Nebraska – a hotbed for new plant diseases

Robert M. Harveson
Extension Plant Pathologist
Panhandle R&E Center, Scottsbluff

Though plant pathology was not officially recognized as distinct from botany until 1920, the study of plant diseases had been practiced in Nebraska since 1884 with the arrival of the eminent botanist Charles Bessey.

Since then, individuals associated with the University of Nebraska have made an astonishingly high number of “first-time” reports pertaining to diseases of plants. Here is a list of some of those events, along with descriptions and their significance to the advancement of discipline.

Violet Root Rot

In 1890 two students of Bessey’s, Roscoe Pound and Herbert J. Webber, independently identified and reported a root disease of alfalfa known as violet root rot (caused by Rhizoctonia corcorum) from two distinct locations in Nebraska. This proved to be the first Rhizoctonia disease reported from the United States.

Beet Curly Top

Another student of Bessey’s, George Hedgcock, is credited with observing distinct cases of a disease that is today assumed to be the leafhopper-transmitted virus disease beet curly top, based on his symptom descriptions. Curiously it was found in garden beets in Nebraska before 1888.

Wheat Streak Mosaic

Nebraska Experiment Station Plant Pathologist and department chair George Peltier generally is credited with the discovery and first recognition of wheat steak mosaic virus in 1922. He was able to mechanically transmit the pathogen to healthy wheat and corn plants from infected, symptomatic wheat plants, providing proof that symptoms were caused by a biotic factor. The disease was first observed from research plots on the UNL farm in Lancaster County.

Sugar Beet Seedling Rust

While with the U.S. Department of Agriculture, Venus W. Pool, a former graduate student, discovered the sugar beet seedling rust pathogen (Puccinia subnitens) in Colorado on sugar beets for the first time, reporting its unique life cycle and several alternate hosts in 1914. Bob Harveson identified and reported a widespread epidemic in 2010 of this same rare disease throughout the Nebraska panhandle, making it the first documentation of the disease from field epidemics in almost a century.

False Root-Knot Nematode

Max Schuster of the UNL Plant Patholody Department first identified the false root-knot nematode (Naccobus aberrans) infecting sugar beets near Mitchell, Neb. He later found this same pathogen infecting prickly pear cacti from uncultivated range lands in multiple locations in Scotts Bluff and Sioux counties, suggesting it was a nematode native to western Nebraska.

Bacterial Wilt of Dry Beans

The original report from South Dakota described bacterial isolates (Curtobacterium flaccumfaciens pv. flaccumfaciens) that were yellow-pigmented. Max Schuster first reported the disease from Nebraska in 1949. In 1957 he discovered a different color variant (orange) of the pathogen. In 1967, Schuster and Anne Vidaver, of the UNL Plant Pathology Department, including stints as Department Chair), found and characterized a purple-colored variant. Finally, Bob Harveson discovered a fourth color variant (pink) of the pathogen in 2008. Interestingly, all four color variants were first found in Scotts Bluff County.

Soilborne Wheat Mosaic

Myron Brakke (USDA/UNL Plant Pathology Department) was the first researcher to determine that the soil-inhabiting fungus Polymyxa graminis was the biological vector for this virus disease. It was identified from Nebraska in 1966.

Goss’ Wilt of Corn

In 1969, Plant Pathologist David Wysong, Anne Vidaver, and Max Schuster first discovered and reported a bacterial disease of corn completely new to science, caused by Clavibacter michiganensis subsp. nebraskensis, from central Nebraska near Lexington. It inexplicably disappeared in the early 1980s, but reemerged again widespread throughout the central high plains in the mid-2000s.

Bacterial Mosaic of Wheat

Randall Carlson, a graduate student, and Anne Vidaver discovered and characterized another new bacterial disease of wheat, caused by Clavibacter michiganensis subsp. tesselarius. This disease suddenly appeared in 1976 and between 1976 and 1979 it was widely identified from Scottsbluff County in the west to Crawford County in western Iowa. In a manner identical of Goss’ wilt, it also mysteriously disappeared by the early 1980s, but unlike Goss’ wilt has not reappeared in commercial production since.

Polymyxa betae of Sugar Beets

William Langenberg (USDA/UNL Plant Pathology Department) and Eric Kerr, UNL Plant Pathologist, determined that the known vector for a viral disease called rhizomania was infecting sugar beets in five of six fields from Scotts Bluff County that were surveyed in 1982. This was the first time this organism was identified directly infecting the roots of this plant in the western hemisphere. However, it was also determined that the vector was not carrying the pathogen at that time from those samples that were tested. The virus disease rhizomania was not identified from Nebraska for another decade.

Dry Rot Canker

Bob Harveson and Melvin Bolton (USDA colleague) discovered through both that the dry rot canker disease of sugar beets was caused by a binucleate species of Rhizoctonia (anastomosis group – AG F). This proved that the fungal pathogen was distinct from the R. solani that causes the well-known Rhizoctonia root and crown rot (RRCR), which was previously assumed. This disease had been noted sporadically occurring for several years prior to this due to atypical symptoms of RRCR, but was not identified and published until 2103.

Bacterial Leaf Streak of Corn

In 2014, Tamra Jackson-Ziems and Kevin Korus of UNL first found an unknown bacterial disease in corn affecting Nebraska production fields. In 2016 it was identified as bacterial leaf streak (BLS) caused by Xanthomonas vasicola pv. vasculorum. This was the first report of this disease in the U.S. and the Western Hemisphere, being previously reported only from South Africa.

Orobanche ludoviciana

In 2014, Bob Harveson identified and documented the parasitic plant Orobanche ludoviciana (Louisiana broomrape) infecting sunflowers within a production field in Kimball County. It was the first report for any Orobanche species found parasitizing commercial sunflowers in the western hemisphere.

Unnamed Virus Disease of Sunflowers

Since 2010, a disease with symptoms characteristic of a virus disease was observed from multiple sites and multiple years throughout the Panhandle of Nebraska, by Bob Harveson. In 2017 it was identified as a member of the family Tombusviridae, based on molecular diagnostics as well as electron microscopy and morphological features. This represents the first report of a virus disease from commercial sunflower production in North America.

Fusarium boothii of Wheat

In 2015, widespread epidemics of Fusarium head blight (FHB) occurred in wheat in Nebraska. During a wheat disease survey, Stephen Wegulo of UNL identified three isolates of Fusarium collected from widely separated regions of western Nebraska as Fusarium boothii. This was a first report for this pathogen species causing head blight. Until this finding, F. graminearum was the only known pathogen in the U.S. FHB. ❖