- 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: Ben Faber
Drought Induced Problems in Our Orchards
Abiotic disorders are plant problems that are non-infective. They are not caused by an organism, but through their damage, they may bring on damage caused by organisms. Think of a tree hit by lightning or a tractor. The damage breaches the protective bark which allows fungi to start working on the damaged area, eventually leading to a decayed trunk. It was the mechanical damage, though that set the process in motion.
Too much or too little water can also predispose a plant to disease. Think of Phytophthora root rot or even asphyxiation that can come from waterlogging or too frequent irrigations.
Salinity Effects from Lack of Water
Lack of water and especially sufficient rainfall can lead to salinity and specific salts like boron, sodium and chloride accumulating in the root zone. This happens from a lack of leaching that removes native soil salts from the root zone or the salts from the previous salt-laden irrigation from the root zone. These salts cause their own kind of damage, but they can also predispose a tree to disorders, disease and invertebrate (insect and mite) damage.
Lack of water and salt accumulation act in a similar fashion. Soil salt acts in competition with roots for water. The more soil salt, the harder a tree needs to pull on water to get what it needs. The first symptom of lack of water or salt accumulation may be an initial dropping of the leaves. If this condition is more persistent, though we start to see what is called “tip burn” or “salt damage”. Southern California is tremendously dependent on rainfall to clean up irrigation salts, and when rain is lacking, irrigation must be relied on to do the leaching
As the lack of leaching advances (lack of rainfall and sufficient irrigation leaching) the canopy thins from leaf drop, exposing fruit to sunburn and fruit shriveling.
Leaf drop and fruit shriveling in avocado.
In the case of sensitive citrus varieties like mandarins, water stress can lead to a pithy core with darker colored seeds, almost as if the fruit had matured too long on the tree.
Total salinity plays an important factor in plant disorder, but also the specific salts. These salts accumulate in the older leaves, and cause characteristic symptoms that are characteristic in most trees. Boron will appear on older leaves, causing an initial terminal yellowing in the leaf that gradually turns to a tip burn.
Often times it is hard to distinguish between chloride, sodium and total salinity damage. It is somewhat a moot point, since the method to control all of them is the same – increased leaching. There is no amendment or fertilizer that can be applied that will correct this problem. The damage symptoms do not go away until the leaf drops and a new one replaces it. By that time hopefully rain and/or a more efficient irrigation program has been put in place.
The Impact of Drought on Nutrient Deficiencies
Salinity and drought stress can also lead to mineral deficiencies. This is either due to the lack of water movement carrying nutrients or to direct completion for nutrients. A common deficiency for drought stressed plants is nitrogen deficiency from lack of water entraining that nutrient into the plant.
This usually starts out in the older tissue and gradually spreads to the younger tissue in more advanced cases.
The salts in the root zone can also lead to competition for uptake of other nutrients like calcium and potassium. Apples and tomatoes are famous for blossom end rot when calcium uptake is low, but we have also seen it in citrus. Low calcium in avocado, and many other fruits, leads to lower shelf life. Sodium and boron accumulation in the root zone can lead to induced calcium deficiencies and increased sodium can also further lead to potassium deficiencies. Leaching can help remove these competitive elements.
Drought Effect on Tree Disease
Drought and salt stress can also lead to disease, but in many cases once the problem has been dealt with the disease symptoms slowly disappear. They are secondary pathogens and unless it is a young tree (under three years of age) or one blighted with a more aggressive disease, the disease condition is not fatal. Often times, in the best of years, on hilly ground these diseases might be seen where water pressure is lowest or there are broken or clogged emitters. The symptoms are many – leaf blights, cankers, dieback, gummosis – but they are all caused by decomposing fungi that are found in the decaying material found in orchards, especially in the naturally occurring avocado mulch or artificially mulched orchards. Many of these fungi are related Botryosphaerias, but we once lumped then all under the fungus Dothiorella. These decay fungi will go to all manner of plant species, from citrus to roses to Brazilian pepper.
