Significance

Economic Significance

Stewart's wilt has little effect on yield of resistant field corn hybrids grown in North America, but it can substantially reduce yields of susceptible sweet corn hybrids. Stewart's wilt also can economically impact seed trade because of quarantine restrictions and/or costs associated with breeding for resistance.

In field corn, yield losses due to Stewart's wilt have been inconsequential in North America for the past 50 years except for a few, localized sporadic outbreaks. The minimal economic impact of Stewart's wilt on field corn is primarily due to adequate levels of resistance in corn hybrids that are grown where the disease occurs. Nevertheless, some early-maturing field corn lines are moderately susceptible. Prior to the development of resistant hybrids, Stewart's wilt caused notable economic losses, particularly during epidemics in the early 1930s.

In sweet corn, economic losses continue to be notable because susceptible or moderately susceptible hybrids are grown periodically in areas where flea beetles occur. Yield losses in sweet corn are affected by the level of resistance of the hybrid and by the growth stage at which plants are infected (Table 2). Yield is not adversely affected or losses are minimal in resistant and moderately resistant hybrids; however, losses frequently range from 40% to 100% when susceptible sweet corn hybrids grown under epidemic conditions are infected prior to the 5-leaf stage. Yield losses in sweet corn are related to systemic infection. Yield is reduced about 0.8% for each 1% incidence of plants infected systemically as seedlings (Figure 22).

Figure 22

Stewart's wilt also can have an economic impact on seed trade as a result of phytosanitary regulations imposed by trading partners. Phytosanitary regulations for Erwinia stewartii primarily affect seed commerce by preventing seed from being exported or by creating additional costs for phytosanitary inspections prior to export. During recent epidemics in the 1990s, Stewart's wilt was a significant economic issue for the corn seed industry because of the logistics of trading and exchanging large volumes of field corn seed throughout the world. Stewart's wilt also creates indirect costs for seed producers because resources must be used to screen germplasm and breed corn for Stewart's wilt resistance in order to develop hybrids that efficiently and effectively control the disease.

Historical Significance

Our present understanding of Stewart's wilt is based on a compilation of discoveries by several plant pathologists, entomologists, and geneticists who added to existing knowledge that often included erroneous facts or misconceptions. This evolution of knowledge continues today as researchers learn more about the bacterium, its insect vector, and the genetics of host resistance.

In the late 1880s, a new bacterial disease of corn was described from fields in southern Illinois by T. J. Burrill, whose work on fire blight was instrumental in establishing the concept that plant diseases could be caused by bacteria. Burrill and an entomologist, S. A. Forbes, associated these symptoms with dry weather, tissues from which bacteria were observed, and large populations of chinch bugs. Despite isolating bacteria from symptomatic plants and insects, Burrill was unable to complete Koch's postulates. Although Burrill's description of plants suffering from this affliction included many symptoms that are not attributed currently to Stewart's wilt, Burrill probably was the first to study Stewart's wilt.

In 1895, F. C. Stewart observed a bacterial wilt of sweet corn on Long Island, New York. Stewart gave an accurate account of the symptoms, isolated the bacterium, and reproduced the symptoms by inoculating sweet corn. Stewart believed the disease was confined to sweet corn, and, thus, was different from the disease that Burrill described from field corn. Stewart sent a culture of the bacterium to E. F. Smith who in 1898 described and named it Pseudomonas stewartii. Smith later supported Stewart's suggestion that the bacterium was disseminated readily by seed. Twenty-five years later in 1923, F. V. Rand and Lillian Cash implicated the corn flea beetle, Chaetocnema pulicaria, as the primary vector responsible for the mid-season spread of the leaf blight phase of the disease. The ability of Erwinia stewartii to overwinter primarily in the insect vector rather than in seed was established by F. W. Poos and Charlotte Elliott in 1934.

In the midst of severe epidemics in the early-1930s, Stewart's wilt was largely responsible for the development and enthusiastic acceptance of the first widely-grown, single-cross corn hybrid, 'Golden Cross Bantam.' 'Golden Cross Bantam' was among the first great economic contributions of hybrid corn. In 1923, Glenn Smith, a USDA scientist working in the Department of Botany and Plant Pathology at Purdue University (Figure 23), began inbreeding lines from a popular, open-pollinated sweet corn cultivar, 'Golden Bantam.' Like many early-maturing sweet corn cultivars derived from Northern Flint corn, 'Golden Bantam' was extremely susceptible to Stewart's wilt (Figure 24). In 1930, Smith tested his first set of 'Golden Bantam' hybrid crosses in the midst of one of the most destructive epidemics of Stewart's wilt ever experienced in northern Indiana. Two of Smith's inbred lines, P39 and P51 derived from two different versions of 'Golden Bantam,' combined to produce a single-cross hybrid that was resistant to Stewart's wilt (Figure 25). Following a second year of successful performance during another Stewart's wilt epidemic in 1931, the hybrid was named 'Golden Cross Bantam.' Within a few years after it was released in 1932, 'Golden Cross Bantam' had virtually revolutionized the sweet corn canning industry and 70 to 80% of all sweet corn canned in the US was Glenn Smith's Stewart's wilt-resistant hybrid.

Figure 23	Figure 24
Figure 25

Fifteen years after his remarkable accomplishment, Smith modestly commented on his work in The Hybrid Corn Makers, "Our project was well planned, and I just carried out the work as best I could. Some of it, to be sure, was monotonous, but take the job as a whole and it has been a lot of fun. I shall never cease to be thankful that we had that epidemic of bacterial wilt from 1930 until 1933, for without that bit of good luck we might never have discovered the possibilities of Golden Cross Bantam." In addition to Stewart's wilt resistance, the "possibilities of Golden Cross Bantam" included uniformity that was unknown among sweet corn cultivars prior to the development of hybrids. Because it was a single-cross hybrid produced from two inbred parents, 'Golden Cross Bantam' was more uniform than open pollinated cultivars or the three-way and double-cross field corn hybrids of the 1930s which were produced from three or four inbred parents. This new variety of "hybrid corn" was extremely popular among sweet corn processors well into the 1950s, and sweet corn inbred lines with P39 and P51 in their backgrounds continue to be used as parents of sweet corn hybrids grown today. Resistance to Stewart's wilt also continues to be a valuable attribute of sweet corn hybrids grown in areas where the disease occurs in order to prevent notable reductions in yield (Figure 26).

Figure 26

Stewart's wilt was again epidemic in North America in the early 1950s and in 1992, 1995, and 1998-2000 following unusually warm winters and/or during summer droughts. These epidemics prompted a resurgence of applied research on host resistance, insecticidal control, flea beetle population dynamics, and seed transmission. Erwinia stewartii also has recently been a model organism for basic biological research on the genetics and biochemistry of pathogenicity and the role of quorum-sensing regulation of gene expression.

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by The American Phytopathological Society