Small, rust-colored spots on starthistle leaves. Click the image for more information about it.

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Rust Fungus Unleashed to Fight Exotic Thistle

There's a showdown at the Mead Ranch in California's Napa Valley this summer. In one corner is a biocontrol rust fungus; in the other, the invasive weed yellow starthistle (YST).

Agricultural Research Service and California Department of Food and Agriculture (CDFA) scientists are refereeing the affair, but they're far from being impartial. In fact, they're rooting for the rust fungus, Puccinia jaceae var. solstitialis, whose spores they sprayed onto YST-infested pasture at the Mead Ranch earlier this spring.

Since arriving in the United States from abroad in mid-1800s, YST, Centaurea solstitialis, has elbowed aside native plants and established itself as an invasive weed in many of the 48 contiguous states. California has the largest infestation with 14-plus million acres.

According to ARS plant pathologist Bill Bruckart, the Mead Ranch release marks the first U.S. field use of a fungus to biologically control YST. Mowing, herbicide spraying and prescribed burning are some of the ways that landowners now battle the invasive weed. The rust fungus is appealing because it's self-spreading, host-specific and unrelenting once it becomes established on YST populations, according to Bruckart, at the ARS Foreign Disease-Weed Science Research Unit at Frederick, Md.

Bruckart and CDFA collaborators Dale Woods and Mike Pitcairn first released the fungus in July 2003 after obtaining federal, state and regional approval. Since spraying the fungus' spores again this spring, the team has been closely monitoring the ranch site's YST populations for disease symptoms, such as orange-brown pustules on the thistle's stem and leaves. The rust's unaided spread beyond the spray zone is also of interest.

The fungus, whose release caps 25 years of quarantined research, joins five other previously released biological control organisms against YST. These include two exotic flies and three weevils that diminish YST's seed-producing ability.

Read more about the research in this month's issue of Agricultural Research magazine.

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