- Author: Ben Faber
There have been a lot of new avocado orchards planted during the last few years. These often have been in old ‘Valencia’ orchards or lemons that had poor production. In order to save money, growers have just cut the trees at ground level and replanted the avocados near the stumps. Avocados have recognition of being resistant to Armillaria, but in this environment of high disease pressure, they can fail.
Armillaria root rot is common, yet is an infrequently identified and poorly understood disease. It is capable of attacking most species of trees and other woody plants growing in California. It is sometimes called “shoestring root rot” and the causal fungus is often referred to as the “honey mushroom.” Because oak is one of the preferred hosts, it is also called “oak root fungus.”
If a tree undergoes a slow to rapid decline without any obvious reason, suspect Armillaria as the cause. Certain areas, such as drainage areas from chaparral or woodlands are likely areas for this disease. Old roots left underground provide a food base for continued fungal growth and survival.
General symptoms of Armillaria resemble those of other root disorders. These symptoms are disrupted growth, yellow foliage, branch dieback, and resin or gum exudates at the root collar. Trees may die rather abruptly without showing any decline symptoms. Avocados typically have a rather protracted death, but in citrus it can be rapid.
Only rarely can the disease be diagnosed without examining the larger buttress roots and root collar of the tree. After carefully removing the soil, examine for the presence of:
1) Rhizomorphs, or fungal ‘shoestrings’ attached to the wood under the bark. These may occur beneath the bark for some distance above the soil line in advanced cases, rarely they may radiate from the wood into the soil. Rhizomorphs may also grow out from the larger roots, resembling feeder roots in appearance. They are about the diameter of pencil lead and vary in color from black to reddish brown. The interior consists of white mycelial tissue.
2) Decayed areas of wood at the root collar or on the crown roots. Armillaria causes a white rot and the wood develops a stringy texture. Roots in advanced stages of decay may be soft, yellowish and wet.
3) Veined, white mycelial fans between the bark and wood where the cambium has been killed. Sometimes this fan (or fans) extends quite far above the soil line beneath the bark.
4) Soil remaining attached to the roots.
5) Characteristic mushrooms on the lower trunk or on the ground near the infected roots. These short-lived annual fruiting structures of the disease-causing fungus may develop during the fall or winter rainy season and may occur in small clusters or in large numbers. The stalk is typically yellow and 3 inches or more long. Usually a ring is connected to the stalk just below the cap. The cap is 2-5 inches across and often honey-yellow. It may be dotted with dark brown scales. The underside is covered with loosely spaced white or yellow gills radiating from the stem.
After the disease has been identified, the grower should study the situation to determine the role Armillaria root rot has played in causing the decline or death of the tree. Frequently the fungus is only involved in a secondary manner by invading and destroying roots after the tree has been exposed to stress of some form, such as severe drought, water logging, or soil fill over the roots. The fungus can also act as a saprophyte feeding on dead wood. It is frequently involved in the decay of old tree stumps and roots.
Many oaks are lightly infected with the disease for years with no resultant damage except for isolated pockets of buttress root rot which are walled off by the tree and have no ill effects. Other infected trees show no damage until subjected to stress. Accumulating evidence suggests the type of root exudate that is produced influences the susceptibility of the tree. Certain forms of stress cause a shift in exudates that promote rapid development of the fungus and may hasten tree invasion and decay.
Spores are produced by the mushroom fruiting structures (mushrooms) and disseminated by air currents and introduced into new area. Once the fungus enters the cambium and bark tissues, mycelial fans develop during the parasitic phase of the attack. Subsequently, mycelium invades and decays the woody tissue of the roots and sometimes also the base of the trunk. Under proper conditions the fruiting structures form at or near the base of the infected tree, completing the life cycle.
Direct control of the fungus in a diseased tree is not possible with present technology. However, in many instances the fungus is incapable of causing severe damage unless the tree is first subjected to substantial stress. Thus, keeping the tree healthy and avoiding severe stress is one important approach in preventing loss of trees to Armillaria.
