By: David N. Appel and Sheila A. McBride
Pierce’s Disease (PD) of grapevines, caused by the bacterium Xylella fastidiosa, is spread by sap-feeding insects called sharpshooters. When the bacterium invades the water-conducting vascular tissues (xylem) in grapevines, the vines respond with distinct, characteristic symptoms. Diagnosing PD is often difficult since symptoms in the field may vary due to different grape varieties, the time of infection, or the vineyard location in Texas’ diverse climates and regions.
Diagnosis of such a complex disease can involve several steps that require observations over time and on a variety of scale (from the entire vineyard to individual leaves and canes). The following guide can help determine whether X. fastidiosa is infecting diseased vines. It is not necessary to complete all of the steps for a definitive diagnosis, but fulfilling multiple steps increases the details available for confirming a disease identification with confidence.
Step 1. Analyze the disease pattern in time and space—populations in vineyards.
A PD epidemic in a vineyard develops according to the source of inoculum, the behavior of the sharpshooter vectors, and the cultural characteristics of the grapes. The pathogen develops and spreads more quickly in susceptible grapes than resistant ones. In some cases, contaminated sharpshooters (Fig. 1) spread the bacteria by hopping from infected vegetation growing adjacent to the vineyard to cause an “edge effect” pattern (Fig. 2). Over time, randomly distributed vines become infected throughout the vineyard. The subsequent infections often spread faster along rows than across them, contributing to the characteristic “patchy” distribution of the disease.
Step 2. Analyze the disease pattern in time and space—individual vines.
Grape variety greatly influences the rate of disease development in individual vines. In resistant varieties such as the native Vitis mustangensis (mustang grape) or V. rotundifolia (muscadine), infected vines might exhibit little or no symptoms and survive indefinitely even when grown in high-risk areas. Very tolerant varieties include American hybrids such as Blanc du Bois and Black Spanish. They sometimes appear to be resistant, but may become infected and slowly decline, showing light to moderate symptoms and subsequent dieback over several years. All V. vinifera are susceptible, but tolerance levels vary. For example, Cabernet sauvignon (Fig. 3) and Chenin blanc sustain infections better than the highly susceptible Chardonnay or Pinot noir. In these varieties, dieback can occur within a year of initial symptoms, and the vine can be dead within 2 to 3 years (Fig. 4). Dieback of diseased vines can appear as winter kill when weakened canes and cordons on infected vines fail to emerge in the spring.
Step 3. Analyze leaf, cane, and fruit symptoms.
Marginal scorching of foliage is the most common symptom of PD in grapes and usually appears as an irregular, necrotic margin with a reddish-brown line demarcating the area of brown necrosis from the green, unaffected tissue (Fig. 5). The reddening may not be as prominent in white grape varieties as it is in the red-fruited grapes (Figs. 6a and 6b). Foliar symptoms appear with rising temperatures in June and July. Once the entire leaf succumbs to the encroaching necrosis, the blade falls from the petioles retained on the cane (matchsticks) (Fig. 7). Another typical PD symptom is the formation of green “islands,” resulting from irregular or incomplete periderm maturation on the canes (Fig. 8). Premature shriveling of ripened berries in the clusters, known as “raisining” is also common on infected vines (Fig. 9).
Step 4. Submit samples to a diagnostic clinic.
There are three laboratory approaches for diagnosing PD:
The first, and oldest is the isolation of X. fastidiosa on a growth medium in sterile conditions. Isolation from diseased tissues is the least expensive method, but it is slow (approximately 3 weeks), and most diagnosticians consider it to be a difficult process. X. fastidiosa is extremely slow-growing and easily overwhelmed by other contaminating organisms in laboratory conditions. Another difficulty with this and other laboratory diagnostic methods is that the distribution of the bacterium in the vine is uneven. The bacterium may not be recovered from symptomatic tissues from a diseased vine, giving a false-negative result.
The second diagnostic protocol, ELISA (enzymelinked immunosorbent assay), is a biochemical technique using principles of immunology to develop a test that detects antigens (pathogen properties) in petiole tissues from a vine. ELISA is relatively fast (within a day) and increases the chance of detection when compared to isolations. However, just as with isolations, proper sampling is critical to obtain infected tissues and avoid a false-negative result.
The third and most sensitive technique for PD diagnosis is PCR (polymerase chain reaction). Usually formatted for rapid diagnosis as qPCR (quantitative PCR), this technique is also the fastest (a few hours). It is based on amplifying and quantifying DNA of the pathogen extracted from diseased tissues. Through elaborate sample preparation, an instrument detects and quantifies the pathogen DNA.
Collecting and submitting samples for laboratory diagnosis
Because bacteria do not distribute evenly throughout the vine, a plant can exhibit symptoms while the pathogen might not be present in a tissue sample. All clinical diagnostic procedures require proper sampling from the suspect vine as well as correct procedures for shipping the tissues to a plant diagnostic clinic:
- Give priority to petioles on symptomatic leaves. Leaf petioles, the portion of the leaf that attaches the blade to the stem, are the best tissues for detecting X. fastidiosa.
- Target the first (oldest) leaf on a cane for sampling.
- Wrap the petioles with the leaves attached in a dry paper towel, place it in a plastic bag, and ship it to the Texas Plant Disease Diagnostic Laboratory (TPDDL) via overnight delivery to keep the sample from deteriorating. If a delay is a concern,keep the samples cool (not frozen) until they are shipped.
- Include with all samples thoroughly completed forms supplied by the TPDDL. The Pierce’s Disease Testing Form (TPDDL 1004) and instructions are available at http://plantclinic.tamu.edu under the Submission Forms tab.
The most crucial tool in managing PD is the proper diagnosis. Clinical diagnosis is a valuable aid in determining the nature of a disease problem in grapevines because symptoms of many abiotic and biotic stresses such as drought, nutrient deficiencies, and cotton root rot mimic those of PD. However, the results of every diagnostic step require interpretation in the context of vineyard conditions and other factors influencing vine health. For a successful outcome, proper diagnosis requires knowledgeable observations in the field and communication between the vineyard manager and the clinical diagnostician.
A thorough treatment of all aspects of Pierce’s Disease of winegrapes in Texas can be found at https://aggie-horticulture.tamu.edu/fruit-nut/files/2010/10/
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