Ustilago maydis

Ustilago maydis (DeCandolle) Corda is a biotrophic pathogen of corn (Zea mays). It is called “common smut” of corn because it is quite common throughout North America, though it is more commonly found in dry conditions. Other Ustilago spp. cause smut on other plants, but U. maydis is the only species that causes smut on corn, and this host specificity makes it easy to identify the fungus to the species level. The fungus has an interesting life cycle containing dikaryotic and haploid stages. The white “blisters” often seen on a developing corn cob (Fig. 1) are actually tumor-like galls produced by the fungus, containing many black teliospores.

Figure 1. Corn field (left) harboring many cobs infected with Ustilago maydis (right).

 

As the gall matures, the fleshy surface begins to break down, releasing the teliospores that can be spread through wind, contact with humans and animals, or surfaces of field equipment. The teliospores are circular,  dark-brown to black, and echinulate, or “decorated with many spines” (Fig. 2). These teliospores are used for overwintering. They are diploid, but germinate and develop a short hyphal tip that can produce many haploid basidiospores. These haploid basidiospores can fuse to form a dikaryon, which then proceeds to form an appressorium that can infect living corn tissue. It is seemingly most effective at invading rapidly growing corn tissues, such as fertilized corn cob silks and ovaries, resulting in smut commonly found on developing kernels. Though culinary appeal of the fungus may be debatable, the fungus is edible and is considered to be a delicacy to some Latino cultures, known by the name “huitlacoche”. The genome for Ustilago maydis was sequenced in 2006 and is a model organism for studying biotrophic plant-fungal pathogenic interactions.

Figure 2. Round, dark brown-black, echinulate teliospores from erupting U. maydis galls.

Phylum: Basidiomycota
Class: Ustilaginomycetes
Order: Ustilaginales
Family: Ustilaginaceae
Genus: Ustilago
Species: maydis
Collection Number: PLP847_2018_99

References:
Kämper, J. et al. 2006. Insights from the genome of the biotrophic fungal plant pathogen Ustilago maydis. Nature. 444:97-101. doi: https://doi.org/10.1038/nature05248
Pataky, J. K., and K. M. Snetselaar. 2006. Common smut of corn. The Plant Health Instructor. DOI:10.1094/PHI-I-2006-0927-01
Brefort, T. et al. 2009. Ustilago maydis as a Pathogen. Annu Rev Phytopathol. 47:423-445. doi: 10.1146/annurev-phyto-080508-081923

09.17.18

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