Ustilago maydis

Scientific Name Ustilago maydis (DC.) Corda
Synonyms Lycoperdon zeae Beckm., Hannov. Mag., Uredo maydis (DC.), Uredo segetum f. zeae-maydis DC., Uredo segetum var. mays-zeae DC., Uredo zeae Schwein. (DC.), Uredo zeae-mays DC., Ustilago carbo-maydis Phillipar., Ustilago zeae (Schwein.) Unger, Ustilago zeae-mays (DC.) Magnus
Common Names English: Common smut, blister smut, boil smut, gall of maize, maize smut; Spanish: carbón del maíz; German: Maisbeulenbrand, Maisbrand; French: charbon du maïs
Description Ustilago maydis is a basidiomycetous fungus, order Ustilaginales (smut fungi). The teliospores (also known as chlamydospores) are globose or subglobose to ellipsoidal, prominently and bluntly echinulate, 8 to 11 µm in diameter.

Biology

Damage

All above-ground parts of the plant are susceptible, especially young, actively growing tissue or meristems. Smut galls may form on leaves, stalks, ears, or tassels.

Young galls are covered by a glistening, greenish-white to silvery-white membrane of fungus and host tissue which soon ruptures. Initially firm and light in color the galls darken while the interior turns into masses of powdery, dark olive-brown to dark brown spores. When the membrane ruptures, dry, brown powdery spores are released. Galls on leaves remain small (6 to 12 mm) and greenish-white and senesce without rupturing.

Mature galls may reach a diameter of 150 mm. Early infection affecting the plant's growing point may kill young plants. Severe infections may cause stunted plants. Corn with infections of the lower stalks may be infertile or produce small ears.

Lifecycle

Basidiospores are disseminated by wind and water droplets and infect plants. They germinate and produce a fine hypha that may enter epidermal cells directly. Fungal development is limited to initial growth unless two haploid hyhae from basidiospores of different mating types (a and b) fuse to form a dikaryotic hypha. This infection hypha enlarges in diameter and grows intercellularly into the plant tissue.
Corn cells surrounding the infection site are stimulated to enlarge and to proliferate, and gall formation may precede hyphal growth - U. maydis has been shown to produce the plant growth regulator indole acetic acid. In galls the mycelial growth is largely intercellular before sporulation. Then, the enlarged corn cells are colonized and used for further fungal development. In later stages the galls consist of dikaryotic fungal mycelium which is transformed into teliospores. After rupture of the membrane these spores may infect again meristematic corn tissue in the same season, but most spores survive in plant debris or in the soil for several years. In inoculation tests on corn seedlings, symptoms appeared three days after inoculation. After 10 days, host tissue became destroyed and smut spores were formed.

Cause

Soil-borne teliospores are the inoculum source and may remain viable in the soil for at least one year. They germinate by forming a septated promycelium from which four hyaline, fusiform haploid basidiospores are produced. The optimum temperature for germination is in the range 20 to 25 °C.

Occurrence

Additional Crop Information

The development of smut from U. maydis is restricted to corn

Agricultural Importance

Corn losses vary greatly and are usually < 1 % when resistant plants have been grown, but they may be considerably higher in sweet corn.
In some parts of Mexico, fruiting bodies of U. maydis are a seasonal delicacy known as 'huitlacoche'. The influence of environmental conditions on the incidence of common smut is not clear. Some reports indicate that dry weather and temperatures >25 °C favor the disease, other studies report on an association of rainy, humid weather with infection.
Being a biotroph plant pathogen which can be temporarily grown on artificial media, U. maydis has become an important model system in plant pathology, e.g. in basic studies on pesticide resistance and signalling pathways in pathogenicity.

Control

Integrated Crop Management

Although crop rotation, sanitation, seed treatment, application of foliar fungicides, fertilization and biological control has been investigated, the most effective and practical control method is the use of resistant plant material.

Corn lines vary in susceptibility to U. maydis. Resistance to smut seems to be associated with functional, physiological and morphological characters of corn, and is likely to be inherited polygenically with each gene contributing a small amount to overall plant resistance. Sweet corn seems to be highly susceptible. Weed control with several herbicides seems to increase disease incidence by facilitating the spread of inoculum to the leaves. High plant densities, heavy fertiliser use, irrigation and mechanical injury are also likely to favor smut incidence. There is evidence that pollination protects silks from smut development. Bacterial antagonists against U. maydis have been tested as soon as in the 1930s, but are not pursued as control agents. The use of a mycovirus producing a toxin killing susceptible U. maydis cells is under investigation.

Chemical Control

Generally, specific control measures of common smut with fungicides are hardly (cost-) effective in most regions. Because only few spores are found on the seed, seed treatment with fungicides such as carboxin or thiram is of value only for hygiene reason in order to limit further spread of the disease.
Overall, fungicidal control measures should be embedded in an integrated strategy combining several control strategies such as crop rotation or the use of resistant cultivars.

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