Drought and leaf defoliation are two major forms of stress that favor Armillaria. In dry years it is advisable, as in all years, to make sure irrigation scheduling is appropriate. Stresses such as defoliation from persea mite, soil compaction and physical injury can exacerbate the disease. Nutrient management may minimize Armillaria effects, although little research information exists on this subject.
The second most important means of minimizing Armillaria damage is to avoid or eliminate the fungus inoculum before planting. Trees planted in former orchards will quite possibly be exposed. Since these sites cannot be avoided, here is a suggestion that will be helpful: remove stumps and old roots from the old orchard to the greatest extent possible.
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Armillaria mushrooms and hyphal plaques under the bark
- Author: Ben Faber
"We don't need to irrigate, it's winter." This is a commonly held idea, and many years it is true. Adequately timed rains will often meet the needs of avocado trees during the winter period, and in times like last year, even satisfy much of the spring requirement. And the calls are coming in – “What’s wrong with my trees, they have all these brown leaves?”. This from San Diego to San Luis Obispo.
In a low rainfall year, irrigation can be as necessary as at other times of the year. This is because a subtropical evergreen like avocado continues to use water regardless of rainfall patterns. At the time of writing this article in March, we have had a scant 4 inches in Ventura and this is on top of a low rainfall year in 2011-12. Rain is necessary to leach the salts that have accumulated from the last irrigation season.
The driving forces for plant water use are light intensity, wind and relative humidity, as well as temperature. Remember how cold, dry winds can dry your skin or freeze-dry backpack food. Even during the winter, the trees are quite capable of losing large amounts of water with clear skies and cold winds.
Dry Santa Ana conditions are also more common in winter than in the past. This winter, a time of drought, I went out to see an orchard to evaluate it for pruning. On arrival, my first concern was for the water stress in the trees. The grower, however, was unconcerned. The trees had been dutifully irrigated the previous Friday. But over the weekend, a Santa Ana had blown for three days and completely dried the soil in the top 10 inches. Digging around the roots convinced the grower of water stress. Do not take irrigation for granted.
Contributing to the problem is the determination of what amount of rainfall is effective. Effective rainfall is defined as the amount of water that is retained in the root zone after rain. Avocados, especially on shallow soils, do not have much of a root zone. Most soils can be expected to hold about 2 inches of available water in the top 2 feet, less the more sandy, more the more heavy.
If rainfall exceeds the holding capacity within the root zone, it is lost to the plant. Just imagine if all the year's expected rain fell during one storm. It would not be long before irrigation would be required with no more rain coming. The extra water may, however, perform the all-necessary function of leaching accumulated salts from the root zone. When the rain gauge says that 2 inches fell, it is quite possible that all that rain will not be available to the tree. This also goes for the quarter inch storms we get that do not even make it through the leaf litter. It is not effective rainfall, even though it may wash the persea mite off the leaves.
One of the best ways to assess the effectiveness of rainfall within the root zone is with tensiometers. These trusty instruments are most commonly used to schedule irrigations. A good rainfall should return the 8- and 18-inch depth gauges to close to 0 cbars. This will tell you whether the rain thoroughly wetted the root zone. It will not tell you how much may have passed through the root zone, however.
If you are using soil sampling to assess the depth of rain infiltration, simply squeezing a handful of soil can help. Regardless of soil texture, a wetted soil will form a ball or cast when thoroughly wetted. Water moves as a front through the soil. After a rain, take soil samples with depth to find where the potential to form a ball abruptly ends. This will tell you the depth of effective rain.
How well a soil holds together can also be an indication of when to irrigate. Even a sandy loam texture will retain a ball that does not hold together well when there is still adequate moisture for the tree. The possibility of forming a ball decreases with water content. When visible cracking of a soil ball is obvious, it is time to irrigate.
Winter irrigation is something we do not commonly perform, but in low rainfall years it is an activity we need to consider, especially for controlling the salts that accumulate from our previous irrigation season.
