- Author: Joey Mayorquin, Mohamed Nouri, Florent Trouillas, Greg Douhan and Akif Eskalen
Recently, an outbreak of shoot and twig dieback disease of citrus has been occurring in the main citrus growing regions of the Central Valley of California (Fig 1). The causal agents of this disease were identified as species of Colletotrichum, which are well-known pathogens of citrus and other crops causing anthracnose diseases. At this time, it is unclear how wide-spread the disease is in California citrus orchards, but surveys are being conducted to evaluate the spread of this disease in orchards.
The disease was first noticed in 2012 by several growers and nurserymen in various orchards in the Central Valley. Symptoms included leaf chlorosis, crown thinning, gumming on twigs and shoot dieback, and in severe cases, branch dieback of trees (Fig.2). The most characteristic symptoms of this disease are the gum pockets which appear on young shoots either alone or in clusters and the dieback of twigs and shoots (Fig.3). These symptoms were primarily reported from clementine, mandarin, and navel orange varieties. In order to determine the main cause of this disease, field surveys were conducted in several orchards throughout the Central Valley. Isolations from symptomatic plant samples frequently yielded Colletotrichum species.
Field observations indicate that symptoms initially appear during the early summer months and continue to express until the early fall. Trees showing dieback and gumming symptoms characteristic of this disease are usually sporadic within an orchard and generally only a few twigs or shoots are affected within a tree. Morphological and molecular phylogenetic studies allowed the identification of two distinct species of Colletotrichum (Colletotrichum karstii and Colletotrichum gloeosporioides) associated with twig and shoot dieback. Interestingly, these Colletotrichum species were also isolated from cankers in larger branches. Although C. gloeosporioides is known to cause anthracnose on citrus, a post-harvest disease causing fruit decay, it has not been reported to cause shoot dieback of citrus. C. karstii however has not been reported previously from citrus in California and our laboratory is currently conducting field and green house studies to determine the pathogenicity of this species in citrus.
At present, it is unclear how widespread this disease is in California orchards or how many citrus varieties are susceptible to this disease. Pest control advisors are advised to remain alert and monitor citrus trees for the presence of the disease in the Central Valley (particularly clementine, mandarin, and navel varieties) during the early summer months. Continuing research lead by Dr. Akif Eskalen (UC Riverside) in collaboration with Dr. Florent Trouillas (Kearney Agricultural Research and Extension Center), Dr. Greg Douhan (UCCE Farm Advisor Tulare County), and Craig Kallsen (UCCE Farm Advisor in Kern County) is focused on further understanding the biology of the fungal pathogens as well as factors influencing disease expression in order to develop management strategies against this emerging disease.
Shoot dieback symptoms on Clementine
Branch dieback symptoms on Clementine
Gumming symptoms on Clementine
(photos: A. Eskalen)
- Author: Jeannette E. Warnert
Three citrus trees that produce inedible fruit at the UC Lindcove Research and Extension Center in Visalia may be a game-changer for the citrus industry, reported Ezra David Romero on Valley Public Radio.
The trees are thought to be resistant to huanglongbing, a severe disease of citrus that has devastated the Florida industry and could become a serious problem in California. The citrus-saving potential of the three 34-year-old trees was outlined in an article by UC Agriculture and Natural Resources writer Hazel White in the most recent issue of California Agriculture journal.
UC Riverside citrus breeder Mikeal Roose collected seed from the trees and will test seedlings as soon as they are large enough.
"So what (breeders) have to do is cross this with some edible varieties and eventually create something that has the gene for resistance, but also the genes for good fruit," said Beth Grafton-Cardwell, Lindcove director and research entomologist.
Huanglongbing disease has cut citrus production in Florida by more than half. It's been found in residential citrus trees in Southern California, but hasn't reached the state's vast commercial orchards yet. Grafton-Cardwell said she expects the disease will arrive in 4 or 5 years.
The rootstocks ‘Bitters', ‘Carpenter' and ‘Furr citrandarins were developed at the USDA Date and Citrus Station in Indio, California. Having mandarin genetics with different horticultural properties and being more tolerant of calcareous soils than some other commonly used rootstocks, their effect on ‘Pixie' mandarin is being evaluated. These three are being compared to the mildly dwarfing ‘C-35' rootstock and to the standard sized ‘Citrumelo' to see how their growth might be used to control tree size, also to see how well they do in an alkaline soil. In 2014, five of each of the rootstock/'Pixie' combinations were planted in randomized blocks at two different sites on mildly alkaline soils (pH 7.3 -7.8) in the Ojai, CA area. Trees were monitored for growth on a yearly basis. At both sites ‘Citrumelo' is the largest in height with the greatest shoot length. All three of citrandarins are smaller than ‘C-35” at both sites. Shoot length is the shortest for ‘Bitters', ‘Carpenter' and ‘C-35' at both sites. At the site with the highest soil pH (7.8), two of the five ‘Bitters' show iron and zinc chlorosis. The only trees to do so. This trial will be monitored for another five years to evaluate their performance. Growth characteristics on other varieties of citrus, such as orange and lemon will probably be the same.
