- Author: Ben Faber
Under our feet, in the soil, is a wealth of microbial activity. Just like humans have different metabolisms and food choices, so do those microbes. In fact, microbes play an important role in making nutrients available to plants.
A recent review paper from Xinda Lu and his team looks at different roles that various soil microbes have in soil's nitrogen cycle. Lu is a researcher at Massachusetts Institute of Technology.
According to Lu, "Soil microbes catalyze most of the transformations of soil nitrogen into plant-usable forms. Diverse microbes use different processes - and sometimes work together. Knowing the various styles of soil microbes, and linking microbes to specific soil processes, can be important knowledge for farmers."
Modern nitrogen fertilizers are applied in the form of ammonium. Through a biological process called nitrification, soil microbes convert ammonium to nitrates that plants can absorb. In order to be efficient at this process, microbes need oxygen. Researchers are studying nitrification because it can be linked to greenhouse gases and loss of fertilizer.
Although microbiologists have been studying the nitrogen cycle for over a century, not all steps were well understood. New microorganisms have recently been identified. A type of prokaryote (single-celled organism) called archaea has also been playing a role in nitrification.
Archaea are not technically soil bacteria, due to their structure. These are newly a newly classified group that really do some amazing things. There are many more archaea that contribute to nitrification in some soils than there are bacteria responsible for the same activity. Including the role of archaea in nitrification has broadened the understanding of scientists and researchers.
Researchers reviewed various studies of soil nitrification. This included the abundance of microbes in soil in relation to various environmental factors. Soil pH, temperature and the ratio of soil carbon to soil nitrogen were all compared to the number of microbes in each soil sample. Soil depth and other factors also influence microbe abundance.
Previous studies have shown, for example, that nitrification archaea are more abundant than bacteria in warmer temperatures. Other microbes thrive in lower temperatures.
Soil pH also influences how active soil microbes are in the nitrification process. Soil bacteria Nitrospira were more dominant in acidic soils, including forests and farm fields.
Researchers have also studied how various microbes "talk" to each other. This keeps the nitrification process running smoothly. Various mechanisms have been proposed, including cell signaling. The presence of nitric oxide in soils may enhance interactions between microbes.
Soil scientists are sure they have not found all the microbes that contribute to the vast array of services soils provide. Just as astronomers discover new stars in the sky as tools advance, so will soil microbiologists find new soil microbes. Some may be involved in nitrification.
Collecting and cataloging the type, abundance and location of soil microbes will continue to advance the knowledge we have about the soil nitrogen cycle.
Crenarchaeota are involved with nitrification in the soil. Here's a cell of this group infected by virus STSV1 observed under microscopy.
- 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.
- Author: Ramadugu, Keremane, Halbert, Duan, Roose, Stover and Lee
Citrus huanglongbing (HLB) is a destructive disease with no known cure. To identify sources of (HLB) resistance in the subfamily Aurantioideae to which citrus belongs, we conducted a six-year field trial under natural disease challenge conditions in an HLB endemic region. The study included 65 Citrus accessio9ns and 33 accessions belonging to 20 other closely related genera. For each accession, eight seedling trees were evaluated. Based on quantitative polymerase chain reaction analysis of the pathogen titers and disease symptoms, eight disease-response categories were identified. We report two immune, six resistant and 14 tolerant accessions. Resistance and tolerance observed in different accessions may be attributed to a multitude of factors, including psyllid colonization ability, absence of pathogen multiplication, transient replication of the bacterium, lack of paht\\thogen establishment in the plant, delayed infection, or recovery from infection. Most citrus cultivars were considered susceptible: 15 citrons, lemons and limes retained leaves in spite of the disease status. Resistance and high levels of field tolerance were observed in many non-citrus genera. Disease resistance/tolerance was observed in Australian citrus relative genera Eremocitrus and Microcitrus. which are sexually compatible with citrus and may be useful in future breeding trials to impart HLB resistance to cultivated citrus.
2016 The American Phytopathological Society, vol. 100, N0. 9, p 1858-1868
Photo: Starch Accumulation in Infected Leaves
- Author: Georgios Vidalakis and Greg Douhan
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.
- Author: Rock Christiano
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.