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Good enzyme may defeat sugar beet leaf spot disease

Agricultural Research Service scientists have applied for a patent on their discovery of a “good” fungal enzyme that may prevent leaf spot, a widespread disease of sugar beets that reduces beet yields and sugar content.

This biocontrol discovery is especially timely because scientists now believe that Cercospora beticola, the fungus that causes leaf spot disease, is beginning to develop resistance to some fungicides. The researchers envision possibly applying some form of the enzyme to sugar beet leaves to starve the “bad” fungus.

In 2001, U.S. farmers grew more than 25 million tons of sugar beets, providing about half of the country's sugar supply. They also applied thousands of pounds of fungicide to sugar beet leaves to battle leaf spot.

Plant pathologist Robert Lartey and microbiologist TheCan Caesar-TonThat of ARS' Northern Plains Agricultural Research Laboratory in Sidney, Mont., discovered an enzyme called laccase in the beneficial fungus Laetisaria arvalis. This fungus is well known as a wood- and leaf-decay fungus of forests. But it is also found in farm soils where sugar beets grow.

Laccase is one of many enzymes produced by the fungus to break down deadwood and leaves. In lab experiments, it was very good at detoxifying the toxin produced by the leaf spot fungus. The toxin, called cercosporin, kills plant leaf cells. Leaf spot gets its name from the spots on leaves that are actually colonies of fungi feeding on leaf tissue killed by the toxin.

Next, Lartey and Caesar-TonThat will test the enzyme on potted sugar beet plants in a greenhouse. If that goes well, they will move on to sugar beets growing in the field.

More information about this natural biocontrol research can be found in the May issue of Agricultural Research magazine, on the World Wide Web at:

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