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ISSUE 13 JULY 29, 1999



A. Plant leaves show damage or affects from a detrimental cause:
    1. leaves are eaten
        * look for tracks of livestock or wild animals
        * insect damage (consider armyworms, grasshoppers, corn borers or even katydids)
     2. leaves are shredded
        * hail damage
        * wind damage
        * disease (sorghum downy mildew)
    3. leaves have a dead "frosted" appearance
        * frost or freeze
        * drought
        * disease (stalk and root rots, leaf blights or anthracnose)
        * insects (corn leaf aphid, spider mites or corn borers)
    4. leaves are spotted or dead
        * disease (leaf blights, rust, Physoderma brown spot, eyespot, Goss's wilt, gray leaf spot,
            Helminthosporium leaf spot or anthracnose)
        * chemical damage (check for possible spray drift)
    5. leaves show a discolored sheath
        * insects (corn leaf aphid)
        * disease (purpling at these later stages can be caused by saprophytic organisms)
        * leaf injury
    6. leaves are red or purple
        * mechanical injury
        * barren plant (sugars which cannot be translocated to an ear will accumulate in the leaves with
            the anthocyanins expressing color)
        * disease (maize chlorotic dwarf virus, corn stunt or rust)
        * genetics of the hybrid
    7. leaves show white striping with white, downy growth on the upper and lower leaf surfaces
        * soghum downy mildew
B. Plant silks are affected
    1. silks are eaten off
        * insects (corn rootworm beetles, corn earworm, caterpillars, katydid, western bean cutworm,
            grasshoppers or armyworms)
    2. no silks or they are delayed many days after tasseling
        * heat
        * drought
        * nutrient deficiency or imbalance (usually nitrogen or phosphorus)
        * temperature variation (usually cold followed by warm)
        * insects (corn leaf aphid or spider mites)
        * excess stand population for conditions
    3. silks are trapped or balled within the husk
        * drought
        * nutrient deficiency
        * insects
        * temperature variation (usually cold followed by warm)
        * genetics of the hybrid
    4. silks are red or green in color
        * fresh silk color is genetically controlled
C. Plant tassel is affected
    1. tassel failed to emerge
        * drought or heat stress
        * nutrient deficiency (usually boron)
        * insects (corn leaf aphid pressure has stressed the plant)
    2. tassel is replaced by a mass of leaves
        * disease (crazy top, sorghum downy mildew or head smut)
    3. tassel is replaced by one or more small ears
        * genetic and environmental interaction (will be more common on tillers)
    4. the tassel is broken off and the corn stalk is tunneled
        * insects (European corn borer)
D. Corn stalk shows symptoms
    1. stalk is red or purple
        * mechanical injury
        * barren stalk (anthocyanins are expressing color due to sugar accumulation)
        * genetics of the hybrid
    2. barren plant or no ear enlargement
        * drought
        * heat
        * nutrient deficiency or imbalance
        * insects (corn leaf aphid or spider mites)
        * silks were eaten off prior to pollination
        * disease (head smut, viruses or mildews)
        * excess plant population for the conditions
        * tillers (seldom carry a full ear as long as the main plant has dominance)
        * silking or pollen shed disrupted (environmental stress)
        * mechanical damage
    3. stalk is broken below the ear
        * wind (green snap)
        * disease (stalk and root rots)
        * nutrient imbalance (usually excess nitrogen, insufficient potassium)
        * insects (corn borer)
        * weather stress (drought, heat or other conditions limiting photosynthesis)
        * mechanical damage (machinery, livestock, other animals)
        * excessive plant populations for the conditions
        * delayed harvest
    4. stalk is broken above the ear
        * wind
        * insects (European corn borer or armyworm)
        * mechanical damage
        * delayed harvest
    5. plant has multiple ears at one node
        * genetics of the hybrid
        * mechanical plant injury
        * cold temperatures when the ears formed
        * disease (viruses or mildews)
        * insects (any that cause silk feeding may actually result in multiple, small ears)
    6. plant is leaning (lodged) but not broken
        * wind and wet soil
        * poor root development (drought, fertilizer placement, soil compaction, low soil pH or poorly drained soil)
        * insects (corn rootworms or other root feeders)
        * nematode activity (usually not in the Valley)
        * disease (stalk and/or root rots)
        * nutrient deficiency (usually potassium)
        * herbicide damage (usually a growth regulator)
        * mechanical damage
        * delayed harvest
    7. plant has a premature death
        * disease (stalk/root rots or leaf blights)
        * insects (corn borers)
        * frost or freeze or severe drought
        * lightning (plants killed in a circular pattern)
    8. stalk appear black and moldy
        * saprophytic organisms have built-up under warm, humid weather conditions, harvest early)
E. Corn ears show symptoms
    1. numerous ear shoots that are only leafy (barren)
        * disease (crazy top, downy mildew or a virus)
    2. smut galls appear
        * disease (common corn smut)
    3. ear tips are poorly filled
        * nutrient deficiency ( potassium or nitrogen)
        * excess stand population for conditions
        * insects (clipped silks before pollination)
        * disease (foliar disease reduced photosynthesis)
        * drought
        * favorable growing conditions after pollination (rarely occurs, but allows cob to elongate
            without setting kernels)
        * low light, carbon dioxide or temperature conditions (reduces photosynthesis, tip kernels abort)
    4. ears are small, malformed or even light weight
        * nutrient deficiency (usually phosphorus)
        * drought or heat stress
        * plant damage (mechanical or disease)
        * excessive plant populations for conditions
        * it is a second or third ear on the stalk or a tiller
        * insect-caused plant stress (aphid and others)
        * disease (virus, leaf blight or stalk and ear rots)
        * soil compaction
    5. ear has short husk with tip exposed
        * weather (usually drought or heat that stopped husk development)
    6. barren ear
        * disease (head smut or virus)
        * pollination failure (drought, heat or other stress, insects, chemical damage)
    7. dropped ears
        * drought (shank was weakened)
        * nutrient deficiency (shank was weakened)
        * genetics of hybrid (ears may drop free of husk)
        * insect damage of shank (European corn borer)
    8. ear shows a scattered kernel set
        * lack of viable pollen
        * silks eaten
        * heat or drought at pollination
        * nutrient deficiency
        * chemical damage (growth regulators or thiocarbamates)
    9. ears have kernel feeding
        * insects (corn borer or armyworm)
        * birds or animals
    10. ear has rotten spots on parts or all of the ear
        * disease (ear/kernel rots or Helminthosporium leaf spot)
        * insects ( followed by weathering and disease)
        * bird feeding
        * hail or other mechanical injury
    11. kernels have been eaten from the ear tips
        * birds
        * wild animals or rodents
        * insects (armyworm or bean cutworm)
    12. insect tunneled into the ear, cob, shank or stalk
        * European corn borer
F. Kernels on the ear show symptoms
    1. broken seed coats give the kernels a "popped" appearance
        * popped kernels (a genetic by environment interaction which usually occurs under high temperatures)
    2. seedcoat is split
        * silk cut (a genetic by environmental interaction which is usually not visible until the kernels are shelled)
    3. kernels have pink or red streaking, especially over the crown area
        * kernel red streak (caused by wheat curl mite feeding and the toxin secreted, usually more pronounced
            toward the ear tip, not detrimental)
    4. sprouted kernels
        * high rainfall and warm temperatures while the ear remained on the stalk (usually is accompanied
            by ear mold)

Denise A. McWilliams
Extension Crop Production Specialist

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