Timely Viticulture Timeline: Mid Season
Updated: October 21, 2024
By Mengjun Hu, Ph.D. , and Joseph Fiola, Ph.D.

Pierce’s Disease—Background and Management

Pierce’s Disease (PD) of grapevines is caused by a bacterium, Xylella fastidiosa subsp. Fastidiosa. It was first described in California in the 1800s, is commonly found in vineyards bordering the Gulf Coast, and more recently the Mid-Atlantic, including Virginia and North Carolina, and the southern Midwest. It is typically found in hot climate regions but has been slowly moving into more northern regions. In the South it can infect and quickly kill entire vineyards but as it moves north to areas experiencing cold winters, it tends to spotty and sometimes transient appearance in vineyards.

The Pierce’s Disease Pathogen and Vector

  • Xylella fastidiosa is a gram-negative bacterium that survives and multiplies in the water-conducting elements (xylem) of grapevines and produces a biofilm that eventually blocks the xylem from transporting water and nutrients through the plant.
  • The bacterium has a broad host range with more than 100 known plant hosts, but strains show specialization for certain hosts.
  • Only specific strains of the bacterium are capable of infecting grapevines.
  • The bacterium that causes PD is native to North America where there are native grape species.
  • The bacterium is typically transmitted from plant to plant by an insect vector, including spittle bugs and leafhoppers.
  • There is also a low risk of mechanical transmission of the pathogen.
  • The risk of spreading the bacterium through pruning practices is also considered low during the winter months.
  • The risk is much higher when pruning practices occur during periods of active vine growth, so in high risk vineyards, it is recommended that pruning equipment be sanitized after working on each vine.
  • The bacterium can also be transmitted through use of propagation material taken from infected grape vines.

Pierce’s Disease Symptoms

  • As the pathogen multiplies in and block the xylem, symptoms are expressed as severe water stress or drought including wilting and premature defoliation.
  • Symptoms tend to appear later in the summer when it is hot and dry but can appear earlier under dry conditions and drought stress.
  • Leaf symptoms include interveinal and edge leaf scorch with a yellow border/transition between the brown and green tissue. The yellow border is the defining symptom to separate from simple drought stress (Figure 1).
  • A ‘matchsticks’ symptom is created where the leaves have dropped but petioles remain attached to the shoot (Figure 2).
  • The peduncle of the berry on the rachis might also show a red coloration on certain cultivars (Figure 3).
  • Maturing fruit clusters may color and shrivel prematurely; this may occur in newly infected vines even before foliar symptoms are observed (Figure 4).
  • Depending on duration and level of infection, productivity can range from reduced yield with poor fruit quality to no production at all.

Incidence and Temperature Threshold for PD

  • Traditionally locations that experienced freeze events were not conducive for the survival of the pathogen, pointing to a low-temperature thresholds for the risk of PD.
  • Research has shown that winter temperatures at or below 10ºF for two to three days were required for a vineyard to be considered at low-risk for Pierce’s disease. However, overwinter curing are both temperature and cultivar dependent.
  • Risk for perennial damage and vine death is much more likely in Southern Maryland, followed by the Eastern Shore, and less in the Piedmont and west.
  • A recent survey of 8 Maryland Commercial vineyards found symptomatic vines that tested positive in all 8 locations. 

Scouting and Managing PD

  • Grape varieties vary in their response to PD, and in general the vinifera will be more susceptible or intolerant.
  • Some American cultivars, or cultivars with large portions of their genetic background comprised of native American species, will typically be more resistant or tolerant to Pierce’s disease. Tolerant vines can be infected with the bacterium but remain symptomless or only slightly affected.
  • Examples of tolerant/resistant cultivars are ‘Black Spanish’ (syn. ‘Lenoir’), ‘Herbemont’, ‘Villard Blanc’ (S.V. 12-375), and ‘Blanc du Bois.’ ‘Victoria Red’ is a tolerant seeded table grape.
  • The grape breeding program at UC Davis has recently released PD resistant cultivars, including ‘Camminare’, ‘Passeante Noir’, ’Errante Noir ‘, ‘Ambulo Blanc’, and ‘Caminante Blanc’.
  • Always plant certified clean stock form a reputable nursery.
  • Infected vines should be marked and removed.
  • In high risk vineyards with a high percentage of infected PD vines,  a systemic insecticide may be necessary to reduce secondary spread of the pathogen between vines.
    • Monitor for insect presence just before budbreak with double-sided yellow sticky traps.
    • Chemical control targeting glassy-winged sharpshooters (leafhoppers) in areas of high risk (where the disease overwinters every year) may include soil applications of systemic neonicotinoid products (e.g., Assail, Belay, Admire Pro, Venom, Platinum) at budbreak.
    • When vectors become active, utilize foliar applied neonicotinoid products (most of the same ones as the soil applications), and/or Sivanto (butenolide) or Danitol (pyrethroid).
    • Surround® may sabotage GWSS preference for grapevines and increase GWSS mortality.
  • Because of the wide host range and number of vectors, a sound weed control program should be practiced in (and possibly around) the vineyard.
  • Because the causal organism is bacterium, is transmitted by insect vectors, and remains systemic in the vines, conventional chemicals are generally not effective.
  • However, novel and promising technologies are underway, bringing some hope for the PD.
    • XylPhi-PD™, a first-ever bacteriophage treatment for PD, developed by Texas A&M AgriLife Research and Otsuka Pharmaceutical.
    • This injectable organic remedy it is a precision treatment that uses viruses (bacteriophage) to infect and kill the bacterium itself within the xylem of the vines.
    • It has been approved by the EPA and is commercially available in California, Arizona and Texas, though it has not been labelled for use in Maryland.
    • Phage therapy and other novel approaches are still in their infancy as far as the plant disease treatment is concerned.  

For more information, contact Dr. Mengjun Hu at mjhu@umd.edu.

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