Salt damage due to lack of leaching
- Author: Ben Faber
The Avocado: Botany, Production and Uses, 2nd Edition
By Bruce Shaffer, Nigel Wolstenholme and Anthony Whiley
This brand new book summarizes avocado science and technology and reviews production practices on a worldwide scale. The book is split into 15 chapters and covers all aspects of avocado production and science and includes: history, distribution and uses, taxonomy and botany, propagation, crop management, diseases and insect and mite pests. This book builds on the 2002 edition and includes the works of 45 writers from all over the avocado world.
Avocado book
- Author: Gary Bender
- Author: David Shaw
OK! Let's Strategize. There are four steps for everybody to consider, it doesn't matter if you have a backyard lawn and landscape or if you have 700 acres of avocados.
1. Maintenance: Irrigation System and Cultural Practices
2. Improve Irrigation Scheduling
3. Deficit Irrigation
4. Reduce Irrigated Area
1. Maintenace
a. Irrigation System.
- Fix leaks. Unfortunately, there are almost always leaks for all kinds of reasons. Pickers step on sprinklers, squirrels eat through polytube, branches drop on valves, coyote puppies like to chew….the system should be checked during every irrigation
- Drain the lines. At the beginning of each year every lateral line should be opened in order to drain the fine silt that builds up.
- Maintain or increase the uniformity of irrigation so that each tree or each area gets about the same amount of water. Common problems include different sized sprinklers on the same line or pressure differences in the lines. Where there are elevation changes, every line should have a pressure regulator, they come pre-set to 30 psi. Having all of your lines set up with pressure regulators is the only way you can get an even distribution of water to all of the trees, and it solves the problem of too much pressure at the bottom of the grove and not enough at the top.
- Clean the filters often. You don't have a filter because you think that the district water has already been filtered? Hah! What happens if there is a break in the line in the street and the line fills with dirt during the repairs? All of your sprinklers will soon be filled with dirt.
- Is water flow being reduced at the end of the lateral line? It could be because scaffold roots are growing old enough to pinch off the buried line. The only cure is to replace the line.
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b. Cultural Management.
- Control the weeds because weeds can use a lot of water.
- Mulch? Mulching is good for increasing biological activity in the soil and reducing stress on the trees, but the mulch will not save a lot of water if you are irrigating often….the large evaporative surface in mulches causes a lot of water to evaporate if the mulch surface is kept wet through frequent irrigation. Mulches are more helpful in reducing water use if the trees are young and a lot of soil is exposed to direct sunlight.
2. Improve the Irrigation Scheduling.
- CIMIS will calculate the amount of water to apply in your grove based on last week’s water evapotranspiration (ET). You can get to CIMIS by using several methods; for avocado growers the best method is to use the irrigation calculator on the www.avocado.org website. If you need further instruction on this, you can call our office and ask for the Avocado Irrigation Calculator Step by Step paper. You need to know the application rater of your mini-sprinklers and the distribution uniformity of your grove’s irrigation system.
- CIMIS tells you how much water to apply, but you need tensiometers, soil probes or shovels to tell you when to water.
- “Smart Controllers” have been used successfully in landscape and we have used one very successfully in an avocado irrigation trial The one we used allowed us to enter the crop coefficient for avocado into the device, and daily ET information would come in via a cell phone connection. When the required ET (multiplied automatically by the crop coefficient) reached the critical level, the irrigation system would come on, and then shut down when the required amount had been applied. Increased precision can be obtained by fine tuning these devices with the irrigation system precipitation (application) rate.
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3. Deficit Irrigation.
- Deficit irrigation is the practice of applying less water than the ET of the crop or plant materials. Deficit irrigation is useful for conserving water in woody landscape ornamentals and drought tolerant plants where crop yield is not an issue. Water conserved in these areas may be re-allocated to other areas on the farm or landscape.
- There hasn’t been enough research on deficit irrigation of avocado for us to comment. We suspect, however, that deficit irrigation will simply lead to dropped fruit and reduced yield.
