Stress - Ear Moulds!
|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.|
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