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Cercospora leaf spot disease symptoms on a sugar beet leaf. Click the image for additional information about it.
Cercospora leaf spot disease symptoms on a sugar beet leaf.  Click the image for additional information about it.

New Protocol Offers Early Diagnosis of Sugar Beet Disease--And More

By Erin Peabody
December 22, 2003

To spot early signs of Cercospora leaf spot disease in sugar beets, an Agricultural Research Service scientist has developed a method to detect the culprit fungus, Cercospora beticola, early on--even in plants lacking any symptoms.

These symptoms include ash-colored spots and dark blotches. When these symptoms appear on the leaves of a sugar beet plant, beet farmers suspect Cercospora leaf spot. The fungus behind this foliar disease reduces both yield and sugar content in sugar beets, the plants that provide the United States with about half of its sugar supply.

But the fungal pathogen doesn't always announce itself, especially as it moves from the soil to the beets' leaves.

Plant pathologist Robert T. Lartey, along with three ARS colleagues, created the pathogen-detecting protocol. It uses real-time polymerase chain reaction (PCR), a technique for rapidly producing many copies of a fragment of DNA so that specific genes can be identified. ARS' John J. Weiland, TheCan Caesar-TonThat and Sarah Bucklin-Comiskey assisted Lartey, who is at the agency's Northern Plains Agricultural Research Laboratory in Sidney, Mont.

The new protocol drastically reduces the time needed to isolate and subculture the pathogen and to purify the DNA for amplification, from a task traditionally requiring up to two weeks to one that can be accomplished in a single day.

The technique relies on a kit developed and sold by Sigma-Aldrich, a St. Louis-based biotech company.

The PCR method can be used to determine how early in the growing season C. beticola may infect nearby plants, and from there spread to sugar beets. Early detection provides a better forecasting of the disease and allows for more precise fungicide applications.

Lartey anticipates the use of the new protocol for disease management in a range of plant pathogen applications. While PCR is widely used to detect fungal, bacterial and viral diseases of crops, for many pathogens there still is no rapid and sensitive test. Scientists are already interested in the technique for identifying diseases in wheat.

ARS is the U.S. Department of Agriculture's chief scientific research agency.