Ustomycetes
Order: Ustilaginales (Smuts)
This class is composed of several orders, but we
will only consider the order Ustilaginales. Species in this order are composed entirely of
fungi that are parasitic on flowering plants. However, unlike the rusts, the smuts are not
obligate parasites throughout its entire life cycle. Only the dikaryon stage is obligately
parasitic. The monokaryon stage, which is yeast, is saprobic. Thus, they are dimorphic.
There are not many species of smuts that occur in Hawaii. Two species that are relatively
common are: Ustilago cynadontis, Bermuda Grass Smut (Fig. 1) and U. maydis,
Corn Smut (Fig. 2).
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Figure 1: Ustilago cynadontis, Bermuda Grass Smut. The black powdery spores are the teliospores It is this dirty, powdery stage that led to this group of fungi being called smut. |
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Figure 2: Ustilago maydis, the Corn Smut. This species can be found where corn is grown in Hawaii. |
The Ustilaginales is similar to the rusts in two significant characteristics: both lack a basidiocarp, during sexual reproduction and produce their basidia from germinating teliospores. However, the smuts do not have sex organs nor do they produce multiple spore stages as is the case of the rusts.
The life cycle is simple and more similar to the Basidiomycetes. We will describe the life cycle of Ustilago maydis, the corn smut, as an explane of a smut life cycle. There are, it should be noted that there are several variations of life cycle.
Following dispersal of the basidiospores, the spores typically germinate by budding to produce yeast cells, which are saprobic. When the yeast cells contact a compatible mating strains, dikaryon formation will occur. Once the dikaryon has formed, it becomes an obligate parasite and must have a host. Once in the host, the mycelial cells become rounded and break apart to give rise to the thick-walled teliospore stage.