Corn Smut: An Update

Corn ear diseases are becoming more apparent as we approach the later stages of corn grain fill across Pennsylvania. Corn smut is caused by the fungal pathogen Ustilago maydis and stands out due to its distinct symptomology and the appearance of galls filled with spores in kernels, leaves, stalks, and tassels (Figure 1).

Corn Smut: Risk and Importance

The incidence of corn smut varies from year to year. It is driven by environmental conditions and is not considered an economically important disease in grain corn production; however, it can be an economic challenge in sweet corn, reducing its desirability for the fresh market, freezing, and canning. Smutty grain is not contaminated with known mycotoxins; therefore, it is not considered harmful to humans and animals when consumed. In fact, in Mexico, immature galls are readily consumed by humans and are considered a delicacy known as 'huitlacoche.'

There is limited information on the impact of corn smut on corn silage quality. However, Cole et al.¹ found that while corn smut did not impact the palatability of corn silage fed to sheep, it slightly decreased digestibility. Their research indicated that protein and mineral composition did not differ between infected and non-infected plants, and silage made with infected and non-infected plants.¹ The authors also indicated that smut-infested silage has similar characteristics to those observed in silage made with corn impacted by drought, which tends to have a reduced grain content.¹ The study also found that TDN (total digestible nutrients) decreases by 0.5% with every 10% increase in corn smut-infected plants.

Corn Smut Biology and Disease Cycle

Fungal infection occurs through silks and into the kernel early in development. The fungal mycelium causes the kernel to enlarge and completely colonizes the kernel tissue. Thousands of black to brown spores, known as teliospores, develop within the gall. Once teliospores are mature, the gall bursts, and spores are exposed to the environment and dispersed by wind or rain (Figure 2). The teliospores can infect nearby corn leaves and stalk tissue during the growing season. Furthermore, teliospores overwinter in the soil and crop residue and are the inoculum source for the following season.²

Management

Because corn smut is rarely economically significant in grain cropping situations, direct management actions are often unnecessary. No chemical control options are recommended for corn smut management; however, resistant sweet corn varieties have been developed and may be deployed.

In addition to genotype, any factor affecting corn pollination can affect corn smut disease development. It has been demonstrated that adequate pollination limits pathogen progression and reduces disease incidence, severity, and gall number, among other variables.³ In addition to poor pollination, a high incidence of corn smut could also indicate that young ear tissues were exposed to mechanical damage from wildlife, hail, or severe insect pest pressure. If this is the case, other ear rot diseases may also be present, some of which may be mycotoxigenic, causing harm to humans and animals. If this is the case, it is essential to scout these fields to identify the different corn ear issues correctly.