Diseases of Field Crops: Corn
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Pub 811:
Agronomy Guide >Diseases
of Field Crops> Corn Diseases
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811: Agronomy Guide for Field Crops
Corn Seedling Diseases
Seed Rot, Seedling Blight, Root Rot
Incidence and Management Strategies: See General
Seed Rots and Seedling Blights.
Disease Cycle: In corn, the most common diseases are
caused by Pythium, Fusarium, Gibberella, Trichoderma and Penicillium,
but other fungi such as Diplodia and Rhizoctonia can also be involved.
Seed, seedling and roots infected by Pythium are most often soft (wet)
and dark coloured, as opposed to roots infected with Fusarium, Gibberella,
Diplodia and Rhizoctonia, which are firm or leathery. The colour of
the roots most often provides a good indication of which organism(s) are
present:
- greyish-white indicates Diplodia
- tan to pink indicates Fusarium or Gibberella
- reddish to brown indicates Rhizoctonia
- blue-green indicates Penicillium or Trichoderma
Pythium, Fusarium, Gibberella, Diplodia, Rhizoctonia, Penicillium
and Trichoderma all live and thrive in the soil. In most cases,
they can affect other crops besides corn. Except for Pythium, all
of these organisms also have the ability to live on or in corn seed.
Corn Leaf Diseases
Anthracnose Leaf Blight (Colletotrichum graminicola)
Incidence: Anthracnose may become severe in
warm, wet years and is often the first corn leaf disease that is noticed.
It begins on the lower leaves, working its way up the plant. Symptoms
often disappear as the corn plant begins its rapid growth phase. The fungus
that causes anthracnose leaf blight is also responsible for anthracnose
stalk rot (see Anthracnose Stalk Rot ). Record
where anthracnose leaf blight symptoms developed early in the season and
return to those areas to scout for stalk rots a few weeks before harvest.
Tillage systems that leave considerable amounts of anthracnose-infected
debris on the soil surface may lead to greater severity of the disease
and an increased presence.
Appearance: Anthracnose may affect both leaves and stalks. The
main symptoms are leaf spotting, top dieback and stalk rot. Leaf spots
are oval, up to 15 mm (6 in.) long, with a tan centre and reddish-brown
border (See Plate 108 ). Individual lesions may
join, forming streaking along the margin or midrib. A general yellowing
of the tissue surrounding the infected areas often develops. With the
aid of a hand lens, small black spots can be seen in the centre of the
lesions. Under close examination, black hairs may be seen protruding from
these spots. The disease is first observed on the lower leaves and later
on the upper leaves. Top dieback can occur late in the season as diseased
leaves wilt and gradually die, taking on the appearance of frost damage.
Disease Cycle: Residue is an important component in
anthracnose development, since the fungus survives (overwinters) as mycelium
or sclerotia within corn residue or seed. Rain splashes spores from the
corn residues onto the lower leaves and stalk. For this reason, second-year
corn is the most prone to anthracnose infection, especially when the weather
is warm and wet.
Management Strategies: Planting hybrids that are resistant
to anthracnose leaf blight can help to manage anthracnose leaf blight.
However, resistance to anthracnose stalk rot is separate from resistance
to anthracnose leaf blight. Hybrid resistance to anthracnose stalk rot
does not guarantee resistance to early-season anthracnose infections on
leaves. In conventional tillage corn fields, removal of corn residues
through tillage will lower the risk to the disease, especially when corn
follows corn. In no-till or reduced tillage fields, management of anthracnose
leaf blight is best achieved with rotations (avoiding second-year corn)
and planting of resistant corn hybrids. Fungicide applications are not
economical in field corn situations, because more than one application
is necessary to control the disease. However, in seed corn fields, fungicide
applications may be cost effective.
Plate 108. Anthracnose
affects both leaves and stalks. The main symptoms are leaf spots, top
dieback and stalk rot.
Northern Leaf Blight (Setosphaeria turcica)
Incidence: Northern leaf blight has traditionally been
one of the most damaging corn leaf diseases. Use of resistant/tolerant
hybrids has limited yield losses from this disease in commercial corn.
However, in recent years the disease has increased, which may suggest
a decline in tolerance levels. Significant losses continue to occur in
seed corn production when highly susceptible corn inbreds are planted.
