Diagnostics
When we speak of diagnosing Pierce's disease, two distinct areas come to mind. For growers and field researchers, identifying symptoms displayed by grapevines is crucial for disease detection and managing the disease in commercial vineyards. The second field of study is laboratory diagnosis. Several procedures can be employed by scientists to confirm or deny the infection status of vines that appear to display symptoms.
Lets first review field diagnosis.
Visual Diagnostics
The first symptom of a grapevine being infected with Pierce's disease is usually scorching of foliage. This scorching is usually quite irregular and appears differently and at different times on each variety. On white varieties, the leaf margin starts to bleach and then becomes necrotic. Scorching usually starts near the point of infection, but is not a reliable indicator of exactly how far the bacterium has traveled within the grapevine. On red varieties, a red "facing" usually accompanies the scorch. As with whites, some varieties show scorching very soon after infection and others may take years to show the first signs of infection.
One should exercise caution when using only leaf scorching to diagnose Pierce's disease. Salt injury, phytotoxicity from agricultural chemicals, drought, fungal pathogens such as Esca or Black Measles and some nutritional deficiencies can also produce leaf scorching. Laboratory diagnosis should be used to confirm visual symptoms where the presence of Pierce's disease is suspected.
The second typical symptom is the abscission of the leaf blade while the leaf petiole remains attached. This secondary symptom termed "match sticks" is a further indication of a vine's infection, but again, this symptom can be caused by other pathogens such as downy mildew.
The third and perhaps most definitive vine symptom is the uneven ripening of canes during the summer. As canes mature, the green tissue further lignifies and becomes brown and woody. Mature wood on one year old canes is termed periderm. With this symptom, periderm forms normally in the internodal area or the part of the cane between developing buds. On infected vines the developing cane remains green at nodes and exhibit an uneven ripening of wood.
As infection progresses through the vine, fruit begins to collapse as it nears maturation and canes and cordons collapse. Because Xylella blocks xylem or water conductive tissue, these conditions usually accompany water stress or the stress of crop ripening.
Finally, vines die. Because water transpiration is inhibited by this bacterium, photosynthesis is reduced, vines often defoliate prematurely and winter hardiness is commonly compromised. Some growers may suspect vines died from winter injury, and in part they are correct. With Pierce's disease however, vines can be predisposed to die as a result of exposure to cold temperatures.
Laboratory Diagnostics
Confirmation of vine infection through laboratory procedures is the definitive way of knowing whether or not your vines indeed have Pierce's disease. There are new emerging techniques, but most labs rely on three main ways of confirming infection. Using laboratory diagnosis is very important and effective in training a grower's eye, but is not cost effective to diagnose infection on a vine by vine basis. All of these diagnostic techniques have their strengths and their weaknesses. While some are more sensitive, others are more reliable or inexpensive.
The oldest and most cost effective laboratory procedure currently used today is ELISA or Enzyme-linked Immuno Sorbent Assay used to detect an antibody or antigen in a sample. ELISA testing for Xylella is not specific to a certain strain and is relatively insensitive to low level bacterial populations. When ELISA samples test positive for a specific antigen, they react and change color. The greater the population of the bacterium, the more intense the color change in the test well, so relative population levels can be estimated. Mobile, hand-held ELISA testing units have been developed that help research personnel confirm infection status in the field.
A more expensive, but much more sensitive method of laboratory detection of Pierce's disease is the use of PCR or polymerase chain reaction is a way of detecting and amplifying a specific sequence of DNA unique to a disease organism. This technique is used to specifically identify Xylella strain relationships, to quantify bacterial populations and to find Xylella in plants and insects when they occur below the threshold of ELISA testing. This procedure is most applicable for research projects looking to detect infection in grapevines before symptoms are evident, or to detect the bacterium in plants and insects at low levels.
A third, and perhaps most definitive way of diagnosing the presence of Xylella in plants is through culture. Plant or insect samples are ground, diluted to exclude high populations of contaminants, then plated on sterile media plates to allow the bacterium to grow. The specific name of this pathogen is fastidiosa, which means difficult or slow to grow, so plating out Xylella cultures is a slow process. Once a colony is identified on a culture dish, it can be analyzed with ELISA testing for definitive confirmation.
Collecting a Sample
To sample for the presence of Pierce's disease its probably best to sample one vine at a time rather than to submit a composite sample of several vines. That way a grower will know definitively about the status of an individual vine. Testing is not inexpensive and remember, this procedure should be done to train your eye, not to test every time you see a suspect vine. Since ELISA testing is somewhat insensitive, choose a vine or two with the most dramatic symptoms, flag the vine and tag the vine with a sample identification number.
Choose twenty leaves that appear to be the most scorched and remove them. Remove and discard the leaf blade and retain the petiole or leaf stem. You should submit about twenty petioles per plant to the lab for analysis. Place the petioles in a polyethylene bag and keep the sample cool, but not cold. You should contact the lab prior to shipping to let them know you are sending a sample and you should plan on over-nighting the sample to make sure it arrives in good shape. Keep good records of what vine corresponds to individually numbered lab samples so you can verify vine infection status.
The most practical approach for growers wishing to test suspect grapevines is to submit petiole samples to the Texas Plant Disease Diagnostic Lab in College Station. Contact information for this Extension branch of the Department of Plant Pathology & Microbiology is as follows:
Web Site • Phone: (979) 845-8032 • Email: Dr. Larry Barnes, l-barnes@ag.tamu.edu
Mailing Address:
Texas Plant Disease Diagnostic Laboratory
1500 Research Parkway, Suite A130
Texas A&M University Research Park
College Station, Texas USA 77845