The 2000 Corn Crop
Stalk Rots and Ear Moulds Enjoyed 2000!

By Albert Tenuta, Extension Plant Pathologist (Field Crops), OMAFRA, Ridgetown

his year will go into the books as one of the most stressful years that an Ontario corn crop has experienced. In many cases, corn plants had to endure not one stress but multiple stresses within the same field. As a result, many producers had to deal with corn plants that lodged, died from the top down or suffered premature senescence. The culprit in many of these incidences was corn stalk rots. The fungi that cause corn stalk rots are good at causing disease. The amount of damage that is created increases when the crop is under stress. In 2000, stresses that contributed to an increase in stalk rot infection included: wet conditions; cool evening and daytime temperatures; cloud cover (in June and July); leaf diseases such as rust and Stewart’s wilt; leaf and ear damage from hail, birds and frost; incomplete pollination; unbalanced fertility; and insect damage from corn borer. All of these factors can increase a corn hybrid’s susceptibility to stalk rots. Unfortunately, stalk rot fungi are not the only disease organisms that prefer these conditions. The fungi that cause ear rots thrive under these same stress conditions. To what degree these ear rots and their corresponding mycotoxins develop depends on the environmental conditions during harvest and in storage.
The distribution and prevalence of stalk and ear rot diseases vary from year to year. There are differences between the fungi that cause these diseases. The following guide will help you to understand how they develop and how to tell them apart.

Stalk Rots
Anthracnose stalk rot
Anthracnose stalk rot is caused by the fungus Colletotrichum graminicola. This fungus survives in the previous corn crop residues and therefore, is most often a problem in second-year corn. The warm, wet and humid weather this year favoured anthracnose development.

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. 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 that is associated with this fungus is “top-dieback”. 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. Last year, many corn plants began to wilt and die from the top down in early September. This year, many plants showed this same dieback which is associated with late season stress.

Gibberella, Fusarium and Diplodia Stalk Rots
All three of these fungi cause general stalk rot and ear symptoms. General stalk rot symptoms include wilting and death of the plants. Affected leaves turn a gray-green colour which resembles frost damage. All three cause a dark external lesion or spots at the lower nodes. Diplodia produces small black spots (pycnidia) that are embedded in the stalk rind. These spots are hard to remove. This is in contrast to Gibberella, which also produces small, round, black spots at the lower node, but these spots can be easily scraped from the stalk surface. The pith is shredded and has a pink to red colour. Fusarium stalk rot symptoms appear as light brown-black lesions near the nodes. Internal stalk symptoms of Fusarium include a salmon-pink fungal growth in the pith.

Pythium stalk rot
Pythium stalk rot gives the same general aboveground symptoms that are associated with the other stalk rot organisms. Pythium is in a unique group of fungi (includes Phytophthora) that are called ‘Oomycetes’ or ‘water moulds’ because of their preference for wet conditions. The unique characteristic feature of this group of fungi is that they produce mobile spores that can migrate or move through the water film in saturated soils. These spores are able to physically move to the corn plants’ roots and, once inside, cause disease. Corn plants infected with Pythium will not have any visible signs of fungal growth atthe base of the plant, which is different than the other stalk rots that produce overwintering structures (black dots) or mould. You must cut the corn plant lengthwise through the stalk base and roots. Pythium-infected tissue will appear wet, soggy and disintegrated (‘a wet rot’) at the root base.

Ear Rots
Gibberella zeae (the sexual reproductive stage of Fusarium graminearium) is the most common and important ear mould in Ontario. 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 differentiate from Fusarium Ear Rot, the two can be distinguished easily when Gibberella produces its 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.

Gibberella Ear Rot is economically important not only because of the potential yield and quality losses, but also 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 effect on their animals. Feed containing low levels of vomitoxin (1ppm) can result in poor weight gain and feed refusal in swine. Zearalenone is an estrogen and can cause reproductive problems such as infertility and abortion in livestock, especially swine. If you plan to feed grain that originated in a field with 5% or more Gibberella ear rot, have a sample tested for these toxins.

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 Ontario 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 in humans.

Diplodia ear rot is the least common of the three primary ear rots that occur in Ontario. Having said that, we are seeing more Diplodia ear rot (caused by Diplodia maydis) this year since it favours cool, wet conditions through grain fill. 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.

1