Recent advances in understanding the history of olive domestication

Elizabeth Fichtner, Farm Advisor, UCCE Tulare and Kings Counties                              

Olives are thought to have first been domesticated in the northeastern Levant, an area near the border of present-day   Turkey and Syria. Map captured from Google Maps.

   With the emergence of the California olive oil industry, the state has witnessed a dramatic diversification in the olive cultivars grown commercially. Our mainstay black ripe olive industry, dominated by the ‘Manzanillo' olive, is now combined with increasing acreage of Spanish, Greek, and Italian cultivars used to create high quality, extra virgin oil. The historic table olive industry of California still represents around 18,000 acres of olives in the state, while approximately 40,000 acres are currently devoted to oil production.

Although olive cultivation in California is relatively new (dating back to the historic Spanish Missions established by Franciscan priests), olives are of key importance in the history and culture of the Mediterranean basin. A recent publication by a group of European, American, and North African scientists has re-evaluated the location of the domestication of the olive, providing genetic evidence that domestication occurred in the northeastern Levant, close to the present-day border of Syria and Turkey.              

To complete the study, researchers collected plant material from nearly 2000 trees, sampling both wild oleaster populations  and domesticated cultivars of olive. World Olive Germplasm Banks in Córdoba (Spain) and Marrakech (Morocco) served as sources of the majority of cultivars included in the study. Researchers utilized the genetic sequences of plastids (ie. chloroplasts) to discern differences between cultivars and wild oleaster populations. Plastids are organelles (structures inside cells) that contain their own DNA. Since plastids are generally inherited from one parent (similar to mitochondria), their genetic sequences are more conserved then that of nuclear DNA, which is contributed by both parents. Since olive is a wind-pollinated crop, nuclear DNA may be disseminated over large distances.

The genetic analysis of wild populations indicates three distinct lineages of olive: the Near East (including Cyprus), the Agean area, and the Straight of Gibralter. These three wild populations are likely linked to refuge areas where populations persisted through historic glaciation events. Interestingly, the geographic distribution of these three populations also corresponds to the subdivisions of the olive fruit fly, suggesting that these regions offered shared refuge habitat for both the host and the pest. The wild oleaster population in the eastern Mediterranean was found to be more diverse than previously thought and ninety percent of the present-day cultivars analyzed in the study matched this group. Common olive cultivars grown in California, including, Sevillano, Arbosana, Arbequina, and Koroneiki, all belong to this group originating in the eastern Mediterranean.

As a result of this study, it is proposed that the initial domestication of olive took place in the northeastern Levant; subsequently, plant material was disseminated to the whole Levant and Cyprus before being spread to the western Mediterranean. After these initial domesticated trees spread throughout the Mediterranean basin, they likely underwent subsequent domestication events by crossing with wild oleasters, thus introducing genetic material from the other two ancient western Mediterranean lineages.

Such studies may appear purely academic; however, they can also address more timely questions and assist in characterizing cultivars. For example, a 2010 study in California made genotypic comparisons between historic olive plantings in Santa Barbara, CA and at Santa Cruz Island, CA. The study elucidated that the olives on Santa Cruz Island, planted in the late 19^th century are different than other historic olive plantings in Santa Barbara, CA. Olives planted at the Santa Barbara Mission in the late 18^th century are the ‘Mission' cultivar, whereas those on Santa Cruz Island (Figure 3) are generally ‘Redding Picholine.' Interestingly, the olives on Santa Cruz Island are thought to have been planted for oil production, but there are no historic reports of harvest or sale of a crop. Additionally, the Santa Cruz Island olives have become somewhat invasive on the island due to their propensity to establish from seed. As a result of genotypic analysis of these populations and the fact that ‘Picholine' makes an excellent rootstock due to its ease of propagation from seed, it is hypothesized that the ‘Picholine' variety was intended as a rootstock, but the grafts never took. Consequently, maturation of a ‘Picholine' orchard may have just been an accident, a mistake, or simply bad luck. The completion of this local population genetics study may have helped explain the unsolved mystery of the historically unharvested trees on Santa Cruz Island.

Find Santa Cruz Island.

Besnard, G., Khadari, B., Navascués, M., Fernández-Mazuecos, El Bakkali, A., Arrigo, N., Baali-Cherif, D., Brunini-Bronzini de Caraffa, V., Santoni, S., Vargas, P., Savolainen, V. 2013. The complex history of the olive tree: from Late Quaternary diversification of Mediterranean lineages to primary domestication in the northern Levant. Proc R Soc B. 280: 20122833.

Soleri, D., Koehmstedt, A., Aradhya, M.K., Polito, V., Pinney, K. 2010. Comparing the historic olive trees (Olea europaea L.) of Santa Cruz Island with contemporaneous trees in the Santa Barbara, CA area: a case study of diversity and structure in an introduced agricultural species conserved in situ. Genet Resour Crop Evol 57:973-984.