- Stumping the avocado tree could be considered a form of deficit irrigation. In this case, the tree should be stumped in the spring, painted with white water-based paint to reflect heat, and the sprinkler can be capped for at least 2 months. As the tree starts to re-grow, some water should be added back, probably about 10-20% of the normal water use of a mature tree.
- Regulated Deficit Irrigation for Citrus is an important method for saving water, and in some cases will reduce puff and crease of the peel. In one orange trial done by Dr. David Goldhammer in the San Joaquin Valley, an application of 25% of ETc from mid-May to Mid July saved about 25% of applied water for the year and reduced crease by 67%, without appreciably reducing yield.
- 4.
- 3. Reduce Irrigated Area.
- Taking trees out of production. Trees that are chronically diseased and do not produce fruit (or the fruit is poor quality) should be taken out of production during this period. Also consider: trees in frosty areas, trees in wind-blown areas, trees near eucalyptus and other large trees that steal the water from the fruit trees.
- Changing crops. You may want to take out those Valencias during this period and replant to something that brings in more money, like seedless, easy-peeling mandarins. The young trees will be using a lot less water.
- Fallow Opportunities. You may decide to do some soil preparation, tillage or cultivation, or even soil solarization of non-irrigated areas.
We have found that this four step process is a logical way to achieve water cutbacks with least impact. It is possible to achieve a ten percent reduction in water by only improving irrigation system uniformity and scheduling procedures. Often, these two measures also result in better crop performance and reduced runoff. Reducing irrigated area or taking areas out of production should be a last resort and a well thought out decision. Plan for the future, hopefully water will be more available in future years.
- Author: Mary Lu Arpaia
- Author: David Obenland
Over the last two weeks Mary Lu Arpaia (Extension Specialist, UC Riverside) and David Obenland (Plant Physiologist, USDA-ARS) had the opportunity to visit Israel and spend several days looking at avocados including a visit with Zvi Mendel and Stanley Freeman, the lead researchers in Israel on the Polyphagous Shot Hole Borer (PSHB) and its Fusarium fungal symbiont. This is their report.
So far the beetle is still largely confined to the central coastal region of Israel and the northern Negev. The beetle has also been found in the Upper Galilee at Kibbutz Hagoshrim in avocado and on ornamental trees in other locations in this region which is quite far from the primary infested area. The infested avocado trees have been destroyed but the beetle population already spread outside of the site of the initial infestation. Interestingly, this find was with a grower who packs their avocados in the coastal area. It is assumed that the beetles arrived in the bins originating from the infested area. This is a reminder to California growers and packers that to minimize the spread of pests clean bins are essential. The spread of avocado thrips and persea mite in California is also assumed to have been by bins containing vegetative material.
The Israeli researchers have continued searching for materials that will either control the beetle or the fungus. They have had reasonable success in the lab when they test materials under controlled conditions but application out in the field is not effective. There are no chemical treatments on the horizon that growers can use.
We visited infested avocado orchards in the Hefer Valley and the region southwest of the Carmel mountains (south of Haifa). We visited a Reed orchard which is believed to have been infested approximately 5 to 6 years. Three years ago this grove showed heavy infestation in the entire grove. What we saw on our visit was severe limb dieback, many broken branches scattered on the orchard floor, dropped mature fruit and smaller than normal fruit size for the fruit remaining on the trees. Signs of the beetle boring as evidenced by sugar exudates were easy to find wherever we looked. Dr. Mendel told us that the grower is giving up on this orchard and plans to bulldoze the orchard after harvest. We went on to see several other groves; in all except a 2-year-old orchard it was easy to find limb dieback, fallen fruit and sugar exudate up and down branches. We were told that they do not often find infestation of young groves but when they do, it is usually on the base of the trunk (either rootstock or scion).
We visited a plot with some growers along with Leo Winer (an extension officer) and Udi Gafni (head of the research and development unit of GRANOT) where insecticide applications to infested trees had been made last fall (2012). Unfortunately, signs of continued beetle activity were relatively easy to find. The growers told us about seeing fruit shriveling as the branch dies back. Since substantial fruit drop occurs of both mature and developing there is also an overall drop in productivity as an infestation spreads throughout a grove. Growers are extremely concerned and frustrated that there are no control measures for the beetle. Similar to California, Israel has historically used minimal pesticide sprays. The growers know that in areas already infested that spray applications are key to their continued orchard viability.