Appearance: The disease appears as long, elliptical,
2-15 cm (1-6 in.), greyish-green or tan streaks. Lesions most often begin
on the lower leaves (See Plate 109). As the disease
develops, individual lesions may join, forming large blighted areas. In
some cases, the entire leaves may become blighted or "burned"(See
Plate 110). Losses due to northern leaf blight
are most severe when the leaves above the ear are infected at or slightly
after pollination. The disease is often confused with Stewart's wilt (see
Bacterial Leaf Blight or Stewart's Wilt).
Disease Cycle: The fungus survives in corn residue as
either spores or fungal strands (mycelium). The spores of the fungus are
spread from the ground residue to the developing corn plant through wind
or rain "splashing." Plants that become infected act as a secondary
source of infection and may spread to other fields. Disease development
is favoured by moderate temperatures (18°C-27°C) with prolonged
periods of humid or rainy weather.
Management Strategies: There are various races of northern
leaf blight. Most of the commercial corn hybrids have resistance or tolerance
to the common races. An increase in northern leaf blight symptoms in an
area could indicate the potential for a new race developing and should
be reported. Crop rotation and tillage will reduce inoculum levels in
surface residues. In reduced tillage systems, rotation and the use of
resistant hybrids is necessary. Foliar fungicides are not usually economical
in field corn but may be warranted if a susceptible hybrid is planted
and disease develops early in the season.
Plate 109.
Northern corn leaf blight showing long, elliptical, greyish-green or tan
streaks.
Plate 110.
Northern leaf blight on a susceptible variety (left) and a resistant variety
(right), showing fewer symptoms.
Eyespot (Aureobasidium zeae)
Incidence: Although eyespot normally causes only minor
losses in corn, the disease has been increasing in Ontario with the shift
to higher corn residues remaining in the field.
Appearance: The disease produces characteristic round
or oval spots, 1-4 mm (1/16-1/8 in.) with a tan/brown centre and a brown
or purple margin (See Plate 111). A translucent
yellow halo forms around the margin, and when held to the sun, the lesions
resemble an eye. Leaf blighting may occur when these lesions join, killing
large portions of leaf tissue. The disease may be confused with non-infectious
physiological leaf spots or insect damage.
Disease Cycle: The disease is more prevalent under continuous
corn and reduced tillage systems, since the fungus overwinters in corn
residue. Disease development is favoured by cool, wet conditions.
Management Strategies: Resistant varieties, crop rotation
and clean plowing of crop debris help to reduce disease severity. Foliar
fungicides are rarely recommended for the disease.
Plate 111. Eyespot
causes round or oval leaf spots with a tan/brown centre, a brown or purple
margin, and a translucent yellow halo when held up to the sun.
Bacterial Leaf Blight (Stewart's wilt
)(Pantoea stewartii)
Incidence: Bacterial leaf blight occurs throughout Ontario, but
the disease is only of concern in Southwestern Ontario. Essex and Kent
Counties, where the majority of seed corn production fields are located,
tend to be especially affected by bacterial leaf blight. Warmer-than-normal
winters in this area have allowed the corn flea beetle, which is a vector
of bacterial leaf blight (Stewart's wilt), to survive in higher numbers.
Appearance: There are two distinct phases of the disease:
the wilt phase and the late wilt phase. The wilt phase primarily affects
highly susceptible seed corn inbreds and sweet corn hybrids early in the
year (V2-V4). The first noticeable sign of the disease appears as long,
yellow streaks that extend along the length of the leaf (See Plate
112). These streaks will take on a water-soaked appearance and eventually
become brown, dead streaks. The bacteria interrupt the water and nutrient
movement in the plant by plugging the vascular system of the plant. The
result is a rapid wilting and even death. Since the new growth is affected,
the wilting and death occur from the top down. Cutting the plant lengthwise
will reveal a discoloured, rotted or hollowed-out growing point.
The leaf blight phase or late-infection stage often occurs after tasselling
and is the most common phase. Symptoms include pale-green to yellow streaks
with irregular or wavy margins that run parallel to the veins. These streaks
may run the full length of the leaf. Infected leaves eventually become
dry and brown. The marks of corn flea beetle feeding are often visible
within the lesions. Premature leaf death can result in reduced yield and
an increase in stalk rots, since weakened plants are more susceptible
to stalk rots.
