- Author: Sonia Rios
Photo: Sun Blotch Branch
Sun-blotch, a viroid disease affecting the avocado, was observed as early as 1914 in Southern California. Sun-blotch is widespread in California, and likely to be found in all districts where avocados are being grown. The main counties in which the disease is found are San Diego, Ventura, Los Angeles, Orange, Riverside, and Santa Barbara.
Symptom's and signs
Sunblotch causes a wide variety of symptoms or may exhibit no symptoms in some hosts. The rate of spread of sunblotch virus appears to be extremely slow due to improved cultural practices in the nursery and field, and the apparent absence of an insect vector. Symptoms of sunblotch include necrotic, red, yellow, or white discolorations on fruit, often in depressions or scars in the fruit surface. Twigs can develop narrow, necrotic, red or yellow streaks on their surface or in shallow lengthwise indentations along the twig. Leaves may have white or yellowish variegated areas and be deformed, but leaf symptoms are uncommon. Rectangular cracking and checking of the bark, called "alligator bark," often occurs on the trunk and larger branches. Infected trees may be stunted and have a disproportionate amount of horizontal growth or sprawling lateral low limbs. Trees with visible sunblotch symptoms often have reduced yields. Infected trees can also be symptomless, although large reductions in yield of previously vigorous trees may indicate the presence of the viroid in otherwise symptomless carriers.
Photo: Sunblotch fruit
Sunblotch is caused by dozens of variants of submicroscopic particles of genetic material (viroids, smaller than a virus with no protein coat and only nucleic acid) that alter development and growth of infected plants. Sunblotch viroid can move systemically within avocado, and it persists in host tissues. Trees that do not show symptoms even though the viroid is present are known as "symptomless carriers." Nearly all cuttings and seed from symptomless carriers will be infected with viroid. However, seedlings from symptomless carriers do not show symptoms of sunblotch when they are used as rootstocks, but the disease often appears on scions grafted to them. Conversely, most seed from trees with symptoms are not infected, and budwood and shoot cuttings from sunblotch leaf symptomatic trees often do not contain viroid. [BF1] The viroid is transmitted in pollen, but pollen only infects the fruit and seed produced from it. Unless a tree is infected by grafting or some way other than through pollen, there will be no viroid in budwood, root grafts, and shoot cuttings from that tree. [BF2] Transmission of the viroid most often occurs during grafting by using infected budwood or rootstock seedlings from infected trees with or without symptoms. Natural root-to-root grafts are important in transmitting sunblotch in groves. Mechanical transmission through wounds caused by contaminated harvest clippers, pruning tools, and injection equipment can also be important if infected trees are in the grove. [BF3] Spread via pollen from an infected tree to the flower ovule of a noninfected avocado, resulting in infected seed, can cause fruit to be culled, but does not further spread the disease unless seed is propagated. There is no evidence of insect transmission.
Management
Nursery
Careful propagation of nursery stock to eliminate viroid has greatly reduced sunblotch to a relatively minor disease. However, ongoing monitoring and management is required in nurseries and established groves. Sunblotch can be easily overlooked, and there are many ways that trees can become infected. Look for diseases and disease-promoting conditions regularly throughout the grove. In the nursery, carefully select disease-free scions and seed sources. Use stringent sanitation and frequent disinfection to avoid spreading pathogens. Periodically confirm that propagation sources are disease-free (indexing) by grafting propagative source material to young Mexican seedlings and observing leaves and twigs for sunblotch symptoms, or by performing a genetic test. Plant only indexed nursery stock that is registered as disease-free.
Field
Promptly remove symptomatic trees from the grove and chemically kill the stumps. Do not retain infected, symptomless trees just because yield does not seem to be affected; symptomless carriers are a highly infective source that can dramatically reduce yield on other trees. If only fruit and seed are infected (from infected pollen), it may not be necessary to remove that tree if indexing indicates the rest of the tree is not infected. However, trees with only fruit and seed infection indicate that other infected (possibly symptomless) trees nearby need to be indexed or removed. The danger of spreading viroid increases in established orchards where mature trees are pruned to reduce tree size and restimulate or maintain fruit production. Severe pruning of symptomless carriers, and perhaps other severe causes of tree stress, are suspected of causing viroid to become active in the new growth, inducing previously symptomless trees to exhibit symptoms. Disinfect pruning tools, harvest clippers, and injection equipment before beginning work on a new tree. Scrubbing tools clean and then soaking them in a 1.5% sodium hypochlorite solution is effective. Growers must use a registered disinfectant and follow label directions.
