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Stress - Ear Moulds!
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Figure 1 - Gibberella ear rot - Infection in most cases occurs from tip down. Note the dark pink to red colour. |
| Figure 2 - Fusarium ear mould - Note the white fungal growth and the "starbusrsting" of the kernels | |
| Figure 3 - Diplodia ear rot- Diplodia in most cases starts from the base of the ear up. Produces a white fungal growth that appears similar to toothpast being smeared on the cob. |
To
put it mildly "this year has been stressful to the corn crop".
You name the stress it probably occurred. The crop had to deal with cool
temperatures, wet conditions, less sunlight resulting in reduced photosynthesis,
cool evening temperatures, leaf diseases, corn borer, stalk rots to name
a few. These conditions may result in a significant increase in ear rots
or moulds this fall. Ear rot fungi are similar to stalk rot fungi: They
thrive under stressful conditions. To what degree, will be determined
by the environmental conditions from now until harvest since the fungi
that cause ear rots are weather sensitive. The three primary ear moulds
that occur in Ontario are: Gibberella Ear Rot, Fusarium Ear and Kernel
Rot and Diplodia Ear Rot. Their distribution and prevalence varies from
year to year but in most years they are present even though it may be
at low levels.
The most common and important ear mould in Ontario is Gibberella zeae
which is the sexual reproductive stage of Fusarium graminearium. This
fungus not only infects corn but also small grains such as wheat. Actually,
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. Although, the fungus can produce a white colour mould
which makes it difficult to tell apart from Fusarium Ear Rot, the two
can be distinguished easily when Gibberella produces it's characteristic
red or dark pink colour mould.
Infection begins through the silk channel and thus, infection in most
cases starts at the ear tip. In severe cases most of the ear may be covered
with mould growth. Corn silks are most susceptible 2 to 10 days after
initiation and this year, the cool and wet weather during this period
was ideal for infection.
Begin scouting for Gibberella Ear Rot in fields which have a susceptible
hybrid planted. If you are not sure how your hybrid rates for Gibberella
contact your seed supplier. You should also scout fields with a large
number of non-pollinated ear tips or have significant European corn borer
or bird damage since this ears are also prone to Gibberella infection.
Gibberella Ear Rot is economically important not only because of the potential
yield and quality losses but because Gibberella zeae and Fusarium graminearum
produce two very important mycotoxins that occur in Ontario, deoxynivalenol
(vomitoxin or DON) and zearalenone. These mycotoxins are especially important
to swine and other livestock producers since they can have a detrimental
affect on their animals. Feed containing low levels of vomitoxin (1ppm)
can result in poor weight gain and feed refusal in swine. Zealalenone
is an estrogen and cause reproductive problems such as infertility and
abortion in livestock, especially swine. If you have Gibberella ear rot
(5% or more) and are planning to feed the grain, you should have the grain
tested for these toxins.
Another common ear rot this year is Fusarium Ear Rot. Unlike Gibberella,
Fusarium infected kernels will be scattered around the cob amongst healthy
looking kernels or on kernels that have been damaged for example by corn
borer or bird feeding. Silks are susceptible to infection during the first
five days after initiation. Fusarium infection produces a white to pink
or salmon-coloured mould and if you examine infected kernels you will
observe a "white streaking" or "star-bursting" on
the kernel surface. Although many Fusarium species may be responsible
for these symptoms, the primary species we worry about in Ontaio is Fusarium
verticillioides (formerly Fusarium moniliforme). The significance of this
fungus is that it produces a toxin called fumonisin that has been shown
to cause cancer (carcinogen) in humans.
Of the three primary ear rots that occur in Ontario Diplodia ear rot caused
by Diplodia maydis is the usually the least common. Having said that we
are seeing more Diplodia ear rot this year since it favours cool, wet
conditions through grain fill but in no way to the same extend that surrounding
states such as Ohio are experiencing. According to Pat Lipps (Extension
Plant Pathologist - Ohio State University), some fields in Ohio have up
to 35% of the ears damaged. The characteristic ear symptom is a white
mould that begins at the base of the ear and will eventually cover and
rot the entire ear. Mould growth can also occur on the outer husk which
has small black bumps (pycnidia) embedded in the mould. These reproductive
structures are where new spores are produced. Unlike Gibberella and Fusarium,
Diplodia does not produce any known toxins.
If you have mold and will be feeding the grain, begin by examining 20
ears at five different locations (100 ears total). Record the number of
infected ears and if possible, which disease is present. To make it easier,
combine the mycotixin producing Gibberella and Fusarium into one group
and Diplodia, which does not produce any known mycotoxins into another.
When dealing with moulds and mycotoxins it is best to be conservative
and therefore, if you have any significant (5%) amount of mould have the
grain tested.
The conditions just prior to harvest and in storage are important when
discussing these diseases and toxins. Toxin production tends to increase
when these ear moulds are allowed to develop as a result of delayed harvest.
In most cases, proper storage of harvested grain will stop any further
mould growth and toxin production. One concern I have is that if we continue
to have cool, wet conditions over the next few weeks, dry down of the
corn is going to be slowed. This will be a favorable condition for the
mold that is already on the ears to continue to grow