- 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
- Author: Jim Downer
All the media are full of stories about the impending El Nino effects and possible flooding rains in California. As I write this, it is the day before our first significant storm (Jan. 4) with a 100% chance of rain throughout Ventura county for tomorrow. Still long range forecasts for Jan and Feb show near normal or less than normal rainfall possibilities for the Avocado growing areas of California. The “real rain”, they say, will come in March. Although nobody can accurately predict the occurrence of rain, we are certainly due for an increased rain year based on statistical models and estimates of the El Nino effect by NOAA, JPL and NASA. If heavy rains are in the near future, nurserymen and growers will need to take measures to protect their operations from the many and disastrous effects of downpours. Physical displacement of soil as erosion and loss of soil are common during heavy rains. While runoff water should not leave agricultural properties, it has to move somewhere when rains come, so ditches should be cleared of weeds and other obstructions to permit efficient flow of water. Most operations will have already done this by now. So what can be expected when the rains come and after they leave? It will likely be a banner year for diseases both biotic and abiotic.
Many pathogens are splash inoculated from plant to plant or soil to plant, so it is imperative to prevent the development of flooded or puddled ground near growing areas. Now or between storms would be a great time to lay down additional gravel under container beds or other outside nursery areas. Keeping containers off soil, either with a gravel or fiber mat and gravel system is imperative when trying to control Phytophthora in nurseries. Compacted walkways and beds may become saturated this winter and create ideal sporulation conditions for Oomycetes or water molds which may then move in water flows to new areas of the nursery causing infections where never seen before. Consider boardwalks or additional gravel in known low spots and walkways so that workers don't move infested mud from one part of a nursery to another. For citrus and avocado growers it is vital to give water a place to go on flat or low lying areas. When we get the expected deluges, trees can suffocate from extended soil saturation and defoliate rapidly due to anoxic conditions.
For Phytophthora sensitive crops, it may be wise to increase the calcium by adding additional gypsum now to reduce sporulation and potential spread of disease. It is also wise to use preventative fungicides such as mefenoxam, and phosphorous acid to increase plant readiness for Phytophthora increases following wet weather.
This is also a great time for woody plant growers to prune any diseased or dead materials from plants ahead of winter rains because many Ascomycete canker fungi that cause disease in woody plants will have inoculum in dead twigs. This has been a banner year for Botryosphaeira fungi which have caused canker diseases in citrus, avocado and ornamentals at record levels due to drought stress. When rain comes, spores are splashed to new plants and can cause new infections. Since this is an El Nino year, it is warm, and warm rains are best for disease promotion. Remove inoculum now, cull and remove weak, diseased or dead plants ahead of the rains to cut down on disease spread. Even though it may be inappropriate to plant some crops now, it is always a good time to remove weak trees, plow diseased row crops and chip up the waste to be used as mulch.
With rains often come strong winds. Greenhouse and tunnel growers should consider the effects of wind this winter on their operations and possible crop loss from this damage. Tunnel houses used in berry and other production are at risk but other greenhouse materials such as polycarbonate sheeting can be detached by wind. Now is a great time to inspect and repair these structures or apply new sheeting as necessary. Wind can also move woody plants to rub against each other, causing injury to the main stem or fruit if tightly spaced. Trees that are blown over due to high winds can be damaged and devalued. Spend time now inspecting trellis systems and staking of woody plants to minimize damage that may be coming.
Outdoor nurseries that have planting media storage piles should start now to downsize these piles or provide new tarps in advance of wet weather. Greenhouse operations with media stored outside should ensure that bales are properly covered with new tarps to prevent saturation of the media. Media bales should be stored off the ground on raised pallets to avoid contamination with soil or mud flows.
The challenge of a wet and potentially stormy winter is to envision what excess water can do in your operation and then try to prepare. Flooding conditions create a time of potential pathogen movement and the best protection for plants is to keep them elevated above the mud and keep workers from spreading it with the movement of machinery or foot traffic. It is also useful to imagine invasion of soil from adjacent land owners who may have diseases or weeds not on your own property. Money spent now on infrastructure will prevent disease loss later this spring or summer.
- Author: Ben Faber
The disease pyramid describes how disease can eventually destroy a plant.