Photo: Sunblotch leaf
As sunblotch is systematic in the avocado tree, it can exist in the roots as well as the above-ground parts. Thus, it might be transmitted through natural root graftage between a healthy and diseased tree. It is known that tree roots will sometimes become forced together in some manner until finally a natural root graft has taken place. The natural grafting between roots of trees has been attributed to a number of causes, some of which are as follows: (1) Roots growing in a shallow soil underlaid by a hardpan when crowded for growing space will sometimes become grafted together. (2) Two roots growing between two rocks or other barriers when wedged together tightly may form a graft. (3) Roots from mature avocado trees which are planted close together in a grove may, if crowded for growing space, form a graft. It is believed, through evidence existing in certain avocado orchards, that sun-blotch disease has infected healthy trees through natural root graftage.
- Author: Ben Faber
Carbonateceous? Gypsum? Read on.
Soil pH or the “acidity” or lack of acidity of a soil can be confused by the different uses and chemistries that surround the term pH, or the power of hydrogen. This can be further confused by the terms “alkalinity” or “basicity” of the soil or the soil solution which can further confuse the situation by whether the solid or liquid phase of the soil is being measured. So, in short, a soil is acid if it has a pH below 7 (more hydrogen ions) and basic when above 7 (fewer hydrogen ions). Big numbers are more basic, small numbers more acidic.
The natural world has a pH scale of 1 to 14. Knowing soil pH is important because it can tell you the inherent fertility of a soil. Usually the higher the number between 5 and 8, the more “basic” nutrients are present, like calcium, magnesium and potassium. When the numbers get lower than 5 and larger than 8, the nutrients may not be there or they may be tied up. Changing the pH can often release nutrients that are not available. Iron and zinc plant deficiencies are most often controlled by soil pH and once pH is neutralized or made acid, the deficiency disappears. In a way, pH is one of the most important nutrient indicators of a soil's fertility and managing should be an essential practice.
Soils that have an elevated pH, those above 7, are usually dominated by carbonate and most commonly this is calcium carbonate. As a rock, we call this limestone which is derived from sea shells or coral. As a mineral, it is called calcite. And in various manipulated forms it is called,calx, lime, calcium hydroxide, calcium oxide,calcined lime, quicklime. Maybe other names, as well, depending on how it is made and used. Mixed with quartz sand it is made into glass. When an acid is added to calcium carbonate, like citric acid, sulfuric acid or rain water (yes pure rain water is acidic), the reaction gives off carbon dioxide and water. (When you respire, burning sugar which is a form of carbonate, you do the same thing, giving off CO2and H2O). When calcium oxide or lime is mixed with water and left to harden, it forms cement when it absorbs carbon dioxide from the air. So, lime, limestone and calcium carbonate are not very soluble. But it does dissolve. Calcium carbonate is a salt and dissociates into calcium ions and when surrounded by water molecules, the carbonate becomes bicarbonate ions. When the water dries up, the bicarbonate becomes carbonate again. A soils report may refer to this cement as “free” or “diffuse lime”, indicating that there is a lot present, and it may even be possible to see the old shells there.
A soil that is dominated by calcium carbonate is called a calcareous soil. It is the carbonate that defines the soil, it has an elevated pH, usually between 7.5 and 8, depending on other minerals in the soil (minerals are naturally occurring chemicals). A high pH leads to plant nutrition problems. It's not until the soil is acidified to drive off the carbonate as carbon dioxide that the pH will drop and the nutrient deficiency disappears.
