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Symptoms and Signs

Ustilago maydis infects two hosts: maize (Zea mays) and teosinte (Zea mexicana). Like other smut diseases, common smut of corn derives its name from the sooty masses of teliospores found on infected host plants (Figure 2). The most obvious symptoms are tumor-like galls that vary in size from less than 1 cm to more than 30 cm (0.4-12 inch) in diameter. All meristematic tissues are susceptible to infection. Galls are found most frequently on ears, tassels, stalks, nodal shoots, and mid-ribs of leaves (Figures 3-6). Even though galls may form on many above-ground parts of the plant, infection is local (i.e., the host is not colonized systemically). Occasionally, galls develop beneath the soil surface when the apical meristem of a young plant is infected.

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Smut galls consist of fungal and host tissues. Young galls are white, firm and covered with a semiglossy periderm. As galls begin to mature, interior tissue becomes semifleshy and streaks of black tissues occur as teliospores begin to form. With further maturation, galls become a mass of powdery teliospores and the periderm ruptures releasing the spores (Figures 2 and 3). Galls usually are obvious within 10 to 14 days after infection. On ears, galls mature about three weeks after ovaries are infected. Slightly discolored or disfigured kernels are obvious 3 to 6 days after infection (Figure 7). Small, white, firm galls develop 9 to 10 days after infection, and galls begin to enlarge resulting in a tumor-like growth 11-12 days after infection (Figure 8). Galls start to have a gray, silvery appearance as streaks of blackened tissues (teliospores) begin to form 14 to 15 days after infection (Figures 9a,b,c). About 70% of the gall tissue is blackened while galls retain a semi-fleshy, mushroom-like integrity 16 to 18 days after infection (Figures 10 a,b). By 21 to 23 days after infection, the periderm ruptures and galls become a sloppy, wet, mass of teliospores. With dehydration, galls become sooty, powdery masses of teliospores.

Figure 7

Figure 8

Figure 9a

Figure 9b

Figure 9c

Figure 10a

Figure 10b

The number, size and location of galls depend on the age of plants at the time of infection. Leaf galls differ greatly in size but usually are small when compared to stalk and ear galls. Galls 20-30 cm (8-12 inches) in diameter are common on the stalk. Rudimentary ear shoots below the fertile ear are commonly infected. Galls can replace most of the tassel or individual florets depending on the time and severity of infection (Figure 5). Ear galls usually result from infection of individual ovaries. Commonly, a few kernels at the basal or tip ends of the ear are infected (Figure 11), although nearly every kernel on an ear may be replaced by smut galls if most ovaries are infected.

Symptoms of head smut of corn, caused by Sphacelotheca reiliana, are somewhat similar to common smut (Figure 12). Head smut occurs when maize plants are infected systemically by S. reiliana. Floral structures (tassels and ears) are converted to sori containing masses of powdery teliospores that resemble mature galls of common smut. Vascular bundles of the host usually are present as thread-like structures in sori of S. reiliana.

Figure 11

Figure 12

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