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Testing positive for Pierce’s disease

Grapevine-killing bacteria found in Humboldt for the first time

Grape leaf samples taken by the Humboldt County Department of Agriculture have tested positive for Pierce’s disease (PD).

In late September, the Humboldt County Department of Agriculture submitted grape leaf samples to the California Department of Food and Agriculture’s (CDFA) plant pathology lab to be tested for diseases. The samples were taken from domestic and wild grapevines from a small neighborhood located in Willow Creek.

Three samples tested by CDFA tested positive for PD, which is the first time this disease has been detected in Humboldt County. PD is catastrophic to grapevines as it clogs their water conducting vessels known as xylem. PD is caused by a strain of the bacterium known as Xylella fastidiosa, a C-rated (widespread) pest in California.

The Department of Agriculture took additional grape leaf samples in the same area in November and a small number of those subsequent samples also tested positive for PD. Due to plant dormancy, the Department of Agriculture will conduct further research and take new samples with the spring bud break.

The Department of Agriculture is committed to protecting Humboldt County’s viticulture industry and continues to work to educate and assist growers in identifying and managing the disease as needed. As such, the Agriculture Department will host a meeting and Q&A session to address concerns surrounding PD. The meeting will be held at the Lower Trinity Ranger District Station, located at 580-CA 96 in Willow Creek, on Thursday, April 13 at 1 p.m.

Dr. Rodrigo Almeida, Hildebrand-Laumeister Chair in Plant Pathology at UC Berkeley, and an expert on the disease, will be the featured speaker at the meeting.

Due to space constraints, the meeting is limited to 25 people. Reserving your space in advance is recommended. For more information and to RSVP, please contact David Juliano, Senior Agricultural Inspector, by email at

For more information on Pierce’s disease, please visit the following links here and here.


For more information:
David Juliano

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