Photo: A young Pixie on Bitters.
Topics in Subtropics
January - March Topics in Subtropics 2017
In this issue:
- Revisiting an old study on high density citrus orchards
- Shoot and Twig Dieback in Citrus
- Alternative Crops or ......
- Referendum Comments Citrus Research Board
A new ruling from CDFA to modify the announcement from March 2. Read on:
CDFA suspends enforcement of new citrus rule
The California Department of Food and Agriculture announced March 2 that it was suspending enforcement of a new regulation regarding transport of bulk citrus, the day after the new rule took effect.
The department said "the decision was made as a courtesy to the citrus industry to allow for additional time to prepare citrus operations to comply with the new rule, which requires all bulk citrus loads to be fully covered regardless of the origin or the destination. Additionally, CDFA is continuing to process and return compliance agreements."
Although enforcement has been suspended indefinitely, CDFA is expecting haulers to voluntarily abide by the new rule. Compliance can be achieved several ways, including but not limited to the use of a shipping container, tarp, enclosed vehicle (including curtain vans), or another method that completely covers bulk citrus during transport. If using a tarp, tarps must reach the bed of the truck.
From March 2, 2017
From AgNet West, http://agnetwest.com/2017/02/27/citrus-tarping-starts-march-1/
As of March 1, 2017, all citrus loads traveling throughout the state of California have to be tarped. This regulation aims to reduce the accidental transportation of the Asian citrus psyllid (ACP).
The Citrus Pest and Disease Prevention Program (CPDPP) is helping to get the word out about the new regulation. “That regulation is stating that every load, regardless of origin or destination, must be tarped,” CPDPP Grower Liaison Erin Betts said. “You don't have to use a tarp. You can use a van or something that is completely covered on all four sides, down to the bed.”
Counties north of the grapevine region have been noticing a trend with ACP finds in the area, and the new tarping regulation hopes to limit some of the accidental transportation the industry is seeing. “When we started having finds, they were all along the major transportation corridors,” Betts said. “So if we cover these loads that are coming and going from wherever, we are preventing that psyllid from hopping off … at a stop light or a stop sign in the middle of the Citrus Belt, Kern County or anywhere.”
All citrus loads being transported in California will now have to be fully covered by tarps. The state passed an emergency law that makes tarping mandatory in an attempt to reduce the accidental spread of the Asian citrus psyllid (ACP).
From the Citrus Pest and Disease Prevention Program (CPDPP):
The California Office of Administrative Law approved an emergency rule that requires all bulk citrus loads to be fully tarped during transport regardless of where the load originates from or its destination. The statewide mandatory tarping regulation is in response to a recommendation from the Citrus Pest and Disease Prevention Committee (CPDPC) to prevent the spread of the Asian citrus psyllid. The California Department of Food and Agriculture will begin contacting growers, haulers and packers to re-sign compliance agreements that include the tarping requirement. These entities are urged to begin preparations now while they wait to receive new compliance agreements.
Why did the Citrus Pest & Disease Prevention Committee ask for a mandatory tarping regulation?
The statewide mandatory tarping regulation is a preventive action to address the spread of the Asian citrus psyllid, and is in response to an analysis conducted by the University of California that looked at Asian citrus psyllid find patterns along transportation corridors. As the number of psyllid finds increase in commercial citrus regions and along major transportation corridors, the CPDPC felt the action was a necessary step to help prevent the spread of Huanglongbing – the deadly citrus tree disease that the psyllid can carry.
What are the regulatory requirements?
The new requirement is a statewide regulation that restricts the movement of regulated articles from “or within” a quarantine area. Revised compliance agreement exhibits will require all bulk citrus loads to be fully tarped regardless of where the load originates from or its destination, even loads that are traveling within a county. Specifics of the requirements will be released soon. In the meantime, producers can review USDA's tarping compliance requirements for general guidelines. Read the full press release from the CPDPP.
New tarping rules are in effect for California citrus. The industry must comply, or it will face costly penalties. Tarping fines could add up to $10,000.
A new regulation requires citrus loads to be tarped or fully enclosed as they travel through the state. “There were new compliance agreements that were mailed out,” Citrus Pest and Disease Prevention Program Grower Liaison Erin Betts said. “The new regulation states that every load, regardless of origin or destination, must be tarped.”
Betts said not complying with the new rules can be very costly. “The fine is not only for if you don't have your load tarped properly, but also if you do not have your compliance agreement with you,” Betts said. “That violation could be up to $10,000.”
According to Betts, the industry has many ways to make sure they are properly complying with the regulation. “(The industry) can contact the county, local grower liaisons and also the California Department of Food and Agriculture.”