Sampling for Xylella fastidiosa the Cause of Bacterial Leaf Scorch

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Sampling for Xylella fastidiosa the Cause of
Bacterial Leaf Scorch

• Dr. Frank Wong
• Dr. Naveen Hyder
• Department of Plant Pathology Microbiology

An effort of USDA-CREES ProjectCombating the
threat of the plant pathogenic bacterium Xylella
fastidiosa using genome-based methods linked to
national and international monitoring
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• Bacterial leaf scorch and dieback are becoming
  increasingly common in California
• Agricultural crops such as grapes, almonds and
  olives are at risk
• Landscape plants such as oleander, liquidambar
  and purple plum are also being affected
• The disease causes scorch and dieback and is
  often fatal to the plants

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• These diseases are caused by the xylem-limited
  bacteria Xylella fastidiosa which grows inside
  the plant and kills it by limiting or blocking
  water movement in these vessels
• The main method in which these bacteria spread
  are by sharpshooter insects which vector the
  bacteria though their mouthparts from plant to
  plant as they feed

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Testing for X. fastidiosa

• Confirming the presence of Xylella fastidiosa in
  a plant is important for making the right
  management choice
• Symptoms can be often confused with water stress
  or other environmental factors

• Xylella fastidiosa is too small to be diagnosed
  from plants by eye or most microscopes
• There are three testing methods available
• Enzyme Linked Immunosorbent Assay (ELISA)
• Polymerase Chain Reaction (PCR)
• Direct isolation and culturing of the bacteria

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ELISA

• The most common test used
• Similar to HIV testing
• Takes 1 to 2 days to complete
• Bacteria is extracted from plant tissue and can
  stick to antibodies bound to plastic plates
• A detectable, second antibody is used to tag the
  bound bacteria

APSnet.org
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PCR Testing

• Bacterial DNA is extracted from plant tissue and
  diagnostic primers are used to amplify only the
  Xylella fastidiosa DNA
• Amplified products are visualized by gel
  electrophoresis
• More sensitive than ELISA, but more expensive

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Isolation Culture

• The most time consuming testing method (often 4
  or more weeks)
• Bacteria is extracted from the plants and put
  onto specialized media
• The bacteria is hard to grow in culture and can
  often be overgrown with contaminants
• Isolation is essential for characterizing new
  strains

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Sampling

• Good samples with live bacteria are needed for
  any of the testing methods
• Live bacteria are most likely to be found in
  diseased, but live plants
• Green stems, petioles and leaf midveins are often
  the best choice for tetsig

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• The best samples are those that are symptomatic
  but still intact and turgid (top)
• Samples from very symptomatic plants can often be
  too dried out or full of secondary microbes
  (middle)
• Testing cannot be done on tissue that is
  completely dead (bottom)

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How to Send Samples?

• Only a few grams of green stems or petioles are
  needed for testing
• A few 6-inch pieces of good samples are often
  more than enough
• Tissue must be kept moist and not allowed to mold
  in transit
• Place samples in a plastic bag with a moist towel
  and sent by overnight mail for the best chances
  of having a good test done

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Where to Send Samples?

• Dr. Frank Wong
• 900 University Avenue
• University of California
• Department of Plant Pathology Microbiology
• 204 Fawcett Lab
• Riverside, CA 92521
• frank.wong_at_ucr.edu
• naveen.hyder_at_ucr.edu
• 951-827-2936
• Walk-in samples are always welcome
• Fill out the information forms available at
  www.xylella.org or here