Vomitoxin, also known as deoxynivalenol (DON), is a toxic substance produced by certain fungi that commonly affects grains, including corn. Understanding the factors that lead to vomitoxin contamination in corn is important for maintaining agricultural safety and ensuring grain quality for both human and animal consumption. This mycotoxin can cause issues such as reduced feed intake and weight gain in livestock, and at higher levels, it may induce vomiting.
The Fungal Origin
The primary fungal species responsible for producing vomitoxin in corn is Fusarium graminearum. This fungus is also known by its sexual stage name, Gibberella zeae. Fusarium graminearum is a common plant pathogen that causes Gibberella ear rot and stalk rot in corn, as well as head blight in wheat. It is a mold frequently encountered in agricultural environments, capable of surviving in crop residue from previous seasons. This fungus thrives in a wide range of conditions, making it a prevalent contaminant of corn crops globally. Its ability to persist in plant debris ensures a continuous source of inoculum for subsequent crops. The fungus itself can appear as a pinkish or reddish mold on infected kernels or plant tissues.
Environmental Conditions for Toxin Production
Several environmental factors promote the growth of Fusarium graminearum and the subsequent production of vomitoxin in corn. High moisture levels are particularly conducive to fungal development and toxin synthesis. Prolonged periods of wetness, such as heavy rain or high humidity during critical growth stages like silking and early grain fill, significantly increase the risk of infection. The timing of rainfall is often more influential than the total amount of precipitation.
Temperature also plays a significant role, with optimal conditions for Fusarium graminearum growth typically ranging around 25°C. However, perithecia, the structures that produce spores, can form and mature across a broader temperature range, often between 20°C and 25°C, when humidity is high.
Plant stress can predispose corn plants to fungal infection and toxin production. Drought stress, for example, can weaken plants and make them more susceptible. Insect damage is another important factor, as pests like corn earworms or European corn borers create wounds on the plant, providing direct entry points for the fungus. Physical damage from hail can also create openings for fungal entry.
The presence of infected crop residue from previous growing seasons serves as a major source of inoculum for Fusarium graminearum. The fungus overwinters in this residue, and its spores are then dispersed to new corn plants. Fields with corn-on-corn rotations or reduced tillage practices, which leave more residue on the surface, tend to have a higher risk of infection.
How Corn Becomes Infected
The infection process begins with the dispersal of fungal spores from infected crop residue or other plants. These spores, primarily ascospores, are spread by wind and splashing rain, traveling from their overwintering sites to corn plants. The presence of moisture is essential for spore production and dispersal.
Spores gain entry into the corn plant through various points. The most common entry route is through the silks during pollination. The moist, nutrient-rich environment of the silks provides an ideal pathway for fungal penetration and colonization of the developing kernels. Wounds created by insects, such as corn earworm feeding, or physical damage from hail, also serve as effective entry points for the fungus directly into kernels or other plant tissues.
Once inside the corn plant, the fungus begins to colonize the kernels or plant tissue. This colonization involves the growth of fungal mycelia within the plant. The fungus can grow and spread within the kernels, sometimes without immediately causing visible symptoms.
Vomitoxin is produced by Fusarium graminearum as a secondary metabolite during its growth and colonization of the corn plant. This toxin is a byproduct of the fungal metabolism. The production of vomitoxin continues as the fungus develops within the infected corn.