Gibberella ear rot (Fusarium graminearum / Gibberella zeae)

Gibberella ear rot: biology

Gibberella ear rot is the most common ear mould in Ontario. The fungus overwinters on crop residue, and the inoculum that causes Gibberella ear rot is present every year.

As with any disease, the pathogen (Fusarium graminearum) needs a susceptible host (corn) and the right environmental conditions at the right time (silking) to initiate infection. Primary infection takes place through the silk channels when warm, wet or humid weather occurs at silking. Corn is most susceptible to infection when weather conditions are cool and wet two to 10 days after silk initiation.

If weather conditions at silking are conducive to infection, environmental conditions during grain fill determine the severity of infection. Warm temperatures coupled with high humidity, heavy dew or frequent rains and increased cloud cover are ideal for ear mould to thrive.

Secondary infection can occur through damage to cobs (ie. insect feeding, bird feeding, hail, mechanical damage) when a pathway is created for the pathogen to enter.

Gibberella ear rot in corn is characterized by pinkish-orange or white mould; infection begins at the cob tip and, in severe cases, moves down towards the base. Generally, reddish mycelium will colonize only part of the ear, but in severe cases, the ear husk and cob can fuse together.

Sometimes the fungus can appear as a white-coloured mould, which makes it difficult to distinguish from Fusarium ear rot. This pathogen can produce many toxins, including deoxynivalenol (vomitoxin or DON), zearalenone (ZEN) and T-2 toxin. If grain is to be used for feed, test mycotoxin levels.

Looking back at 2018, many areas in Ontario experienced a tough early season and drought stress prior to tassel. Any stress on plants left them more vulnerable to disease. Here are some other significant factors that contributed to high DON levels in many crops in 2018:

1. Several fields emerged unevenly, largely due to tough planting conditions. Uneven emergence can cause uneven silk emergence from plant to plant. Cobs from later emerged plants tend to have smaller ears. The smaller ears would have a tighter husk.
2. Drought stress at silking can slow silk emergence or cause uneven silk emergence, lengthening the time during which the cob is exposed to the Gibberella inoculum.
3. Anything that prevents cobs from filling to the tip will cause husks to close more tightly. Husks that closed more tightly were more prone to higher infection. This can be influenced by hybrid characteristics or factors such as drought stress, low fertility, or compaction.
4. Upright ears showed more visible ear mold than ears that hung down, which allowed moisture to escape the cob. Upright ears can be a hybrid characteristic.
5. If there was a second, smaller ear on a plant infection levels seemed to be much worse.