ISSUE 9 JULY 1, 1999
IS YOUR CORN KNEE-HIGH BY THE FOURTH OF JULY?
Early planted corn should be ahead of the expected growth for the season this year. Corn planted within the first week of May that has not had any replanting problems should be looking good and well above knee-high. Low insect pressure this year in corn has helped in preserving crop health (even on late-planted and replanted fields) and reports from the Midwest suggest low levels of corn borer are being seen.
From germination to the four-leaf stage, corn normally is at risk from sod webworms, slugs, stalk borers, true armyworms, billbugs, flea beetles and corn root aphids on the plant with wireworms, white grubs, black cutworms, seed corn beetles and seed corn maggots still having the potential for a go at the root of the seedlings. Once into corn's fifth-leaf stage, the insects to watch for through silking include: European corn borer, corn rootworm, true armyworm, fall army worm with still some potential damage from stalk borer, sod webworm, black cutworm, white grub and wireworm if numbers increase beyond what we have seen during spring.
Any severe wilting or death of plants may be due to lightning (usually a circular pattern in the field) or with dieback of the leaves (wilting, drying up of leaf tissue beginning at the leaf tips) the problem may be molybdenum deficiency (younger leaves also twist) .
Corn plants that become discolored may be revealing some production problems such as yellowing of the plants (beginning with the lower leaves) due to nitrogen deficiency, drought conditions or ponded conditions (standing water can produce nitrogen deficiency). Yellowing of leaves beginning at the tips may harold potassium deficiency. Purpling or reddening of leaves from the tip back and affecting the lower leaves first may show phosphorus deficiency. Yellow to white interveinal striping on corn leaves may just be genetic or may be magnesium deficiency or if the leaves have white spots that graduate to striping, boron may be deficient. Pale green to white striping shows iron deficiency (some is being seen this year due to the ample moisture) or with this symptom along with the lower leaves very dark (almost olive) colored, magnesium deficiency may be a problem.
Stunting or very fine chlorotic stripes in the whorl leaves will suggest that the plants be checked for maize dwarf mosaic or maize dwarf chlorotic disease. Individual stunted, tillered plants with twisting and rolled leaves probably indicates isolated plants with crazy top fungal disease (possible this year due to the very wet spring).
Plants that have top leaves fused so leaves have difficulty emerging ("rat-tailed" plants) may have been exposed to some growth regulator herbicide or may be mechanically injured during a cultivation or sprayer pass. Corn leaves that are very tightly rolled and very erect show symptoms of growth regulator injury or can simply be exhibiting drought stress. Lodged plants or plants showing "sledrunner" or "goosenecked" shapes may have problems with corn rootworm larvae, corn nematode feeding (usually not a problem in ND), an earlier herbicide damage residual symptom (from a dinitroaniline or a growth regulator herbicide), simple mechanical injury or from hot, dry weather with winds which prevents normal brace root development.
Corn stalks that show a brown, soft rot on the lower internode with some stalks twisting and lodging will probably be pythium stalk rot or bacterial stalk rot, or heavy European corn borer or stalk borer damage which has weakened the stalks.
Fused brace roots, as they emerge on the plant, will be due to growth regulator herbicide injury from an application which was applied after the corn was taller than eight inches.
Appearance of soft, glistening white galls that soon become black and dusty on corn stalks, leaves and later the ear or tassel will be from common smut (a possibility this year, from the early, wet spring affect on the early planted corn).
Ragged plant tissue due to shredding or tearing of leaves in fields may be due to hail damage, wind damage, Western corn rootworm beetles (the green upper layer of leaf tissue is stripped) or corn blotch leafminers (window effect, interior leaf is eaten out). Whole leaf or large chunks of leaf removed may be due to armyworms, grasshoppers or browsing livestock or wildlife. Holes bored into the stalks are due to European corn borers or stalk borers.
General lesions, spots or color streaks initially on small areas can be Northern corn leaf spot, gray leaf spot, holcus bacterial spot, fungal leaf spots, Stewart's bacterial leaf blight, physoderma brown spot, or herbicide (contact) damage. Late-applied granular fertilizers or even air pollution can also cause spotting. Anthracnose leaf blight, bacterial leaf stripe, eyespot and common corn rust also cause lesions on plants that are very distinct in color and shape.
Carefully scout your corn fields through tasseling to determine production needs in order to optimize your corn yields.
INVENTORY FIELDS FOR PROBLEM MANAGEMENT
Cool, wet spring weather requires additional crop scouting on fields that have been planted throughout the summer. One way to keep track of fields and problem areas is to map the field and to include with this map a field observation sheet of factors affecting crop growth. On the map, write in the year, crop, date and stage of the crop with each observation. List any unusual, good or bad, crop conditions during each scouting review. Make a note of the hybrids or varieties most affected. Diagram the unusual or problem areas on the field map. Next, use the following check list with your map to further document your crop nonperformance by checking off the probable causes of problems in the field:
lack of light
high clay areas prone to flooding
sandy area prone to drought stresses
organic matter differences in the field
root growth impedance (rocks, compaction, etc.)
previous crop residue acting like high organic matter
low area (flooding, frosting, freezing)
high area (drought, freeze, frost, wind damage)
maturity range not suited for the location (too early, late a variety planted)
erosion problems (water, soil)
disease (symptoms, suspected disease)
weed competition (weed appearance or mix, specific species)
insect damage (what symptoms, which species)
lodging (and suspected reason)
herbicide injury (what was used; rate; resulting damage; probable cause overlapping
applications, over application, miscalibration, synergistic effect of chemical mixture)
commercial fertilizer (plant burn, plant stress, salting)
manure (plant burn, plant stress, salting)
stand/density loss due to chemical (what chemical, why?)
lodging of crop plants from chemical application (what chemical, why?)
residue decomposition affecting the present crop (allelopathy, decomposition tie-up) other?
Physical Damage or Tillage or Planting Influences
mechanical damage (cultivator blight?)
stand or density problems (too thin a stand; too thick a stand, other cause of planter calibration or operator misjudgement?)
fertilization injection applied too late (root/plant pruning from late application)
lodging (suspected reason?)
tassel or flower drop (why?)
ear or pod drop (why?)
kernel, soybean or plant loss (physical damage cause?)
compaction due to equipment, tillage or other?
root growth impedance due to physical damage?
seeding impedance due to heavy residue, planter depth not set correctly, planter not heavy enough to cut through soil or residue properly?
Denise A. McWilliams
Extension Crop Production Specialist