Disease Cycle: The bacteria overwinters in the gut
of adult flea beetles, which hide through the winter in protected areas
(See Corn Flea Beetle). Mild winters
can result in higher beetle numbers. Overwintering adult flea beetles
feed on corn in the seedling-to-whorl stage, and susceptible varieties
will develop a stem wilt, resulting in complete plant loss. This occurs
rarely in hybrids but occasionally in susceptible seed corn parents. The
next generation of adult beetles emerges after corn silking and causes
leaf wilting symptoms, which are commonly seen in many hybrids. Seed transmission
is rare. Most often, late infections after silking are associated with
high beetle populations. Sweet corn is often more susceptible than field
corn and can serve as a reservoir for the bacteria. The disease is often
found in the best fields, and fertility seems to play a part. Susceptibility
to the disease increases in fields that have high nitrogen and phosphorous
levels.
Management Strategies: Field corn has good tolerance to Stewart's wilt
and therefore no control is required. Certain seed corn inbreds are susceptible
and are rated for disease tolerance. This disease is controlled by managing
the corn flea beetle. See Corn Flea Beetle.
Plate 112. Stewart's
wilt (bacterial leaf blight) occurs after tasselling. Wilt phase occurs
at V2-V4 growth stage. Corn flea beetle is a vector of Stewart's wilt.
Grey Leaf Spot (Cercospora zeae-maydis)
Incidence: Grey leaf spot is a destructive and economically important
disease that has been increasing over the past 10 years in the states
surrounding the Great Lakes. Significant losses can occur from this disease
under warm, wet and humid conditions.
Appearance: Soon after tasselling, the symptoms develop
on the lower leaves. The disease has unique, elongated, 2-7 cm (1-3 in.)
long, narrow, light-tan, rectangular lesions. These lesions run parallel
to the leaf veins. As the lesions mature, they become grey and join, killing
or blighting entire leaves.
Disease Cycle: Grey leaf spot is most problematic when
corn follows corn in fields with a considerable amount of corn residue.
The fungus survives as fungal strands (mycelium) in corn residue. Spores
produced on the residue are dispersed by wind and rain splash. Warm, humid
weather favours spore and disease development.
Management Strategies: Crop rotation and tillage will
reduce inoculum levels in surface residues. In reduced tillage systems,
rotation and use of hybrid resistance may be necessary. Chemical control
is not usually needed but if a highly susceptible hybrid is planted and
disease development begins early in the season, it may be warranted.
Common Rust (Puccinia sorghi)
Incidence: Common rust does not overwinter in Ontario. It originates
from infected corn in the southern U.S. and Mexico. Rust spores are blown
into Ontario. In most years, rust is of minor economic importance. However,
sometimes spring storm fronts bring in spores and cause early-season infection.
The disease is favoured by high humidity with cool evening temperatures
(14°C-18°C), followed by moderate daytime temperatures.
Appearance: Early symptoms of rust infection are yellow
flecks or spots on either side of a leaf. These develop into small, brick-red
pustules that break through the leaf surface (See Plate
113). The brick-red colour is the result of spores being released
from these oval or elongated 2-10 mm (1-4 in.) lesions. Yellowing of the
leaf occurs around these lesions. Dead, brown areas of the leaf develop.
In severe cases, the entire leaf dies. The brick-red spores mature and
turn black as they mature, causing the lesions and leaf surface to appear
black.
Management Strategies: Since common rust does not survive
in Ontario, cultural practices such as reduced tillage and crop rotation
do not influence disease development. Commercial corn hybrids have good
tolerance. However, many seed corn inbreds, sweet corn and specialty corn
hybrids are very susceptible to the disease. Foliar fungicides in field
corn are not usually needed but can be economical in highly susceptible
corn hybrids, seed corn inbreds or specialty corn hybrids.
Plate 113. Common
rust symptoms range from yellow flecks to red pustules.
Common Smut (Ustilago zeae)
Head Smut (Sporisorium holci-sorghi)
Incidence: Two corn smut diseases, common and head smut,
occur in Ontario. Common smut occurs most frequently. In severe cases,
over 25% of the plants in some fields can have smut galls.
Appearance: Common smut overwinters in the soil and
in corn residue. The spores are spread by wind and rain through splashing.
