Robert M. Harveson, Extension Plant Pathologist University of Nebraska, Panhandle REC, Scottsbluff
History and Background
Goss’ wilt and leaf blight is a bacterial disease of corn, caused by Clavibacter michiganensis subsp. Nebraskense. It was first found on two farms in south central Nebraska (Dawson County) near Lexington in 1969, making it a true Nebraska native. Over the next decade, it was identified from at least 53 other counties in the state and additionally spread into surrounding western Iowa, South Dakota, Kansas, and Colorado. Over the next 10 years the disease was identified from additional U.S. corn-growing states (the “I” states), but the disease disappeared just as suddenly as it first appeared.
The disease has been known by many names over the years, including Nebraska bacterial wilt and leaf freckles, or leaf freckles and wilt. However, the most frequently used and currently most widely accepted name is Goss’ bacterial wilt and blight to reflect the symptoms and form of disease manifestation associated with the disease, or just simply “Goss’ wilt”. This name was proposed in honor of Robert W. Goss, a former chair of the Department of Plant Pathology and the first full-time Dean of the Graduate College for the University of Nebraska, who pioneered the early development of modern plant pathology in Nebraska in the early 1920s.
The bacterium can cause two major types of symptoms or disease phases, including a leaf blight phase and a systemic vascular wilt. The most commonly observed stage is the leaf blight, characterized by dark green and water-soaked foliar lesions that are similar in appearance to those associated with Stewart’s disease. Small dark green spots (freckles) can develop at the edges of some developing lesions and mature lesions are frequently shiny and orange discolored because of the secretion of bacterial exudates onto the surface of the leaf, both of which aid in identification. Lesions may progress to blight and kill large portions of the canopy. Severe necrosis and tissue death may be confused with leaf scorch symptoms, associated with drought stress or hot, drying winds. The pathogen is the only known gram positive bacterium known to infect corn, so this test and other enzyme and pathogen nutritional tests have been routinely used for preliminary dentification of samples submitted to the Panhandle Plant Disease Diagnostic Clinic in Scottsbluff.
The disease is also characterized by the less common wilt phase. In these situations, the pathogen infects the vascular system early in the season and moves systemically within the water-conducting elements, resulting in discoloration of xylem tissues, a water-soaked, slimy stalk rot, and leaf wilting and death of plants. Systemic infection can kill plants at any time during the season, including young seedlings.
Favorable Conditions for Disease Development and Pathogen Survival
Infection by this bacterium requires injury to the plant and is most common in Nebraska following hail, wind damage, and sandblasting. Infection can also be systemic and cause the vascular bundles inside of cut stalks to be necrotic and discolored orange. The pathogen survives in infected residue on the soil surface, which serves as the primary inoculum source for future corn crops. Infection and spread throughout fields is enhanced by reduced tillage practices, continuous corn production, tissue injury on leaves, and sprinkler irrigation or rainfall; but insects or any other potential biological vector are not thought to be involved with disease spread or development. Once infection occurs, optimal temperature for disease development is 80 F, and the pathogen can be killed at temperatures >=100 F. The pathogen is also carried in seeds, both on the outside of the seed coat, and internally, which may also explain its movement and distribution throughout the corn-growing regions of the U. S.
New Disease Outbreaks
After seemingly disappearing from the region for almost 20 years, Goss's wilt and blight has now increased in incidence since 2004 where is was observed from approximately one dozen fields in northeastern Colorado and western Nebraska, before becoming particularly severe in 2005 and 2006. In 2006 alone, the pathogen was confirmed from more than 50 fields with many more suspected but unsubstantiated reports being made. Most of the recent disease outbreaks in the mid-2000s have occurred in western Nebraska, northeast Colorado, and southeast Wyoming, and it has become the predominant disease of corn in this tri-state region. During the 2007-2008seasons, the disease in western Nebraska decreased, while at he same time, an increase in incidence and severity was detected in counties of eastern and central Nebraska where it hadn't been reported in recent years. Goss’ wilt and leaf blight was confirmed from 15 and 24 counties in 2008 and 2009, respectively.
During 2009 an increase was additionally observed in many counties of western Nebraska which represented the first documentation of the disease being found statewide in more than two decades. In addition, the disease developed in other Midwest states, including for the first time, Indiana. Increased incidence of the disease can partially be attributed to the more frequent hail storms experienced in 2009 that created wounds, which are required by this pathogen for infection.
Until 2008, the disease had been centered in the western half of the state along with southeastern Wyoming and northeastern Colorado, before suddenly becoming problematic throughout eastern Nebraska. During 2010 and 2011, the disease has been particularly destructive across the Midwestern Corn Belt states with new reports emerging from Minnesota and Texas.
The most susceptible hybrids can sustain yield losses in excess of 50% with severe infections. Resistant dent corn hybrids are available and are strongly recommended for management in areas where the disease is severe, thus the use of tolerant hybrids offers the best management opportunity for Goss’ wilt. Several disease resistant hybrids are available, but should only be used in situations where the disease has previously been problematic and of major concern, due to the poor performance of certain hybrids in the absence of disease. Since the bacterium survives in infected residue, some type of tillage operation to bury and encourage the decomposition of infected residue may also be effective for managing new infections. Rotating out of corn into other crops such as soybean, dry bean, small grains, or alfalfa will help to reduce primary inoculum sources in corn residue. Additional hosts for the pathogen include green foxtail, shattercane, and barnyard grass, so weed control may also provide an effective method for reducing inoculum for secondary spread within fields planted with susceptible cultivars.
Why Has the Disease Returned to the Central High Plains?
We are uncertain as to why the disease has recently re-appeared in this region after many years’ absence. However, we can speculate that changes in cultural practices over the last thirty years have been major contributors to this phenomenon. We do know that reduced and minimum tillage practices have increased substantially throughout the area for a number of reasons, including soil moisture savings as a result of our extended decade of drought (1999-2008). In addition, there has also been a tremendous increase in continuous corn production and change of irrigation methods from primarily gravity or furrow systems to center pivot, sprinkler irrigation. All of these practices have been documented to enhance pathogen survival and disease severity and progress.
Lastly, the creation and development of new disease resistant cultivars have undergone changes as well. When the disease disappeared, many seed companies simply stopped evaluating for and providing new information concering resistant cultivars. In 2005 (coinciding with the new outbreaks), less than 25% of seed companies (5 out of an incomplete list made up of 22 companies) in Nebraska provided ratings of their hybrids for their reaction to Goss’s wilt. Since this time growers in this region that have been harmed by this disease have refused to buy seed unless they contained good levels of disease resistance to Goss’ wilt. This then enticed the seed companies to return to routine testing and breeding processes for producing new resistant cultivars for affected areas. The most likely explanation for the disease’s return to this region is a combination of all factors discussed above coming together simultaneously to create a perfect storm of conditions that favored the disease and its spread and development.