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Common Smut of Corn Disease Management

Management

Many methods of controlling common smut have been recommended or evaluated, including crop rotation, sanitation, seed treatments, application of foliar fungicides, modification of fertility, and biological controls. In spite of these frequently mentioned control tactics, host resistance is the only practical method of managing common smut in areas where U. maydis is prevalent. Nevertheless, no corn line is immune from infection by U. maydis.

Resistance

The first significant advance in the control of common smut occurred when the first generation of corn hybrids were developed from inbred lines that where selected for lower incidence of smut. Although resistance to common smut has been studied for nearly 100 years, specific mechanisms of resistance and genes that condition resistance have not been documented clearly.

Early corn breeders observed that varieties of flint corn tended to be more smut-susceptible than varieties of dent corn. In 1918, just a decade after George Shull and Edward East independently discovered heterosis (hybrid vigor) in corn, Donald Jones demonstrated that inbred lines of corn developed by him and East at the Connecticut Agriculture Experiment Station differed strikingly in the number of smut infected plants. During the next ten years, early corn breeders such as H. K. Hayes, R. J. Garber and F. R. Immer reported that self-fertilization of open-pollinated maize varieties rapidly led to the isolation of inbred lines with noticeably different reactions to common smut. Not only did inbred lines differ in the incidence of infected plants, susceptible lines could be differentiated by the plant parts on which galls formed. For example, some inbreds were uniformly infected on lower nodes of the stalk while others had a high incidence of ear or tassel galls. The rapidity with which selfed lines became homogeneous for smut reactions led Immer and J.J. Christensen to speculate that “a relatively small number of genetic factors are involved in determining smut reactions,” and that “composite crosses [of smut-resistant lines] will undoubtedly yield smut-resistant lines in a large percentage of cases.” Fortunately, the latter of these two hypotheses proved to be correct.

In 1928, Marion Griffiths, a USDA plant pathologist, demonstrated that selfed lines of corn and crosses between them that were resistant to natural infection by U. maydis in the field were very susceptible when inoculated. None of the resistant lines were immune from infection when Griffiths used a needle to inject sporidial suspensions of U. maydis into young plant tissues. She hypothesized that “resistance or susceptibility is largely a matter of relative accessibility of the susceptible [plant] parts to the invading organism.” Other researchers soon began to corroborate Griffith’s observations by demonstrating that protection of susceptible host tissues by non-susceptible tissues played an important role in smut resistance. After observing a strong association between smut susceptibility and the open or closed condition of apical buds (i.e., tightness of leaf whorls), A. H. Eddins concluded that “open buds hold the inoculum and allow [spore] suspensions to seep down the plant and come in contact with young, actively growing and susceptible tissues.”

J.M. Walter and others observed that injury to leaf and stalk tissues or injury resulting from detasseling plants increased the incidence of common smut although the smut galls did not occur on the injured tissues themselves, but instead, occurred on meristematic tissues in close proximity to the injured tissues. After evaluating 200 inbred lines grown under smut-epidemic conditions for 10 years and crosses between those lines and genetic linkage testers, M. M. Hoover concluded that smut reactions probably were conditioned by two sets of genetic factors: one controlling physiological behavior and one concerned with gross morphology of the plant.

By the early 1940s when double-cross hybrids were beginning to be grown on a significant amount of the corn acreage in the U.S., corn breeders had substantially improved smut resistance simply by selecting against smut susceptibility in breeding populations and among inbred lines. When G. H. Stringfield and D. H. Bowman compared the incidence of smut on 340 open-pollinated varieties and 1,052 corn hybrids in 48 experiments throughout Ohio from 1933 to 1938, they observed that 75% of the hybrids had less smut than the open-pollinated varieties (Figure 24). Hybrids derived from one of twelve inbreds lines that Stringfield considered to be smut resistant had less than a third as much smut as the open-pollinated cultivars. This type of progress, resulting from selection against smut susceptibility without knowledge of the specific resistance genes involved or the mechanisms of resistance, has continued through today. Most modern field corn hybrids grown in the U.S. have adequate levels of resistance to natural infection to avoid catastrophic smut epidemics. Nevertheless, no hybrid is known to be immune to infection when injected with sporidial suspensions, and major genes responsible for resistance have not been identified definitively in spite of recent evaluations of mapping populations for quantitative trait loci for smut resistance.

Figure 24

Sweet corn hybrids, whose pedigrees often include New England flint germplasm, tend to be more susceptible to common smut than field corn. This may be due to differences in the genetic backgrounds of field corn and sweet corn and/or differences in the diligence with which susceptible lines have been eliminated from field corn and sweet corn breeding programs. Ear galls are of greatest economical importance in sweet corn. Similar to the conclusions made by breeders and pathologist in the 1920s and 1930s about the association between gross morphology and reaction of corn lines to smut, resistance to ear galls could be morphological or physiological. K. M. Snetselaar and co-workers recently demonstrated that ovaries (kernels) are protected from U. maydis infection when an abscission layer forms at the base of corn silks soon after fertilization (i.e., pollination). The rapidity with which abscission layers form following fertilization, the rate of pollen germ tube growth, synchrony of silk emergence and pollen production, and the height of ear placement (assuming that inoculum is dispersed from the soil) are just a few of the morphological factors that could allow corn plants to escape ear infection and appear to be ‘field resistant’.

Eradication and Exclusion

Although Ustilago maydis occurs throughout the world where corn is grown, the occurrence of common smut is localized in Australia where the disease is known as maize boil smut. For most of the 20th century, Australians attempted to exclude U. maydis from areas where it did not occur. Infected crops were destroyed to eradicate occurrences of U. maydis, and quarantines were enforced to exclude new introductions.

In 1911, when common smut was first reported in Bathurst, New South Wales, Australia, the infected crop was destroyed and corn was not planted on that farm for 10 years. The disease remained localized to that area, reoccurring sporadically between 1935 and 1940. Affected crops were destroyed and a quarantine enacted in 1940 prohibited corn from being grown in that area for 25 years. Also, areas free of the disease could only be planted with maize seed produced from crops free of U. maydis and treated with an approved fungicide. This was considered to be a successful case of eradication and exclusion because additional occurrences of common smut were not reported until 1982 when the disease was found throughout the North Coast district of New South Wales and southeast Queensland (Figure 25). By 1996, the disease spread to other areas of this region and crossed the Great Dividing Range where it spread southward to areas west of Sydney. The disease subsequently was reported from areas in southern New South Wales where corn is grown under irrigation and from at least one location in northern Victoria. According to corn seed companies in Australia, boil smut has continued spread in areas near Bathurst, Dubbo, and the Murrimbigee Irrigation district.

Figure 25

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