All above-ground plant tissue is susceptible, but infection occurs most
often in areas of actively growing tissue. Common smut incidence increases
in fields where the plants have been wounded by hail, frost, drought,
mechanical injury, detasselling, herbicide injury, insects or sandblasting.
High levels of nitrogen and manure promote this disease.
Greyish smut galls up to 10 cm (4 in.) in diameter develop on the stalks,
ears and tassels, while smaller galls often appear on the leaves (See
Plate 114). The galls initially have a white membrane cover that eventually
breaks and releases dark-brown or black powdery spores. On the leaves,
galls develop into a hard, dry growth. Smut galls can replace kernels.
Unlike common smut, head smut occurs only on the ears and tassels (See
Plate 115).
Disease Cycle: Spores released from the galls are well
adapted for Ontario conditions. They survive in soil and crop residues
for many years. In the spring, these spores germinate to produce new spores
that will infect the rapidly growing areas or injured areas of the plant.
The resulting galls will release spores that infect other plants. Disease
development is favoured by rain showers, high humidity and warm temperatures
in conjunction with physical plant injury.
Management Strategies: Most commercial corn hybrids
have sufficient resistance to smut to prevent serious outbreaks. However,
some smut is present in most fields and is still very problematic in many
seed corn fields. Risk is reduced by minimizing mechanical and herbicide
injury, while maintaining a balanced fertility program. Rotation and cultivation
have little effect on the disease, since spores can survive for a long
time in the soil.
Plate 114. Common
smut incidence increases where plants have been wounded.
Plate 115. Head
smut can occur on the ears or tassels of corn.
Stalk Rots
Incidence: Fungi cause corn stalk rots. The amount of
damage they cause increases when the crop is under stress, such as prolonged
wet or dry conditions, cool temperatures, cloudy weather, leaf diseases
(such as rust and Stewart's wilt), leaf and ear damage from hail, birds
and frost, incomplete pollination, unbalanced fertility, insect damage
(e.g., European corn borer), high plant populations, hybrid susceptibility
and poor soil conditions.
The distribution and prevalence of stalk and ear rot diseases vary from
year to year. However, the diseases are present in most years even though
it may be at low levels. The majority of stalk rot damage in Ontario is
caused by three fungi, namely Anthracnose, Gibberella and Fusarium.
However, Diplodia and Pythium have also been observed in
Ontario.
Impact of Stalk Rot: Although these fungi cause different
symptoms, their ultimate effect on the corn plant is the same. They reduce
grain fill and stalk integrity and accelerate senescence. Stalk rot fungi
affect the nutrient movement of the corn plant in three main ways:
- Sugars produced through photosynthesis and carbohydrates in the root
and stalk are diverted to the fungus and not to the ear. These nutrients
allow stalk rot fungi to grow and flourish.
- There is a reduction in stalk integrity. To meet the nutrient needs
of both the developing ear and the stalk rot organisms, the corn plant
will begin to cannibalize itself by moving soluble carbohydrates from
the root and stalk. Problems arise when the plant is unable to meet
the nutrient requirements of the developing ear. The result is a weaker
stalk (prone to lodging) and less resistance to stalk rot fungi.
- Finally, the infection and colonization process inhibits or blocks
many of the pathways that the plant would ordinarily use to move nutrients.
Yield losses (generally 10%-20%) arise from poorly filled ears and harvest
losses from lodging.
General Stalk Rot Management Strategies: Management
begins by reducing crop stresses through:
- planting hybrids that have good resistance or tolerance to leaf diseases
and stalk rots
- managing insects such as European corn borer
- good weed control
- appropriate plant populations
- a balanced N and K fertility program
- crop rotation
- tillage
Scouting for Stalk Rots
Two methods are used to scout for stalk rots.
The Push Test
1. Randomly select 20 plants from five different areas of the field, for
a total of 100 plants.
2. As the name implies, push the top portion of the plant 15 to 20 cm
(6 to 8 in.) from the vertical.
3. Note whether the plant lodged or not.
The Pinch or Squeeze Test
1. Randomly select 100 plants in the field (20 plants from five different
locations).
2. Remove lower leaves and pinch or squeeze the stalk above the brace
roots.
3. Record the number of rotted stalks.
If 10%-15% of plants lodged, or are rotted, harvest the crop early. The
extra drying charges that may result will be covered by increased harvest
efficiencies with less corn left in the field.
