G2223

Smut Diseases of Corn

This NebGuide describes the two smut diseases of corn in Nebraska, common smut and head smut, including their symptoms, life cycle, and management.

Tamra A. Jackson-Ziems, Extension Plant Pathologist

Common Smut

Symptoms

Figure 1. Two smut diseases occur in corn in Nebraska, common smut (upper left ear) and head smut (bottom ear).

Epidemiology
Management
Huitlacoche

Head Smut

Two smut diseases of corn occur in Nebraska (Figure 1). Despite their similar appearance, their disease cycles are very different, as are their yield loss potential and management strategies (Table 1).

Common Smut

The more common of the two diseases is common smut, also known as boil or blister smut because of the galls it produces. It is caused by the fungus Ustilago maydis.

Symptoms

The development of thick, fleshy galls that contain spores is a diagnostic sign of the disease. Galls can develop on any plant part and are especially common on the ears. To some people the galls look like mushrooms (Figure 2). Galls that develop on leaves may appear different than on other plant parts, and often are much smaller and less noticeable than elsewhere on the plant (Figures 3a and 3b). Galls may develop at any point in the season on actively growing plant parts. Initial infections may cause chlorosis of affected parts, as well as rippling on leaves where galls will eventually form. Galls developing on the stalks (Figure 4) are potentially the largest and galls on the roots (Figure 5) are least common.

Figure 2. Common smut galls may look like mushrooms or tumor-like growths.
		Figures 3a and 3b. Common smut galls on leaves are often small and look like blisters.


Figure 4. Galls developing on stalks are often larger than those on other plants parts.		Figure 5. Galls also may develop underground on the developing roots (top center of photo).

Epidemiology

The fungus survives for several years in the soil as dark brown to black, thick-walled spiny spores (teliospores). Germinating teliospores produce basidiospores that must fuse with other basidiospores of the opposite mating type to create infection hyphae (growing strands of the fungus). The infection hyphae infect corn plant cells that are growing and still expanding. Recently infected plant tissue may become yellow and begins enlarging almost immediately to eventually produce the characteristic tumor-like galls that are made up of both fungal and plant tissue. Although galls develop most commonly on the ears, they may also develop on the tassels (Figure 6), stalks, leaves, and less frequently on the roots. As the galls enlarge and mature, fungal strands grow throughout the galls, causing development of dark streaks (Figure 7) until the cells finally mature into large quantities of teliospores. The fleshy, soft gall eventually thins and dries, bursting open to reveal dusty black spores inside. The pigment and thick walls of the teliospores enable them to survive harsh conditions and overwinter in the soil.


Figure 6. Common smut galls may develop on tassels.		Figure 7. Dark streaks may appear inside galls as fungal strands develop into spores.

Spores may be spread by wind or splashing rain/irrigation water onto plants where they may infect through wounds or silks. Common smut can be found in practically every corn field in the Midwest to some extent, although widespread distribution of the disease and yield loss is not common. Severe weather events that cause plant wounds during the early or middle part of the season while plants are still developing can lead to widespread common smut infection and potential yield losses. In addition, the failure of kernels to become pollinated during hot and/or dry conditions that cause crop stress can lead to increased incidence of common smut galls on ears. Although, like other fungi, the pathogen needs moisture for spore dispersal and germination.

Management

This disease is usually not considered economically important; however, some fields may sustain substantial loss with widespread disease after severe weather events. Unfortunately, there are no management recommendations for common smut in field corn. Field corn hybrids are somewhat less susceptible to the disease compared to sweet corn hybrids. Sweet corn that is grown for the fresh market or canning and becomes diseased with common smut may become unmarketable. If common smut is a consistent problem in sweet corn, switching to a hybrid with more resistance can reduce disease incidence.

Huitlacoche

Since the fungus that causes common smut is closely related to other fungi that produce mushrooms, the smut galls themselves have some of the same characteristics that make them desirable for eating. In Mexico, common smut galls are commonly eaten as a delicacy and referred to as huitlacoche (or sometimes cuitlacoche or Mexican and/or maize truffles). Smutted ears are sold for more than 30 times the value of healthy ears in some Mexican produce markets. In the United States, restaurants now offer some selections on their menus and canned cuitlacoche is available in some popular grocery stores.

Head Smut

Head smut of corn is caused by the fungus Sphacelotheca reiliana (syn. Sporisorium holci-sorghi) and occurs in some fields in Nebraska. Yield loss due to the disease is variable and directly dependent upon disease incidence because infected plants usually don’t produce viable kernels. In addition, the disease can indirectly impact grain value because it is subject to export restrictions to some countries.