Another secondary pathogen that clears up as soon as the stress is relieved is bacterial canker in avocado. These ugly cankers form white crusted circles that ooze sap, but when the tree is healthy again, the cankers dry up with a little bark flap where the canker had been.
Drought Effect on Pests
Water/salt stress also makes trees more susceptible to insect and mite attack. Mites are often predated by predacious mites, and when there are dusty situations, they can't do their jobs efficiently and mites can get out of hand. Mite damage on leaves is often noted in well irrigated orchards along dusty picking rows
Many borers are attracted to water stressed trees and it is possible that the Polyphagous and Kuroshio Shot Hole Borers are more attracted to those trees.
And then we have conditions like Valencia rind stain that also appears in other citrus varieties. We know it will show up in water stressed trees, but we aren't sure what the mechanism that causes this rind breakdown just at color break. Could it be from thrips attracted to the stressed tree or a nutrient imbalance, it's not clear?
Water and salt stress can have all manner of effects on tree growth. It should lead to smaller trees, smaller crops and smaller fruit. The only way to manage this condition is through irrigation management. Using all the tools available, such as CIMIS, soil probes, soil sensors, your eyes, etc. and good quality available water are the way to improve management of the orchard to avoid these problems.
Scroll down for Images
Tip Burn, notice sun burn bottom right hand fruit
Endoxerosis with dried out core
Boron toxicity
Nitrogen deficiency
Blossom end rot
Potassium deficiency
Bot gumming in lemon
Black Streak in Avocado
Bacterial Canker
Citrus red mite
Polyphagous Shot Hole Borer damage on avocado
Valencia Rind Stain
- Author: Ben Faber
Recently some 'W. Murcott' mandarins were shown to me. Brown spots in the core of the fruit. Another problem caused by drought and lack of leaching rains? Endoxerosis, also called internal decline, dry core, yellow tip, dry and blossom end decline is often confused with Alternaria rot which frequently accompanies or follows it. Internal tissues back of the stylar end break down, dry and become pinkish or brownish in color. Gum commonly forms in the core and either in or nest to the rind. Green fruits lose luster and frequently but not always develop a yellow color in circular areas surround the stylar end. The cut fru9i shows the gummy pinkish to brownish mass of partially dried and collapsed tissue. Gumming may even extend into the twig bearing the affected fruit. When the fruit turns color, the malady is more difficult to detect without cutting.
The cause is believed to be related to water and the physiological conditions within the tree and fruit and temperature conditions in the air and soil influencing transpiration and water stress. It is suggested therefore that water conditions in the soil be kept as favorable for tree heath as possible and pick on time so that they are not over mature.
From: The Citrus Industry, Volume IV, Editor: Walter Reuther, UC Press
In other words, make sure to leach the root zone of accumulated salts from previous irrigations and pray for rain.
Craig Kallsen in Kern County says he often sees this in young mandarins especially on the south and west sides of the canopy, to the point that growers will not even bother to harvest this fruit until the trees are older. The fruit just transpires so much water when it's not shaded that the fruit just dries out.
If my Latin serves me right: endo - inside, xeric - dry. Dry Inside.
- Author: Ben Faber
Tada
There are some new pest management guidelines out from UC ANR. This is a free publication and you access it as a web page or downloadable PDF document.
These official UC-approved guidelines for pest monitoring techniques, pesticide use, and nonpesticide alternatives for agricultural crops are essential tools for anyone making pest management decisions in the field. This 124-page guideline covers citrus fruit.
A hard copy version of these guidelines can be purchased as Publication 3441P.
The PDF version of this publication is best viewed using the free Adobe® Acrobat® Reader. You can download a free copy of the Acrobat Reader from Adobe Systems Incorporated.
Some users have experienced problems using Preview with these documents; we recommend using the Adobe® Acrobat® Reader.
- Author: Elizabeth Fichtner
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