Anthracnose Stalk Rot (Colletotrichum
graminicola)
Appearance: Anthracnose stalk rot is the easiest to identify.
It appears as large, dark brown-to-black shiny areas or streaks on the
outer stalk rind (See Plate 116). These shiny
or discoloured areas are often found at the base of the stalk. Cutting
the stalk lengthwise will reveal a discoloured and rotted pith. Another
symptom associated with this disease is "top dieback." Typically,
top dieback symptoms begin in late August or early September, as corn
plants begin to wilt and die from the top down. This resembles premature
death due to frost. Premature death occurs above the ear, with the plant
tissue below the ear remaining green. Examination of the stalk in these
dead areas will show the same shiny black areas that are found at the
stalk base. Plants with top dieback symptoms correspond to areas of the
field that had late-season stresses.
Disease Cycle: The fungus that causes anthracnose stalk
rot survives in the previous corn crop residues and therefore is most
often a problem in second-year corn. Warm, wet and humid weather favours
anthracnose development.
Plate 116.
Anthracnose stalk rot. Internal corn stalk tissue is often discoloured
(black), and the pith is rotted.
Gibberella Stalk Rot (Fusarium graminearum/Gibberella zeae)
Fusarium Stalk Rot (Fusarium verticilloides)
Diplodia Stalk Rot (Diplodia maydis)
Appearance: These fungi cause general stalk rot symptoms, including
wilting and death. Affected leaves turn a grey-green colour that resembles
frost damage. All three rots cause a dark external lesion or spots at
the lower nodes. Diplodia stalk rot produces small black spots (pycnidia)
that are embedded in the stalk rind. These spots are hard to remove. In
contrast, gibberella stalk rot also produces small, round, black spots
at the lower node, but these spots can be easily scraped from the stalk
surface (See Plate 117). The pith is shredded
and has a pink to red colour. Fusarium stalk rot symptoms appear as light-brown
to black lesions near the nodes. The internal stalk symptom of fusarium
stalk rot is a salmon-pink fungal growth in the pith.
Plate 117.
Gibberella stalk rot. Inside of stalk is shredded and characteristically
red.
Pythium Stalk Rot (Pythium aphanidermatum)
Appearance and Disease Cycle: Pythium stalk rot gives the same
general above-ground symptoms that are associated with the other stalk
rot organisms. Pythium is in a unique group of fungi (that also
includes Phytophthora) called "oomycetes" or "water
moulds" because of their preference for wet conditions. The unique
characteristic feature of this group of fungi is the production of mobile
spores that can move through the water film in saturated soils. These
spores (infection stage) are able to physically move to the corn plant
roots and, once inside, cause disease. Unlike other stalk rots that produce
overwintering structures (black dots) or mould, corn plants infected with
Pythium have no visible signs of fungal growth at the base of the
plant. When the plant is cut lengthwise through the stalk base and roots,
Pythium-infected tissue will appear wet and soggy and will disintegrate
("a wet rot") at the root base.
Ear Rots or Moulds
Ear Rot Management Strategies: White moulded corn may
or may not contain toxins, but pink or purple moulded corn will likely
be contaminated. Any of the Fusarium or Gibberella rots
can establish after pollination in wounds created by insects or birds.
Warm rainy weather or long dews any time after pollination may lead to
ear rots in these wounded cobs.
The green (Penicillium) and black (Cladosporium or Alternaria)
moulds do not normally pose a problem. However, when found in great abundance,
they may put livestock off feed. Development of ear rots is stopped when
corn is dried or ensiled, but the level of harmful toxins already present
will remain unchanged. The fungi will continue to produce toxin until
corn moisture drops below 20%. More information on Fusarium mycotoxicoses
can be found on the OMAFRA website at www.ontario.ca/crops.
Preventing ear rots and mould is difficult since weather conditions are
critical to disease development. Although some tolerant hybrids are available,
none have complete resistance. Crop rotation can reduce the incidence
of diplodia ear rot. Cultural practices have been shown to have limited
success in preventing ear and kernel rots. Minimize these diseases through
timely harvest and proper drying and storage.
Harvest fields where 10% of the ears have some ear rot quickly to limit
further disease development and potential mycotoxins production.
When ear rot is present, the following harvest, storage and feeding precautions
are advisable:
- Harvest as early as possible.