The latest edition of Topics in Subtropics newsletter is out, Elizabeth Fichtner as editor.  Read on.

TOPICS IN THIS ISSUE:

• Why has California red scale been so difficult to control?

• Navel Orange Nitrogen Fertilization

• Recent Advances in Understanding the History of Olive Domestication

• Upcoming UC Olive Center Events

Elizabeth Fichtner¹, Dani Lightle², Dan Flynn³, Rodrigo Krugner⁴

UC Cooperative Extension (UCCE) Tulare¹, Kings¹, Glenn², Tehama², and Butte² Counties, UC Davis Olive Center³, USDA, Agricultural Research Service, San Joaquin Valley Agricultural Sciences Center⁴

The recent outbreak of the plant bacterium Xylella fastidiosa in southern Italy illustrates the potential for introduced organisms to incite widespread havoc in a short time. Xylella fastidiosa has been found in association with a new disease called olive quick decline syndrome (OQDS), which is affecting over 20,000 acres of olive in the Apulia region of Italy. Symptoms of OQDS include extensive branch and twig dieback, yellow and brown lesions on leaf tips and margins, vascular discoloration, and subsequent tree mortality. In southern Italy, the main insect vector of X. fastidiosa is the spittlebug Philaenus spumarius, which is known to occur in California. While research is underway to determine if the bacterium is responsible for causing OQDS, establishment of X. fastidiosa in Europe leaves growers and researchers asking where it came from. Genetic techniques used to compare pathogen populations have led researchers to hypothesize that the isolates of X. fastidiosa (strain CoDiRO) associated with OQDS, may have originated in Costa Rica. Marking the first European find of X. fastidiosa outside of Italy, the bacterium also was found on ornamental coffee plants at a retail market near Paris in April 2015; the plants had been imported to Europe from Honduras (3). In July 2015, X. fastidiosa was identified on ornamental plants on the French island of Corsica and on the mainland in October 2015 (2). The X. fastidiosa subspecies found in Corsica (subspecies multiplex) is different from that associated with OQDS in Italy (subspecies pauca) (2).

The European Commission has imposed several regulatory actions to prevent further spread and introduction of X. fastidiosa into the European Union, including a specific ban on import of coffee plants from Honduras and Costa Rica; eradication measures in Italy and France; and the potential for strict eradication measures aimed at new outbreaks or finds of the bacterium, including removal and destruction of infected plants, and all host plants within a radius of 100 m. (1)

The new introductions of X. fastidiosa to Europe illustrate the potential for long distance dispersal of the bacterium and a vulnerability of California agriculture to invasion by new organisms. The United States Department of Agriculture-Animal and Plant Health Inspection Service-Plant Protection and Quarantine's (USDA-APHIS-PPQ) New Pest Advisory Group (NPAG) is assessing the biology and potential economic and environmental impacts that the strain CoDiRO may pose to the United States to recommend potential regulatory strategies (Tara Holz, personal communication).  NPAG is designed to inform Federal decision makers regarding potential regulatory actions that may be appropriate to prevent pest introduction.Previous USDA-led research has found X. fastidiosa in California olive trees, but the endemic bacterium has only limited association with disease and is a different subspecies than the CoRiDO strain associated with the OQDS in Italy.

California olive growers and industry stakeholders are encouraged to contact UCCE Farm Advisors to report trees displaying symptoms of OQDS.

References

1. European Commission Press Release, April 28, 2015: http://europa.eu/rapid/press-release_IP-15-4887_en.htm

2. European and Mediterranean Plant Protection Organization. 2015. First reports of Xylella fastidiosa in EPPO region-Special Alert. http://www.eppo.int/QUARANTINE/special_topics/Xylella_fastidiosa/Xylella_fastidiosa.htm

3. French Ministry of Agriculture Food and Safety Press Release, April 29, 2015: http://translate.google.com/translate?hl=en&sl=fr&tl=en&u=http%3A%2F%2Fagriculture.gouv.fr%2Fstephane-le-foll-salue-la-mise-en-place-de-mesures-europeennes-de-prevention-contre-la-bacterie-xylella

Elizabeth Fichtner and Dani Lightle, Farm Advisors, UCCE Cooperative Extension                 

   The report of a new disease on olive in Italy, called “quick decline,” marks the first report of the bacterial pathogen, Xylella fastidiosa, in Europe. This pathogen is not new to the Americas and has been in California for over 100 years. It is perhaps best known as the cause of Pierce's Disease on grape, but also causes citrus variegated chlorosis, peach phony disease, alfalfa dwarf, and scorch on almond, oleander, and pecan. In response to scorch and dieback symptoms (Figure 1 A-C) on landscape and orchard plantings of olives in California, Dr. Rodrigo Krugner, an entomologist with the USDA ARS in Parlier, CA, established a research program to investigate the epidemiology of X. fastidiosa on California olives.

