ISSUE 12 JULY 22, 1999
SCOUT CORN FIELDS TO ELIMINATE WORRY
Corn is now entering the rapid-growth phase that can weaken the corn
stalk even more than was
seen in the early vegetative growth stages. Also, the number of kernels per row on each ear will be
determined from V12 through V14 with rapid dry matter accumulation setting the yield potential for
each plant. Herbicide applications should be over for most fields, unless corn is behind due to late
planting. Scout weekly for disease, insect or physical plant damage up through tasseling (less intense
scouting can commence after tasseling--every two weeks).
Common corn rust thus far has been minor. The cinnamon-brown
(blister-like growths) can occur on any above-ground plant tissue. Rust thrives in moderate to cool
temperatures and high humidity. Luckily, this disease rarely causes economic loss.
Recent reports from the Midwest report some northern corn leaf
blight showing up. This disease will
develop with long, elliptical, gray-green lesions on the leaves that will become tan-brown. Infection spreads
from the lower leaves, up the plant. High humidity and moderate temperatures favor the disease.
High winds and hail in some locations earlier this season may also
spell head smut trouble. The corn
seedling was infected when damaged and the disease systemically developed with the plant growth. Tassels
of affected plants often show the affects. Tassels may not develop (in a sporadically pattern) across the field,
being replaced instead with a mass of leaves or the tassel may be replaced by a black, smutty mass which
will rupture and release spores (to provide more spores to be harbored in the soil for later years when
conditions are ideal). In some cases, ears may be completely replaced by similar smut masses or an ear
may only produce a few kernels. Hot, dry soil at corn seedling stage favors infection so perhaps this problem
will be low in prevalence as it usually is low.
However, crazy top or corn plants with excessive tillering or
rolling/twisting of newer leaves often is more
prevalent when young plants were under flooded conditions (often appearing in low areas). In this systemic
(but usually small incidence) disease, the tassel can also become a mass of leaves, as can the ears. However,
these leaves are often narrow, thick and appear "strap-like." Sometimes with crazy top over-sized plants will
develop and can be easily spotted in fields. This disease is actually only one of numerous downy mildews
that may attack corn.
Many of the stalk rots that may be harbored in corn are not
expressed until the plants reach maturity.
However, stalk rots caused by several fungi and sometimes bacteria organisms can contribute to weaken
stalk strength. Visual identification without destroying plants in the field in order to split stalks to observe
early, interior signs is very difficult. Often only a wilting of plants (sometimes very minor) may be seen.
Later, infected plants may show leaves that are just barely "frosted" with a gray color and may then
progress to definite ears drooping and the outer rind of the corn plant's lower stalks may turn brown. Any
fields with stalk rot will have to be harvested early to reduce grain losses.
Also while scouting, watch for late-appearing herbicide injury
symptoms. Observe surrounding fields as
well as your field's conditions and the season's problems with other probable causes in mind above and
beyond herbicides before making a hasty guess on the problem. Many environmental conditions as well
as environmental by hybrid interactions can mimic drift or post-application injury conditions. Also, review
and analyze any symptoms against any previous herbicides used prior to the current cropping season. Corn
plants frequently outgrow the effects of herbicide injury and final yields may not be reduced, so carefully
evaluate each situation in your fields.
Insects must also be scouted for now in corn. Often damage from
insects is difficult to define and identify.
It is not unusual to find insect damage but then to not find any of the suspected insects! Carefully consider
economic thresholds of damage before treating and definitely consider if timing is appropriate to give control
by spraying. Watch corn for aphids this year, as number have been large in other crops in many areas.
Armyworms have also made an appearance in locations. Corn borer numbers appear down this year, but more
time is needed to determine if the recent surge in moth flights will result in any larvae that are detrimental. Flea
beetles can feed on corn and while often only leaving a "scratch mark" on corn can also be the primary vector
of bacterial wilt of corn. Many other insects can contribute to problems in corn.
Nutrient deficiency can be accentuated by poor root development and
unfavorable soil conditions (such as
water-logged or compacted soil). Nutrient deficiencies are very difficult to diagnose. Soil and plant analysis
can help in identification and determination of the cause of these problems.
Scout your corn fields regularly in order to eliminate worry about
the crop. Check various regions of individual
fields so that you periodically see most of each field.
SOYBEAN DIAGNOSTICS FROM V5 TO R1
Many of the soybeans across the Valley are beginning to bloom;
however, if the beans are just into this
stage or have not yet initiated flowering consider the following symptoms to determine problems in the field.
Soybean plants have leaf or stem damage:
leaf and/or stem damage
leaves are physically torn but show no insect, disease or herbicide injury
leaves show insect feeding/damage
leaves show yellowing, followed by browning of leaf margins and possibly plant death--triazine injury
leaves wilted, dead or dropped (particularly in a circular pattern in the field)--lightning
stems or leaves show discoloration or other disease symptoms
- leaves have a dead, ragged appearance after a hard rain or wind but appears more than weatherrelated--bacterial blight
- white fungal growth and sclerotia (large, black, irregular structures inside the stem)--Sclerotinia blight (white mold)
- lower stem discolored--Phytophthora rot, Rhizoctonia, Fusarium, black root rot (will show discolored stem with reddish fruiting bodies present)
- dead spots on leaves with raised areas on under-leaf surface--bacterial blight
- brown spots on upper leaf surfaces and gray to purple fungal growth on underside of leaves--downy mildew
- leaves with leafspots with light-colored centers and dark margins--frogeye leafspot
- brown spots and/or yellowing on lower leaves--brown spot disease
- leaves crinkled and/or disfigured--soybean viruses (also check if the plants are also stunted for glyphosate injury, manganese toxicity, phenoxy injury, benzoic acid injury, boron toxicity)
stem is discolored and may be burned at the soil line--high soil temperature,
herbicide injury from
post-directed sprays, including shielded contact herbicides such as paraquat
leaves are speckled or burned, new growth is generally unaffected--diphenyl ether
sunburn, air pollution
yellowing along the leaf margins, followed by browning and later necrosis of the leaf
leaf curl--potassium deficiency, triazine injury
interveinal yellowing of young leaves while veins remain green--manganese
chlorosis, chlorimuron, clomazone (may turn white) herbicide injury
yellowing of both old and young leaves--sulfur deficiency, nitrogen deficiency,
nematode (usually not a problem in ND), water damage, zinc deficiency, magnesium deficiency
possible yellowing and some distortion of young leaves, plants may be stunted with
internodes--imazaquin, imazethapyr, imazamox herbicide injury
scorching of leaves along the leaf margins--chlorine toxicity, boron toxicity
Roots show damage on the soybean plants
roots are stunted or showing abnormal growth
Denise A. McWilliams
NDSU/UM Extension Crop Production Specialist