Posts Tagged: viroid

Protecting California Citrus

Department of Plant Pathology and Microbiology, University of California, Riverside, CA 92521-0122, USA. and University of California Cooperative Extension, Tulare County, Tulare, CA 93274-9537

 

The Citrus Clonal Protection Program (CCPP) has its roots in the 1930s, when Professor H. Fawcett of the University of California (UC), Citrus Experiment Station in Riverside, discovered the graft-transmissible and viral nature of the citrus psorosis disease. In 1956, following a request from the California citrus industry, UC Riverside established the “Citrus Variety Improvement Program” which in 1977 became the CCPP. Today, the CCPP stands as a cooperative program between the United States Department of Agriculture (USDA), the California Department of Food and Agriculture, and the citrus industry of California as represented by the California Citrus Nursery Board and the Citrus Research Board.

Since 2009, the CCPP has also been part of the National Clean Plant Network (NCPN) for specialty crops. The purpose of the CCPP is to provide a safe mechanism for the introduction into California of citrus varieties from any citrus-growing area of the world for research, variety improvement, or for use by the commercial industry of the state or any citrus hobbyist and enthusiast. This comprehensive mechanism includes disease diagnosis and pathogen elimination, followed by maintenance and distribution of true-to-type citrus propagative material. The potential problems resulting from the introduction of pathogens into a country or citrus area cannot be overemphasized. Likewise the need for pathogen-tested citrus propagative materials is recognized as basic to the establishment and maintenance of a sustainable and profitable citrus industry. The presence of graft-transmissible pathogens such as viruses, viroids or bacteria in citrus propagative materials can be deleterious to tree survival and fruit production for both existing and future citrus plantings.

Realizing that the availability of pathogen-tested, true-to-type propagative materials are critical for citrus and other vegetatively propagated crops, three USDA agencies (Animal and Plant Health Inspection Service, Agricultural Research Service, and National Institute for Food and Agriculture) came to an understanding in 2005 to create a national network to support the use of clean propagative materials. The NCPN, came into being in 2008 with the mission of "providing high quality asexually propagated plant material free of target plant pathogens and pests that cause economic loss.”

Incorporation of citrus into the NCPN began in 2007 and a charter was adopted in March, 2010 for a "Citrus Clean Plant Network" (CCPN). The CCPN currently has centers in California, Florida, Arizona, Texas, Louisiana, Alabama, Hawaii, Maryland, and Puerto Rico. In a typical year, NCPN Citrus centers conduct over 75,000 diagnostic tests, distribute over 600,000 pathogen-tested plant materials, perform therapeutics on hundreds of plants, and maintain hundreds of foundation plants.

NCPN Citrus has established and enhanced quarantine, germplasm, and extension and education programs in all of the major and minor citrus producing regions. This has facilitated the importation, testing, therapy, and release of pathogen-tested citrus to nurseries, growers, and the public both regionally and globally.

citrus psorosis

Discouraging citrus varieties smuggling by offering easy access to pathogen-tested budwood

Citrus Clonal Protection Program Lindcove Research & Extension Center, University of California

 

When I started working at the Citrus Clonal Protection Program (CCPP) in 2012 (Fig. 1), citrus budwood was distributed three times per year (i.e. January, June and September) and there was a minimum order limit for 36 buds per order. In July of 2013, CCPP began monthly budwood distribution and essentially removed the budwood order limit offering as little as one budstick (6 buds). This was a game changer. In the following three years of monthly budwood distribution, the amount of requested buds has increased by almost 50% (Fig. 2), and most importantly, the orders placed by citrus hobbyists has increased by almost 80%. Citrus hobbyists are growing citrus for non-commercial purposes. Many of the hobbyists have a small “citrus forest” in their backyards, typically of diverse varieties, and they are very proud of their trees. They are typically not interested in purchasing grafted citrus trees, they want to graft their own citrus trees. It is hard to understand the citrus hobbyists' deep affection for their trees! I grew up in a citrus family farm and for me, citrus was as any other crop, a plant for profit. However, after interacting with the citrus hobbyists over the past several years, I have gained a level of respect and understanding that the passion citrus enthusiast have.

I had always been taught that citrus hobbyists are a threat to the citrus industry and their capacity to propagate citrus should be limited or denied. This line of thinking has resulted in some citrus production areas to restrict citrus budwood access for non-commercial use. Today, I see that ignorance is the true threat to the citrus industry. People usually don't understand or comprehend that smuggling plants or plant parts can disseminate diseases and cause severe economic damage to the farmers. Restricting the desire to propagate a citrus variety may force someone to smuggle it. A sad example is Huanglongbing (HLB) in California. This imminent threat to the California citrus industry was first found in a back yard citrus tree that had 23 grafts of unknown budwood origin.