It is comprised of the presents of the pathogen that causes the disease, the plant or host, the environmental conditions that sets up the pathogen to go after the plant and time. It requires all four at the same time to have a disease. So in the case of avocado root rot, it's necessary to have Phytophthora cinnamomi there with a susceptible avocado root, in warm, moist conditions for a period of time for the disease to express itself. For a young tree, the disease may show up within a year but for older trees it may take several years to see the disease symptoms. For avocado crown rot, it may take several years for Phytophthora citricola to appear. In the case of Huanglongbing, not only is the pathogen required, but also the vector for spreading it, the Asian Citrus Psyllid. At this point in California, we have a rapidly spreading vector, but as yet, it appears the psyllid is not infected with the bacteria causing the citrus disease.
Not only time is important, but timing of sampling and point of sampling are important. In the case of avocado root rot, the best sample if from a recently infected root in the spring or summer. A thoroughly destroyed root taken in the winter will often give a negative response. The pathogen has done its ravages and is no longer active. If the wrong leaf or root is sampled for the Huanglongbing bacteria, the assay will show a negative response, even though it is in the plant. (This may have something to do with Einstein's Theory of Relativity in this 100th year of pronouncement, but I doubt it).
And then too, even if a disease is finally diagnosed, it doesn't mean the tree is going to die. If Phytophthora is caught soon enough, it can be treated with phosphites, but the environment needs to be changed at the same time. Meaning, the irrigation needs to be adjusted so that the treatment will work. If the irrigation is changed, the trunk canker will disappear.
It also means that all those terrible disease out there that threatened our trees in other countries won't hurt ours. We have a different environment from Thailand, India, Brazil, etc. and so even though there may be devastating disease there, since we don't have the environment, we wont get those diseases. Yeah, we are going to get huanglongbing and polyphagous shot hole borer fusarium, but we should be happy we aren't getting all the other problems other countries have. This is a way of saying, happy New Year. It could be a whole lot worse and we are doing pretty well.
- Author: Ben Faber
When reviewing possible problems your citrus might have, it's easy to jump to the conclusion that it is a virus. That's because viruses are a major problem around the world in citrus and the effects can be slow, chronic and debilitating or fast and deadly. Images get posted on the web, and if those symptoms look like something your tree has, then by golly you have a virus. Well, actually viruses are everywhere and in most plants, so you probably do have a virus or viruses, but not plant debilitating one. California, has had a pretty thorough nursery inspection procedure in place for many years and the likelihood of a virus causing a problem is less likely here than in many parts of the world.
In most cases viruses are difficult to eradicate in practice, so it is best to remove them before they get out in the field. The Citrus Clonal Protection Program (http://www.ccnb.info/page.php?s=2&c=3) weeds out citrus viruses before they get to wholesale nurseries and into the trade. That does not mean that we don‘t have debilitating viruses in the California industry. We do. Tristeza is in some of our orange orchards and that can lead to significant yield reductions and tree death (http://www.ipm.ucdavis.edu/PMG/r107101311.html.). Tristeza is spread by the melon aphid and is hard to control without good control of the aphid. In many older orchards there is exocortis and psorosis http://www.ipm.ucdavis.edu/PMG/r107100100.html; http://www.ipm.ucdavis.edu/PMG/r107100511.html). These are graft transmissible and why it is not good, in fact unlawful, to propagate trees with uncertified budwood.
In most cases in California if you are having symptoms of unhealthy in your trees it's most likely due to an irrigation problem (too much, too little, poor timing), a nutrient deficiency and possibly a fungal disease (most likely a root one such as armillaria or Phytophthora). Or in this day, it could be the start of Huanglongbing vectored by Asian Citrus Psyllid (http://www.ipm.ucdavis.edu/PMG/r107304411.html). Before jumping to the conclusion that there is a virus in your trees. Check out the most common problems for California citrus first (http://www.ipm.ucdavis.edu/PMG/C107/m107bpleaftwigdis.html). There are enough of those anyway.
Boron toxicity
- Author: Ben Faber
Here are the top 10 blogs from Topics in Subtropics over the last 5 years. The two earliest and latest pertain to how you select products that will truly help you from amongst all those that are being promoted. Some of these have been around a long time, so that's why they are the most read. Do a Ctrl + click on the URL to read the post.
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A predatory mite
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