The Pathogen

X. fastidiosa is a gram-negative, xylem-limited bacterium affecting over 100 known plant hosts. The pathogen multiplies within the xylem and is thought to cause disease by interfering with water and nutrient transport. It is spread naturally from plant to plant by xylem-fluid feeding insects. The pathogen is difficult to culture (Figure 1D); consequently, prompt identification often relies on use of PCR techniques that detect pathogen DNA in plant tissues.

The pathogen may be grouped into subspecies based on host specificity. For example, X. fastidiosa subsp. fastidiosa causes Pierce's disease on grapevine as well as scorch on almond; however, the X. fastidiosa subsp. multiplex, causes disease on almond but not on grapevine.

Vectors associated with X. fastidiosa in California

X. fastidiosa is transmitted by xylem-fluid feeding insects, such as spittlebugs, froghoppers, and sharpshooters. While many of these insects may have the potential to transmit X. fastidiosa, there are four sharpshooter species in California that are recognized to have the greatest role in X. fastidiosa spread. Three of these sharpshooters are native to California and present throughout the state: red-headed sharpshooter, blue-green sharpshooter, and green sharpshooter. The last vector is the invasive glassy-winged sharpshooter (Figure 2A), which became established in southern California in 1990 and is responsible for the rapid spread of X. fastidiosa on grapevine.

Sharpshooters acquire X. fastidiosa when feeding on infected plant material. Once inside the vector's mouthparts, the bacterium multiplies rapidly and the insect is then capable of transmitting the bacterium for the remainder of its life (if it is an adult) or until it molts (if it is immature). Because sharpshooters are strong fliers and typically feed on multiple host plant species, X. fastidiosa may be spread to multiple hosts over the insects' lifetime.

‘Quick Decline' in Italy

In October 2013, X. fastidiosa was reported in the Puglia region of southern Italy, marking the first report of the pathogen in Europe. Characteristic symptoms included extensive leaf scorch and branch dieback, as well as discoloration of vasculature. Along with isolation of several putative fungal pathogens, presence of X. fastidiosa was confirmed by serological and PCR tests. Almond and oleander plants near the infected olives also tested positive for the pathogen. Scientists in Italy are currently surveying the area surrounding the outbreak and regulatory agencies have prohibited the movement of propagation materials from susceptible hosts out of the infected area. Additionally, researchers are working to determine the subspecies of X. fastidiosa associated with symptomatic olives and to obtain pure cultures of the pathogen for pathogenicity tests. Currently, the origin and strain(s) of X. fastidiosa introduced to Europe, as well as the insect species responsible for transmission, are unknown.

Association of X. fastidiosa with California olives

Leaf scorch and dieback symptoms have been observed in commercial olive orchards and landscape plantings (Figure 1 A and B) in California. Krugner's laboratory found that only 17% of the trees sampled tested positive for X. fastidiosa by PCR, with rates of pathogen detection higher in southern CA (39%) than in the Central Valley (2.5%). The pathogen was only successfully cultured from samples collected in southern California, suggesting that the pathogen population on olive is limited in the Central Valley. Reintroduction of the pathogen into multiple varieties of olive resulted in low levels of infection, and asymptomatic infections were common. Dr. Krugner's work also demonstrated that California strains of X. fastidiosa belong to the multiplex subspecies, which is pathogenic on almond, but not grapevine. Consequently, California olives are not considered a source of inoculum for Pierce's Disease on grapevine; however, olives may harbor insect vectors (Figure 2B) responsible for transmission of the bacterium to grapes or other crops.

What does the “'Quick Decline” in Italy mean for California olive growers?

Dr. Krugner's work demonstrated low levels of pathogen recovery from olives in the Central Valley and minimal association of the pathogen with disease upon reintroduction to healthy plants. Further studies, however, are necessary to determine a) the subspecies responsible for the ‘quick decline' in Italy, and b) the pathogenicity of isolates recovered from symptomatic plants in Italy. It is possible that pathogen strains recovered in Italy may be different, and more aggressive on olive, than strains endemic in California. California olive growers and landscape managers should report new incidences of extensive dieback or scorch on olives to farm advisors.

Acknowledgements

The authors thank Dr. Rodrigo Krugner for his critical review of this article and for providing photographs. Dr. Krugner's research was supported by the California Olive Committee and the USDA Agricultural Research Service.

Select References

Carlucci, A., Lops, F., Marchi, G., Mugnai, L., Surico, G. 2013. Has Xylella fastidiosa “chosen” olive trees to establish in the Mediterranean basin? Phytopathologia Mediterranea. 52:541-544.

Krugner, R., Sisterson, M.S., Chen, J., Stenger, D.C., Johnson, M.W. 2014. Evaluation of olive as a host of Xylella fastidiosa and associated sharpshooter vectors. Plant Disease. 98: in press.