As corn producers plan for harvest, they should be accessing their fields for stalk rot and ear moulds, reminds the OMAFRA Field Crop team. The distribution and prevalence of these diseases vary from year to year but they are present every year, especially when the crop is under stress (water stress, insect feeding, etc.). It goes without saying that this year was very stressful and in many cases, corn plants had to endure not one stress but multiple stresses and we are observing ear mould infections (Gibberella and Fusarium primarily) in many fields across the province. They range from light to significant levels therefore, in order to manage and minimize the effects of these ear rot diseases, it is critical to assess fields before harvest. Growers should assess fields each year, because these pre-harvest assessments can alert them to potential problems and provide time for livestock producers for example, to segregate, obtain alternative grain, or hold onto stored corn from the previous year.
Scouting practices are similar for all corn ear rots. Begin scouting fields at late dent stage to determine their presence and severity. When scouting, randomly select plants and pull back the husk to examine the entire ear. A quick method is to select 100 plants across the field (20 ears each from five different areas). For each ear, be sure to peel back the husks and examine the entire ear. Fields with 10 per cent of ears having significant mould growth should be harvested sooner than later.
Leaving diseased grain in the field allows the ear rot fungi to keep growing, which will increase the risk of mouldy grain and mycotoxin contamination. Most ear rot fungi continue to grow (and, if applicable, produce mycotoxins) until the grain has less than 15 per cent moisture. In severely infected fields, it may be worthwhile to harvest grain at higher moisture and then dry it to less than 15 per cent to minimize further mycotoxin accumulation.
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.
When ear rot is present, the following 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. Suspect samples should be tested for toxins.
Gibberella ear rot: 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, and can survive on soybean roots. In most cases, Gibberella begins at the ear tip and works its way down the ear. Also, the husks from infected ears are often tightly adhered to the ear. Although the fungus can produce a white-coloured mould, which makes it difficult to tell apart from Fusarium kernel rot, the two can be distinguished easily when Gibberella produces its characteristic red or pink colour mould. Infection begins through the silk channel and thus, in most cases 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 two to 10 days after initiation. Warm and wet weather during this period is ideal for infection.
Fusarium kernel rot: Unlike Gibberella, Fusarium infected kernels are often scattered around the cob amongst healthy looking kernels or on kernels that have been damaged by corn borer or bird feeding for example. Fusarium infection produces a white to pink or salmon-coloured mould. A "white streaking" or "star-bursting" can be seen on the infected kernel surface. Although many Fusarium species may be responsible for these symptoms, the primary species we are concerned about in Ontario is Fusarium verticillioides (formerly Fusarium moniliforme). 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, two to three weeks after silking.
Diplodia ear rot: The characteristic ear symptom of Stenocarpella maydis and S. macrospora infection 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. Pycnidia (the small, black, spore-producing structures of the fungus) overwinter on corn residue and are the source of infection for the subsequent corn crop. Dry weather before silking, immediately followed by wet conditions, favour Diplodia infection.
Penicillium ear rot: Penicillium rot (Penicillium oxalicum) produces a light blue-green powdery mould which grows between the kernels and cob/husk surface. Infected kernels could become bleached or streaked. Penicillium ear rot can be a serious problem if corn is stored at high moisture levels (greater than 18 per cent).
Table 1. Common ear rots and moulds that occur in Ontario and the primary mycotoxins they produce.
||Signs and symptoms|
|Aspergillus||Aflatoxin||Hot, dry||Olive green spores on ear|
Moderate to warm temperatures during silking,
wet periods prior to harvest
|White to purple mold, scattered across ear;
Starburst pattern in kernels
|Gibberella||Deoxynivalenol (vomitoxin) and zearalenone||Cool, wet weather||Pink to white mycelial growth|
|Diplodia||None currently known in U.S. and Canada||Moderate temperatures and wet during silking||White mycelial growth on ear and husk; black pycnidia in cob|
|Penicillium||Ochratoxin (only some species)||Wet, humid conditions post grain-fill||Blue-gray fungal spores|
|Nigrospora||None||Damaged corn||Black spores, grey mycelia, shredding cob|
|Cladosporium||None||Wet weather near harvest||Dark-green to black kernels|
|Trichoderma||T-2 (only some species)||Damaged corn||Blue-green spores growing in and on kernels; may cause sprouting|
More information can be found in OMAFRA’s Field Crop Agronomy Guide – Publication 811, or Crop Protection Network Publication “Corn Management Disease Series – Ear Rots” at CropProtectionNetwork.org.