Avocado growers in Israel are also seeing problems with Botryosphaeria fungal infections. We visited the northern Negev Desert area where avocados are grown. We went to a large Hass orchard the grower is attempting to control this problem using phosphite injections. We were able to see upper limb dieback and staining on the upper branches.
In the meantime, Drs. Mendel and Freeman are continuing to study beetle biology and the behavior of the Fusarium fungal component. Dr. Mendel is developing a method to raise the beetle in the laboratory. This will be a important breakthrough since it will allow for a better understanding of the beetle life cycle. They know that the female beetle once it flies and seeks a place to burrow has about a 48 hour window to successfully establish itself in the host plant since this is the time period it can survive without feeding. We also learned that the beetle carries the Fusarium spores in its mycangium (a specialized structure at the back of its jaw), rather than hyphae. The larvae and pupa do not have mycangium, only the adults. While the larvae pupate, the Fusarium in the galleries sporulates and the emerging adult as it feeds picks up the Fusarium spores. On a side note, Drs. Mendel and Freeman do not necessarily agree with our use of the name PHSB. They argue that the beetle is monophagous (eats only one kind of food, Fusarium) but uses several tree species as hosts. We will have to see how the final name for the beetle is ultimately settled among the insect taxonomists.
Finally, Dr. Mendel is very worried about the spread of the beetle to native tree species in Israel, especially oaks which are found in many areas throughout the country. The box elder (which also occurs as a landscape tree in California) has been decimated by the PHSB. This is a warning flag for all California residents to take action to safeguard our native oaks, other native species and landscape trees.
These visits reinforced the extreme importance that the California industry must be diligent looking for tree infestation. The industry must work with the landscape industry and forestry service in southern California to understand how fast the infestation is spreading and what hosts are most susceptible. The industry needs to continue funding both applied (surveys, control measures and understanding of the beetle and fungal biology) as well as more basic work such as the origin of the beetle. This latter piece of information may lead to better ideas concerning control measures. Finally California researchers and industry leaders would be well advised to collaborate closely with our Israeli colleagues. They are facing this crisis head on since the PSHB has already spread into commercial groves. From their advanced experience, the California industry can learn much.
A conference on the PSHB and its Fusarium fungal symbiont was held in August 2012 in Riverside. The talks from this meeting are available for review on the website www.avocadosource.com,
Invasive Ambrosia Beetle Conference - The Situation in California ...
www.avocadosource.com/Journals/...2012/IABC_20120814_TOC.ht..
. Video of the conference presentations from the public meeting are available for viewing on YouTube. The science portion of the meeting will be posted on YouTube in the forthcoming weeks.
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Figures 1,2. ‘Reed’ avocado tree in the Hefer Valley, Israel. Note previous limb breakage and branch dieback due to infestation by the PSHB and its fungal symbiont, Fusarium. (Pictures taken 3/9/2013 by M.L. Arpaia).
Figure 3. Infested ‘Reed’ branch in later stages of decline. The white larvae are termites. Once the branch begins to dieback, termite infestation usually follows. Note the staining of the wood and evidence of the PHSB galleries. (Picture taken 3/9/2013 by M.L. Arpaia)
Figure 4. External symptoms of infestation including sugar exudate and bark darkening reveals wood staining due to Fusarium infection of the woody tissue. (Picture taken 3/9/2013 by M. L. Arpaia)
Figure 5. Small branch infestation and dieback in ‘Hass’. Picture taken at Kibbutz Ma’agan Michael along the central coast of Israel. (Picture taken 3/9/2013 by M. L. Arpaia)
Figure 6. ‘Hass’ avocado tree in the northern Negev affected with Botryosphaeria. Note the dieback and the fact that previously infested wood has been removed. (Picture taken 3/9/2013 by M. L. Arpaia)
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