One of the problems with the word “calcareous” is that it can be interpreted as meaning dominated by calcium. A soil or water analysis may reveal high levels of calcium and this can lead to concern. Calcium is a base cation, necessary for plant and human growth. In western states, water and soil tend to have calcium as the dominant cation, balancing anions like sulfate and carbonate and in some cases chloride. The important character to look for, though, is the carbonate or bicarbonate in the soil or water. It's not the calcium that is controlling pH, it's the carbonate. Very commonly calcium is said “to reduce the acidity of soil.” But it's not the calcium, it's the carbonate.
So, does this seem like a big semantic problem, how many dancing on the head of the pin sort of debate? No, because one of the most common recommendations for correcting an alkaline water/soil is to add gypsum – calcium sulfate. In a calcareous soil, calcium is already present, so adding more calcium is not going to change it, but rather increase it. Sulfate does not displace carbonate. Carbonate just stays there because calcium carbonate is not very soluble. Remember it is cement. Actually, adding gypsum is adding more salt which has its own problems - leading to saline soils.
So, this blog all came about because two different people asked me about correcting iron deficient avocado trees in calcareous soils (or a carbonateceous soil as I call it), with gypsum. pH correction is not going to occur with gypsum. An acid needs to be added to drive the carbonate off as carbon monoxide and then the pH comes down. So, you use acids or elemental sulfur that converts to sulfuric acid or urea-sulfuric acid fertilizer, or long-term use of acid fertilizers like ammonium sulfate or organic mulches that gradually create acid conditions.
Soil acidification as a practice is a separate subject all together that needs to be discussed, but gypsum is not normally a part of the process of field acidification. Does that mean gypsum does not have a place in soil management? It does, just not in the case of calcareous soils. And even though you can see bags of lime and dolomite (calcium-magnesium carbonate) on store shelves, for sure don't use that in the calcareous soils of California.
So, what about gypsum and sodic soils – soils dominated by sodium. And what about serpentine soils – soils dominated by magnesium? There is more to tell about gypsum.
Photo: old shells in the soil
- Author: Ben Faber
So where is the highest potential for avocado root rot in California? It turns out that the Natural Resource Conservation Service has taken the soils maps that have been generated for use by growers, engineers, planners and others over the years and used the data to rate soils for their sensitivity to root rot conditions. In many cases the county-based maps have been updated with new information, as well. It is now much easier to see where root rot would be more likely. The two major soil characteristics of soil texture and depth and how they affect drainage are the major parameters used to assess the root rot hazard. The soil surveys for years were only available in printed form, and then became available online about 2005. Just recently the added feature of identifying “root rot” soils was created. Now at the touch of a button, maps of where these soils are found are available. It looks like all California counties can be viewed from an avocado root rot hazard, even though avocados may not be grown in that climate, like Humboldt, but who really knows what can grow there. Soils that are conducive to Phytophthora root rot for avocados, would also be conducive for root rot in other plants species, as well. So, this information is helpful for identifying where rhododendrons or other susceptible species might have problems, as well.
Of course, this is just the first step in assessing the potential for root rot. Irrigation management is critical for creating root rot conditions that can occur even in soils that are not conducive to the disease. So, a soil identified as having a higher potential for root rot does not mean you cannot plant an avocado in it. The key is water management and how the tree is planted and how that soil is managed.
So, go to the Web Soil Survey: https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm
Click on an Area of Interest (type in California and your county or even your address)
Once you have a map of your general area, use the tool at the top of the map to delineate the area you want to know more about
Then click on “Soil Map” tab and you will see the map polygons or soil units for that area
Then click on “Soil Data Explorer” tab to get “Suitabilities and Limitations Ratings” on the left side
Click on “Land Management” and there is “Avocado Root Rot Hazard”
Click on “View Rating” and the map will appear with colored units showing root rot hazard along with a chart showing the root rot hazard of the different soils in the map.
Wow. Watch out for those dark areas.
- Author: Sonia Rios
“Laurel wilt – A new threat to California's avocado industry”
Presented by: California Avocado Society, Inc., California Avocado Commission, University of California Cooperative Extension, and University of Florida's Tropical Research and Education Center. Event is FREE, everyone is welcome!