- If bird damage is evident, harvest outside damaged rows separately.
Keep and handle the grain from these rows separately.
- Adjust harvest equipment to minimize damage to corn. Clean corn thoroughly
to remove pieces of cob, small kernels and red dog.
- Cool the grain after drying.
- Clean bins before storing new grain.
- Check stored grain often for temperature, wet spots, insects and mould
growth.
- Control storage insects.
- Exercise caution in feeding mouldy corn to livestock, especially to
hogs. Pink or reddish moulds are particularly harmful. Test suspect
samples for toxins.
See Appendix D, Feed- Mould- and Mycotoxin-Testing
Laboratories, for a list of laboratories.
Fusarium Ear Rot (Fusarium verticilloides)
Incidence: Fusarium ear rot is common in Ontario. Unlike gibberella
ear rot, kernels infected with fusarium ear rot will be scattered around
the cob among healthy-looking kernels or on kernels that have been damaged
(by corn borer or bird feeding). Silks are susceptible to infection during
the first 5 days after initiation.
Appearance: Fusarium infection produces a white-to-pink
or salmon-coloured mould (See Plate 118). A "white
streaking" or "star-bursting" can be seen on the infected
kernel surface. Although many Fusarium species may be responsible
for these symptoms, of concern in Ontario is Fusarium verticilloides.
Disease Cycle: Fusarium survives in corn debris. The
significance of this fungus is that it produces a toxin called fumonisin
that has been shown to cause cancer in humans. The environmental conditions
that favour disease development are warm, wet weather, 2-3 weeks after
silking.
Plate 118.
Fusarium ear rot. Note the white fungal growth and the "starbursting"
on the kernels.
Gibberella Ear Rot (Fusarium graminearum, Gibberella zeae)
Incidence: The most common and important ear mould in Ontario
is Gibberella zeae, which is the sexual reproductive stage of Fusarium
graminearum. This fungus not only infects corn but also small grains such
as wheat. Many plant pathologists believe that in years with a high occurrence
of fusarium head blight in wheat, the potential exists for increased gibberella
ear rot in corn.
Appearance: Although, the fungus can produce a white
mould that makes it difficult to distinguish from fusarium ear rot, the
two can be distinguished easily when Gibberella produces its characteristic
red or dark-pink (purple) mould (See Plate 119).
Disease Cycle: Infection begins through the silk channel.
Therefore, in most cases, it starts at the ear tip and works its way down
the ear. In severe cases, most of the ear may be covered with mould growth.
Corn silks are most susceptible 2-10 days after initiation. Cool and wet
weather during this period is ideal for infection.
Caution: In addition to its economic importance
due to yield loss, gibberella ear rot is also important because Gibberella
zeae and Fusarium graminearum produce two very important mycotoxins
- deoxynivalenol (vomitoxin or DON) and zearalenone. These mycotoxins
are especially important to swine and other livestock producers since
they can have a detrimental effect on the animals. Feed containing low
levels of vomitoxin (1 ppm) can result in poor weight gain and feed refusal
in swine. Zearalenone is an estrogen and causes reproductive problems,
such as infertility and abortion in livestock, especially swine. Test
feed grain that originates in a field with 5% or more gibberella ear rot
for these toxins. See Appendix D, Feed- Mould-
and Mycotoxin-Testing Laboratories.
Plate 119. Gibberella
ear rot infection usually occurs from the tip down. Note the pink-to-red
colour
Diplodia Ear Rot (Diplodia maydis)
Incidence: Of the three primary ear rots that occur in
Ontario, diplodia ear rot is the least common. Diplodia ear rot is caused
by Diplodia maydis and is favoured by cool, wet conditions through
grain fill.
Appearance: The characteristic ear symptom is a white
mould that begins at the base of the ear and eventually covers and rots
the entire ear. Mould growth can also occur on the outer husk, which has
small black bumps embedded in the mould. These reproductive structures
are where new spores are produced. Diplodia ear rot does not produce any
known toxins.
Disease Cycle: Diplodia ear rot overwinters in corn debris
left on the soil surface from the previous crop. Spores that are produced
during wet weather can infect silks and husks or enter through tissue
damaged by birds or insects. Disease development is favoured when cool
wet weather occurs during the first 21 days after silking.