The California HLB/Asian Citrus Psyllid prevention campaign is doing an excellent job of educating the public. Also the University of California Cooperative Extension is teaching Master Gardeners about the dangers of smuggling plants. On the Internet, there are individuals, such as the pomologist-writer “Fruit Detective” and the citrus hobbyist-blogger “Fruitmentor”, educating people on the correct way to propagate citrus and providing information regarding the threat of importing budwood that that may contain pathogens that could effects citrus production locally. Thanks to this multi educational effort, many citrus hobbyists are now part of the solution and they are actively engaged in the effort to protect the California citrus.

CCPP has over 300 citrus varieties available to anyone interested in propagating citrus trees for commercial or personal use. Orders can be as small as 6 buds (one budstick) per variety at $ 4.50. Therefore, the CCPP offers the incentive to use inexpensive-easily purchased- tested budwood over smuggled or exchanged “dirty” citrus budwood.

Despite all of the above, I still strongly recommend to purchase grafted trees at local stores or online (e.g.

www.fourwindsgrowers.com). Grafting citrus is not as easy as it may look. It requires skill, another citrus tree to be used as a rootstock, and a controlled environment (especially

temperature and water). However, if you are going to do it yourself, make sure the material your using is disease free!

 

To learn more about the CCPP, go to www.ccpp.ucr.edu and remember: CCPP is the place for starting citrus correctly.

 

Panoramic view of the Citrus Clonal Protection Program foundation block operations at the Lindcove Research and Extension Center, Exeter, California. Photo: E. Grafton-Cardwell.

 

CCCP foundation block operations at Lindcove

Avocado Sunblotch Viroid testing by RT-Qpcr Grove Sampling Project

Avocado sunblotch viroid (ASBVd) is a small (247 nt) pathogen that causes discoloration and disfigurement of leaves and fruit. It is transmitted by grafting, pruning tools, pollen, seed, and root grafting. Trees may recover from the symptomatic expression of the viroid and become “symptomless carrier” trees. These trees maintain high levels of the viroid in leaves, fruit and seed, but do not express the characteristic sunblotch symptoms shown above. The viroid is also readily transmitted by the pollen of these trees. One characteristic of these trees is they typically have low fruit set. If such trees were used for the production of nurse seeds, widespread transmission of the viroid could occur.
Our laboratory has developed a real time quantitate RT-qPCR protocol for quickly and sensitively detecting ASBVd in both mature field trees and small greenhouse/nursery trees.
Details of sunblotch testing
Collection of tissue for ASBVd testing is as follows: All leaves should be hardened off and mature, but not excessively old or damaged by insects or wind. Ideally, each leaf should come from a separate branch to ensure a representative sample from around the canopy of the tree. Place leaves in plastic zip lock bags with NO WET TOWELS or other material.
For mature field trees the method is: Harvest 10 leaves from each of the 4 compass points. Place the 10 North and 10 East leaves in one bag, the 10 South and 10 West leaves in another bag, so you will have “sample 1-NE, and 1-SW”, etc., 2 bags for each tree. If you see symptomatic fruit, include the fruit and 2-3 leaves from directly behind that fruit in a separate bag.
For small nursery trees or newly planted field trees (2-4 ft tall, 1-4 branches): if there are only 1-2 shoots, 2-3 leaves from each plant can be collected (at least one leaf from each shoot); for slightly bigger trees, collect 6-8 leaves from each tree (2 leaves from each branch). It is very important to collect the most mature and fully expanded leaves from the small trees. Avoid the young expanding feathery leaves.
If any pruning or cutting tools are used in sample collecting they need to be disinfected for 2-5 minutes between trees with a freshly made (within 2 hours) 20% dilution of 5% household bleach (generally speaking: 2 parts bleach to 8 parts water), then rinsed with water before cutting the next tree.
The total number of leaves collected as above should be 20 or less per bag. Samples should be sent by overnight shipping and in the summer should be placed in styrofoam containers with reusable ice packs. Results are usually available within 10-14 days after receipt, depending on demand.
Contact Georgios Vidalakis (vidalg@ucr.edu, lab: 951-827-4932, office: 951-827-3763, cell: 951-237-9948) for more information and scheduling.

Typical symptoms of ASBVd on fruit and leaves (lower left branch)

 

sunblotch

sunblotch leaves

Read more