- Tuesday, August 1, 2017, 1:00 p.m. to 3:00 p.m.,
UC Cooperative Extension Office Auditorium, 2156 Sierra Way, San Luis Obispo, CA 93401
- Wednesday, August 2, 2017, 9:00 a.m. to 11:00 a.m.,
UC Cooperative Extension Office Auditorium, 669 County Square Dr. Ventura, CA 93003
- Thursday, August 3, 2017, 1:00 p.m. to 3:00 p.m.,
Fallbrook Public Utility District Board Rm., 990 East Mission Rd. Fallbrook, CA 92028
Agenda
|
Speaker(s) |
Title |
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Jeff Wasielewski, Subtropical Fruit Agent, University of Florida Extension |
The current status of laurel wilt in South Florida |
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Randy Ploetz, Professor of Plant Pathology at the Tropical Research & Education Center of the Institute of Food & Agricultural Sciences (IFAS), University of Florida |
Laurel wilt epidemiology and management |
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Bruce Schaffer, Professor of Ecophysiology of Subtropical and Tropical Horticultural Crops,IFAS, University of Florida
|
Vascular physiology and anatomy of different avocado genotypes relative to laurel wilt susceptibility |
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Daniel Carrillo, Professor of Entomology and Nematology in Subtropical and Tropical Horticultural Crops, IFAS, University of Florida
|
Laurel wilt vectors: biology and management |
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Edward Evans, professor in the Food and Resource Economics Department located at the University of Florida's Tropical Research and Education Center |
Economic impact and economics of control strategies |
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Jonathan Crane, Professor of Horticulture / Tropical Fruit Crop Specialist, University of Florida's Tropical Research and Education Center |
Current control strategies, recommendations and issues |
- Author: Ben Faber
The very fact that avocados can be grown in hard to get to places means that the trees are also in areas that are subject to wildfire damage. Recently several hundred acres of avocado burned in the foothills. The fire was fanned by high winds and low humidity.
Every year there are avocado trees that burn, either through careless attention to early morning fires that pickers build, wildfires or car accidents. A grower needs to be patient and observant to bring the trees back into production.
Although injury to foliage and young growth is visible within a few days of the fire, the full extent of the damage may not be known for several months or possibly the next growing season. In the case of severe injury, die-back may continue to occur for several months after the fire. New growth that occurs after the fire may suddenly collapse the following year when the growth is tested by Santa Ana conditions.
The important rule to follow after a fire is to do nothing - don't prune, don't fertilize and maybe don't water. Or rather, water very carefully. Dry winds may have sucked the water out of the ground and may need to be replenished. The fire may have burned the irrigation lines and need to be replaced.
In the meantime, if the tree has been defoliated by the fire, it has lost its ability to transpire water. Watering a tree with no leaves will set up those conditions that are conducive to root rot. Until the tree begins to leaf out, watch soil moisture to decide how much water the trees are pulling out of the soil. The emitters should be capped or plugged on some leafless trees. Then as the tree puts on new growth, shallow, infrequent irrigations should start. This may mean replacing the 10 gph microsprinkler with a 1 gph dripper if only a portion of the orchard has been burned and the rest of the trees need their usual amounts and frequency of water.
The avocado has a tremendous ability to come back from fire and frost damage. However, the tree will tell you where it is coming back. It will start pushing growth where the tree is still healthy. It may take 3 to 6 months for this growth to occur.
Delay pruning until the tree clearly shows where it is going to regrow. By waiting, you save the expense of having to return sometime later to remove more wood and also will be able to save the maximum about of tree.
An activity the grower can perform is whitewashing. The defoliated tree can be further damaged by sunburn after it has lost its protective cover of leaves. The upper surface of horizontal limbs and the south sides of exposed trunks are the most affected. The whitewash can delay the appearance of new growth, but it does not affect total growth. There is usually no value in applying the whitewash to small limbs.
There are various commercial whitewashes on the market. The easiest to prepare is the cheapest white latex paint on the market mixed with water to the extent that it will go through a sprayer.
Avocado trees have a great ability to recover after fire damage. Even trees killed below the bud union will frequently develop into good trees if they are rebudded and given good care. Trees which do not put out vigorous sprouts should be removed. Interplanting avocados would rarely be advisable because of their rapid recovery.
Fire Information:
http://ceventura.ucanr.edu/Agricultural_Threats/Fire_Information/