Did You Know? Nebraska Contributed Significantly to the Detection of Rhizomania in the U.S.
Robert M. Harveson, Extension Plant Pathologist, Panhandle R&E Center
Author’s note: This is the first article in a new series that will document significant achievements or discoveries in plant pathology from University of Nebraska personnel.
Rhizomania is one of the most feared and destructive diseases of sugarbeet, causing severe losses in Europe and Japan since its first report from Italy in the 1950s.
It is also unusual in that it is a soilborne, root-rot disease caused by a virus – beet necrotic yellow vein virus (BNYVV). The viral pathogen is uniquely transmitted to host roots by a fungal parasite called Polymyxa betae.
This fungus has a host range limited to plants related to sugarbeets, including spinach, chard and several weed species such as pigweed, lambsquarters, and Kochia. Without this soil-inhabiting vector infecting plants, the disease cannot occur; however the fungus does not always contain the virus. The vector itself (the fungal parasite) is essentially harmless, causing little damage unless it is carrying the virus and transmits it to the plant roots after infection.
Due to the fear of this disease and the potential damage to sugar beet production, my predecessor at the Panhandle Center, Eric Kerr, conducted a survey in 1976 from western Nebraska sugarbeet fields, searching for root infections by the vector, but found none.
Polymyxa betae was not reported from North America until 1977, although it was not found directly infecting sugarbeet roots in the field. It was detected in the greenhouse from beet plants grown in soil samples collected from two California fields where previous beet crops had exhibited yellowing symptoms. Furthermore, no evidence of the virus or symptoms of the disease (rhizomania) was observed, only the presence of the vector (P. betae) baited from the field soil samples.
The virus pathogen (BNYVV) was encountered in North America for the first time in 1981 from a cherry orchard in Washington – curiously, a location having nothing to do with sugarbeets. In the attempt to connect a potential soilborne virus with the stem-pitting disease of sweet cherry, BNYVV was detected from the roots of Gomphrena globosa (globe amaranth) used as bait plants from soil samples collected from beneath diseased cherry trees.
Due to the reports of both the vector and virus from California and Washington, respectively, Kerr and Willem Langenberg (USDA-Agricultural Research Service, UNL-Lincoln) again conducted a search in 1981, testing plants (sugar beet and redroot pigweed) in low-lying areas of Scotts Bluff County sugarbeet fields. In five of six fields inspected, P. betae was present in the roots of all plants that were collected and examined.
This was the first finding of the fungal vector directly infecting sugar beets in the field from North America. However, the virus causing rhizomania was not detected from this search. It was not for another decade that the disease itself would be found affecting sugar beets in Nebraska.
My Unlikely Role
Ironically, I personally ran the tests on the sugarbeet samples sent to us by Eric Kerr in July 1992 that confirmed the presence of the disease in Nebraska and Wyoming. At that time, I was working as a research technician with the Texas Agricultural Experiment Station in Bushland, Texas, one of only two labs in the western hemisphere that were capable of testing for rhizomania. I also processed additional samples that summer identifying the disease from Idaho as well.
These discoveries from Nebraska, Idaho, and Wyoming were the first documentation of the disease affecting sugarbeet crops in the United States since the initial findings from California in 1983 and Texas in 1985. I had no idea at the time that I would eventually replace Eric Kerr as the extension plant pathologist in Scottsbluff seven years later.