- Author: Elizabth Fichtner
- Author: Dani Lightle
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.
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.
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.
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.
Figures. The glassy-winged sharpshooter Homalodisca vitripennis, is a known vector of Xylella fastidiosa. The winged sharpshooter can reproduce and overwinter on California olives. In southern California landscapes, olives exhibited dieback and leaf scorch. Six strains of Xylella fastidiosa subspecies multiplexwere isolated into pure culture from symptomatic olives trees in southern California. Photos: R. Krugner
Images of the pest and disease are in the attached files below. Click on the link to see them.
California Drought Watch
Although water is in short supply, UC Davis' expertise in water-related issues is not. California Drought Watch brings together the university's globally renowned resources in water sciences, management, law and agriculture to ensure that policymakers and the general public have access to the knowledge, research and technologies that are crucial to addressing the challenge of the state's drought.
Visitors to the site will find the latest drought research and news headlines, as well as drought management tools from UC Davis experts and water organizations throughout the state. They will discover how UC Davis is cutting back on water consumption, learn about upcoming events like the April 25 Drought Summit, and find out how to reach UC Davis' top experts in drought and water management.
“Policymakers and the public need to see, not just be told, that UC Davis is a go-to place on drought,” said Jay Lund, director of the UC Davis Center for Watershed Sciences. “This site should help draw tighter the wide range of work done on drought and related subjects from all across the campus.”
Farmer and Rancher Voices from the Drought
Over the sound of bleating sheep and singing birds, Dan Macon describes how the current drought has affected his sheep ranch, Flying Mule Farm, in the Sierra Nevada foothills: “This is the driest of three dry years in a row for us … Over the last three years, we've reduced the size of our flock by half. Because we've reduced the size of our flock, I have taken a part-time job and will probably take a second part-time job this spring.”
His is one of several audio stories compiled as part of Farmer and Rancher Voices from the Drought.
A team of UC researchers began the project this spring. Led by Ken Tate, UC Davis professor of plant sciences and rangeland watershed specialist in Cooperative Extension, they use digital tools to capture the voices of farming and ranching families who are battling the worst drought of their careers.
Each week, the team posts new tracks to SoundCloud — an audio-streaming service and social network where users can upload, record and share tracks they create. Friends, loved ones and colleagues of ranchers and farmers are also invited to interview them and post tracks to the SoundCloud group page. Several of the farmers and ranchers share practices that have worked for them so that others struggling with the drought can better cope.
Members of the state's farming and ranching community also share stories, photos and comments at the project's Facebook group page.
The project was inspired by Caroline Henderson's “Letters from the Dust Bowl,” which documented the voices of farmers and ranchers leaving dust-covered Oklahoma in the 1930s in search of a better life in California.
Drought summit on April 25
Upcoming events: UC Drought Summit The Center for Watershed Sciences is organizing faculty from all UC campuses and other California universities to present a daylong Drought Science, Policy and Management Summit on April 25, 2014 in Assembly Committee Room 4202 at the state Capitol. Video recordings of presentations will be available soon after the summit at http://www.calchannel.com.Climate Change: Challenges to California's Agriculture and Natural Resources Monday, May 19, 2014 The California Museum 1020 O Street Sacramento, CA ------------------------------------------------------------------------------------------------------------------------
Past events: Labor, Water, and California Agriculture in 2014 Friday, April 18, 2014 UC Davis Law School Click for AIC Director Dan Sumner's slide presentation
------------------------------------------------------------------------------------------------------------------------Media articles:- UC Davis experts: Sources on California's drought
- California Seeing Brown Where Green Used to Be- California Farmers Brace for Drought, Unemployment - Calif. Rancher Still Optimistic Amid Record Drought - Californians brace for year of 'mega-drought' - California farmers brace for drought, unemployment - California Drought Impacts Produce Departments. - California Drought And The U.S. Food Supply ------------------------------------------------------------------------------------------------------------------------/h1>
A recent trip to Spain was an opportunity to look at their cherimoya production practices. One of the most interesting is their ability to manage the tree through pruning to produce fruit off-season (in spring) when the prices are the highest. IN California our low period of production is in the summer. The climate in Spain along the Mediterranean coast is warmer and more humid than coastal California, so most tree crops are about two months advanced in their production. So in the text I refer to a period when something is done and then follow it with another date. The one in parenthesis is the probable time in California if the date in Spain is used. So, to produce fruit in spring (summer) in March/April when prices are high:
Remove all shoots from the previous year in March (May)
With the new shoots, prune them back 6 inches in length around July 15 (September 15)
Pollinate the flowers that are produced in the period of August to September (Sept/Nov)
Pick fruit in March/April (June/Aug)
Fruit is produced when prices are higher
Generally fewer seeds than at other periods
In some cases there is higher sugar content in the off-season frui
Not always consistent with all cultivars
Off-season fruit often has black spots in the pulp
May see increased leaf drop
In some cultivars, the skin is more prone to abrasion, and this is already a very delicate fruit
There are other fruit species that fruiting date can be manipulated by pruning, such as evergreen blueberries, guava, lime, mango and carambola (star fruit). Always it is to find a better market for the fruit.