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Symptoms. Common smut in corn is caused by the fungus Ustilago maydis (also known as Ustilago zeae). Any actively growing portion of the plant may become infected.1 Galls on ears are the most dramatic in appearance; however, potential yield losses are greatest when young seedlings are infected. In this case, plants grow abnormally and may not produce ears or plants die, resulting in reduced stands. Smut galls are a combination of host and fungal tissues. Young smut galls are firm and covered with a periderm.3 They are greenish white or silvery white in appearance as they develop.2 As they mature, the fungal tissues begin to turn black with the development of teliospores (Figure 1). Upon maturity the periderm ruptures and releases the powdery black teliospores. Galls that form on leaves usually remain small and dry out and turn hard without rupturing.
Life Cycle. Spores overwinter in the soil and can remain viable for several years. They are spread by wind and rain. With warm, moist conditions, infection can occur on corn leaves, stalks, tassels, silks, or ears; however, each infection is a separate event. The fungus does not grow systemically in corn plants. Infection of seedlings and plants growing vegetatively is often associated with wind and blowing soil, which creates wounds exposing meristematic tissues to the fungus. Infection of kernels on ears can occur whenever conditions result in poor pollination.1 Conditions include drought or rainy weather that results in poor pollen production and inhibits normal pollen release. Silks that are not pollinated remain susceptible to infection for an extended period of time. Poor pollination, caused by a drought that is followed by warm, rainy conditions that spread smut spores, can significantly increase kernel infection. Additionally, damage from insects, wind, cultivation, herbicide, animals, or hail can create an entry point for the fungus and increases the likelihood of infection of various plant parts.
Symptoms. Head smut in corn is caused by the fungus Sphacelotheca reiliana. The fungus infects corn plants during the early vegetative stages and grows systemically in the plant. Symptoms are not evident until plants reach reproductive growth stages. Infected ears and tassels are replaced by smut sori (spore masses) (Figure 2). Smut sori are covered by a thin membrane, which easily ruptures to reveal masses of dark brown to black spores called teliospores.4 Smutted ears may be rounded and do not produce silks. Infected tassels are completely or partially covered by sori and normally do not produce pollen. Thread-like strands of vascular bundles surrounded by black spores are characteristic of head smut galls. These strands are remnants of the vascular tissue of the corn plant. Individual spikelets of the tassel may be infected, forming a shoot like structure. Infected plants may also produce strange leafy structures on either the ear or tassel.
Life Cycle. Although teliospores can be transmitted on the surface of the seed, the primary source of inoculum is teliospores in the soil. Spores may be transported into a field by contaminated harvesting, planting, or cultivation equipment. They can remain dormant in the soil for at least four years. The disease is favored by low soil moisture and temperatures between 70 and 82 0F. Head smut is more common in clay loam soils and in soils with nitrogen deficiency.
Although it can be difficult to determine the difference between galls of common smut and head smut once they rupture, there are several important differences between the diseases:
Common Smut. Fungicides do not effectively control common smut.1 Crop rotation is not a feasible option because the fungus is widespread and remains viable in the soil for several years. If possible, grow corn in fields with no history of common smut. In fields with high levels of common smut, deep tillage can bury the fungus, which might reduce the level of inoculum available for the following year. During cultivation, avoid injury to roots, stalks, and leaves. Plant products that are less susceptible to common smut. Excess nitrogen tends to increase the incidence and severity of the disease, so maintain balanced fertilization.
Head Smut. Plant corn products with resistance to head smut. Products with rapid seedling emergence may avoid infection. Because infection occurs in the seedling stage, treating seed with a systemic fungicide can reduce infection. In-furrow fungicide treatments can be effective, but may not be economically feasible in areas with only sporadic disease incidence. As teliospores can survive for several years in the soil, crop rotations are not effective in reducing the disease. Where feasible, remove and burn smutted ears before the dispersal of spores. Head smut has been reported to be more serious when there is a lack of nitrogen, so maintain a balanced fertility program.