numerous downywheat curl mite. mildews that attack corn.27 Corn Diagnostic Guide
  29. 29. Sorghum Downy Mildew (Stage II, III) stalk than with other stalk rots. The same organism Leaves are yellow with causes leaf blighting earlier in the season. white stripes; plants are stunted and ears and Bacterial Stalk Rots (Stage II, III) tassels multiply into leafy At least two organisms masses. Often the base have been identified as of leaves are chlorotic causing bacterial stalk rot. and sharply contrast with With either, there is rapid healthy green leaf tips. development of a soft rot White, downy growth of stalks, accompanied appears on either or both by wilting and plantleaf surfaces. Leaves often split or shred. This is death. Stalks twist andprimarily a disease of sorghum. 5361254 collapse while still green, disintegrating into a softSTALK AND ROOT ROT DISEASES mass often accompanied by a foul odor.Stalk rots are diseases that are most commonlyexpressed as plants reach maturity. Stalk rot of Charcoal Rot (Stage III)corn tends to be a complex of several disease- Charcoal rot can becausing fungi and sometimes bacteria; seldom will expected when the croponly one causal organism be isolated and identified. has grown under hot, dryPlants with rotted stalks almost always have rotted conditions; it also attacksroots, too. Usually, but not always, the same causal sorghum and soybeans.organisms are involved. Visual identification is very Lower internodes are affected, causingdifficult. premature ripening,Typically, wilting is the first sign of stalk rot in a field. shredding, and crownIn a few days, leaves turn a “frosted” gray, ears disintegration. Vasculardroop and the outer rind of the lower stalk turns strands remain intact but are black or “charred” inbrown. Fields where stalk rot is developing should be appearance. Stalks become gray-black or charcoalharvested early to reduce grain losses. color.Anthracnose Stalk Rot (Stage II, III) Diplodia (Stenocarpella) Stalk Rot (Stage III) Early infection may kill Lower internodes are plants before pollination, straw-brown, spongy, and but onset usually occurs dry. Pith disintegrates, just before plants mature. leaving vascular strands Sometimes a portion intact. White fungal growth of the plant above the may appear on the stalk ear blanches and dies surface. Minute, dark prematurely (top dieback). bodies embedded just Usually, the entire plant under the stalk surface are is killed and several difficult to remove.nodes are rotted. Late in the season, a shiny blackdiscoloration develops in blotches or streaks on thestalk surface, especially on lower internodes. Internalstalk tissue may become black and soft, starting atthe nodes. Lodging typically occurs higher on the 28
  30. 30. Fusarium Stalk Rot (Stage III) temperatures resulting in darker pigmentation. Rotting of roots, crown, The role of this root rot in stalk lodging is not fully and lower internodes leads understood. to premature ripening and stalk breakage. Split EAR AND KERNEL ROTS stalks may show whitish- These rots can affect ears, kernels, or cobs, pink to salmon color. reducing test weight and grain quality. Some rots Disintegration starts at the are responsible for development of mycotoxins that nodes. may contaminate grain. Positive identification is Healthy stalk, left. Diseased difficult. Rotting observed in the field is often due to a stalk, right. complex of causal organisms, not just one.Gibberella Stalk Rot (Stage III) Most ear rots are favored by late-season humidity. Affected plants wilt, leaves Infections are increased by ear damage by birds or turn dull gray-green, and insects and by stalk lodging that allows ears to come the lower stalk softens and into contact with the soil. becomes straw colored as plants die. Pith tissue Aspergillus Ear and Kernel Rot (Stage III) disintegrates, leaving only Greenish or yellowish- vascular strands. The tan discoloration occurs inside of a rotted stalk is on and between kernels, pink to red. Small, dark especially near the ear bodies are superficial on tip. Symptoms are morethe lower stalk surface and can be scraped off easily. prevalent if the husk does not cover the ear tip. ThePythium Stalk Rot (Stage II, III) rot is favored by hot, dry This organism attacks weather. It may produce individual plants in localized aflatoxins. areas, sometimes before Cladosporium Ear Rot (Stage III) flowering. It is usually confined to the first internode above Dark gray to greenish the soil line, where rind and black fungal growth pith develop wet rot. Stalks causes kernels to appear twist and collapse. Hot, wet blotched or streaked. Initial weather favors development discoloration appears where kernels are attached to the of this stalk rot. cob. Infection eventuallyRed Root Rot (Stage III) progresses upwards and Red root rot often occurs in plants that are also stalk rotted. Seminal roots infected kernels can are most damaged and be seen scattered over adventitious root damage the ear. If completely increases with their age. colonized, ears are dark The root color of affected and lightweight. This plants ranges from pink to disease is often associated royal red, with higher soil with damage due to insects, hail, or frost.29 Corn Diagnostic Guide
  31. 31. Diplodia (Stenocarpella) Ear Rot (Stage III) Penicillium Ear Rot (Stage III) Symptoms include Powdery green to blue- bleached husks, white green mold develops on mold over kernels, and and between kernels. rotted ears with tightly Infection usually begins at adhering husks. Early the ear tips and primarily infection (2 to 4 weeks occurs on ears with after silking) is likely to lead mechanical or insect to complete ear rotting. damage. Infected kernels Later infections may result may become bleached in partial rotting, usually and streaked. “Blue eye”beginning at the base. Since corn is the only known occurs when the embryo becomes discolored duehost, this disease is most severe when corn is to the presence of blue-green fungal spores andplanted following corn in reduced tillage situations. can occur if infected grain is stored at high moisture levels.Fusarium Kernel or Ear Rot (Stage III) Trichoderma Ear Rot (Stage III) Dark green fungal growth is found on and between kernels and husks, often covering the entire ear. Scattered individual or The disease usually occurs groups of kernels show on ears with mechanical or whitish-pink to lavender insect damage. Infected fungal growth. Infected plants tend to be widely kernels may also have distributed within a field. a “starburst” pattern of white streaks on the cap of the kernel or along the base. Infections are more Photo Citations: frequent on damaged ear Clemson University - USDA Cooperative Extension Slidetips, and are favored by dry weather. Fusarium rot Series, (viewed 9/22/10); Corn Stuntmay produce mycotoxins. 1235014 William M. Brown Jr., (viewed 9//22/10)Gibberella Ear Rot (Stage III) High Plains Virus 5366657 Symptoms include reddish Howard F. Schwartz, Colorado State University, Bugwood. kernel discoloration, org (viewed 9/22/10) Bacterial Stalk Rot 5361254 usually beginning at the Holcus spot and Pythium stalk rot photos courtesy of Don ear tip. Husks may rot and White at University of Illinois be cemented to the ear. The organism is favored by cool, humid weather, particularly 2 to 3 weeks after silking. It produces several mycotoxins. 30