- 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: 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
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.
Below:
Armillaria mushrooms and hyphal plaques under the bark
- Author: Ben Faber
- Author: A. James Downer
South African plant pathologists were the first to show that root rot in avocado could be controlled by trunk injection with both phosphorous acid and the patented material Aliette®. Aliette was briefly registered in California in the late 1980’s, but theregistrant soon lost interest in pursuing a full pesticide registration when it became apparent that other researchers believed phosphorous acid could be registered as a fertilizer - a process much less costly and simpler than a pesticide registration. The company continued to hold on to the patents for the product and the breakdown products that were useful in root rot control. By holding onto the patent, this effectively stopped other companies from pursuing a pesticide registration for phosphorous acid. In 1990, a publication reported that phosphite could be used as a source of phosphorus fertilizer and this became the basis for the registration of phosphite as a fertilizer. Subsequently, when the original patent expired, at least two materials have been registered as fungicides containing phosphite – Fosphite® and Agri-fos®. There are, however, numerous phosphite materials that have been registered as fertilizers (for some brands see Brunings et. al., 2005, http://edis.ifas.ufl.edu/HS254), and every day seems to bring more brands onto the scene each making claims of having the best efficacy.
We wanted to see if we could detect an efficacy difference between Aliette, another registered phosphite fungicide and four different materials registered as fertilizers, for a total of six materials. In a greenhouse, three-month old ‘Topa Topa’ seedling avocados with cotyledons removed were planted into a Phytophthora cinnamomi -inoculated organic potting mix. A control was also planted without the inoculum, as well as an inoculated control. One of six different materials was then applied as a soil drench until draining from the bottom of the liner. The materials were applied at the equivalent phosphorous acid concentration. There were 20 replicates for each of the controls and treatments. The experiment was repeated twice.
At harvest, root fresh and dry weights were highest for the non-inoculated trees and lowest for the untreated, inoculated controls, in both trials. All treatments’ associated weights intermediate between these two were statistically the same. Even a repeat application of one of the treatment materials in trial II didn’t result in greater root weights than single application treatments. Shoot weight, both dry and fresh, was much less affected by root rot and treatments. There were no differences in fresh shoot weight in the second trial, not even between the inoculated and noninoculated controls. The root and shoot weights of all the treatments in the second trial were higher than in the first trial, indicating that either the inoculum was not as effective or that the trial was not continued long enough to produce as much damage.
Root rot studies often have dramatic effects on root weights while shoot weights may remain little affected. It is clear from our data that phosphites reduced the severity of root rot in this study, but that there was no benefit of a single source of phosphite relative to any other source.
Below: Healthy and decaying avocado roots.
- Author: Elizabeth Fichtner and Rachel Elkins
Lime-induced Iron Chlorosis: a nutritional challenge in the culture of several subtropical perennial crops in California
Elizabeth Fichtner, UCCE Tulare County and Rachel Elkins, UCCE Lake and Mendocino Counties
Spring, and new leaves are coming out, but this could, but yellow could be a sign of iron chlorosis, as well.
Although iron (Fe) is the 4th most abundant element in the lithosphere, Fe deficiency is among the most common plant micronutrient deficiencies. Fe deficiency in plants is common in calcareous soils, waterlogged soils, sandy soils low in total Fe, and in peat and muck soils where organic matter chelates Fe, rendering the element unavailable for plant uptake. In California, lime-induced Fe deficiency is often observed in soils and irrigation water containing free lime, and is exacerbated by conditions that impede soil drainage (ie. compaction, high clay content), resulting in reductive conditions. Given that over 30% of the world's soils are calcareous, lime-induced Fe deficiency is a challenge in numerous perennial cropping systems including: grapes, pears, apple, citrus, avocado, pecans, and stone fruit (prune, almond, apricot, peach, nectarine, cherry).
In most soils, Fe oxides are the common source of Fe for plant nutrition. Solubility of Fe oxides is pH dependant; as pH increases, the free ionic forms of the micronutrient are changed to the hydroxy ions, and finally to the insoluble hydroxides or oxides. In calcareous soils, the bicarbonate ion inhibits mobilization of accumulated Fe from roots to foliage and directly affects availability of Fe in soil by buffering soil pH. When irrigation water is also high in bicarbonate, probability of Fe deficiency is enhanced because bicarbonate is continuously supplied to the soil, and more importantly, the roots may become crusted with lime as water evaporates, thus inhibiting root growth and function. Inside the plant, bicarbonate inhibits nutrient translocation from roots to aboveground plant parts. The adverse effects of high bicarbonate levels are exacerbated in very saturated, very dry, or compact soils, where bicarbonate levels increase concurrent with diminished root growth and nutrient uptake.
Symptoms of Fe deficiency in plants
Fe is immobile in plants; therefore, symptoms appear in young leaves. Interveinal chlorosis (Figure 1) is the main symptom associated with Fe deficiency, followed by reduced shoot and root growth, complete foliar chlorosis, defoliation, shoot dieback, and under severe conditions may result in tree mortality. Overall productivity (yield) is reduced, mainly from a reduced number of fruiting points.
Plant Adaptation
Plant species and cultivars vary in their sensitivity to Fe deficiency, and are categorized as either "Fe-efficient" or "Fe-inefficient". Fe-efficient plants have Fe uptake systems that are switched on under conditions of Fe deficiency. Fe-inefficient plants are unable to respond to Fe deficient conditions. All Fe-efficient plants, except grasses, utilize a Fe-uptake mechanism known as Strategy 1. Strategy 1 plants decrease rhizosphere pH by release of protons, thus increasing Fe solubility. Some plants may excrete organic compounds in the rhizosphere that reduce ferric iron (Fe3+) to the more soluble ferrous (Fe2+) forms or form soluble complexes that maintain Fe in solution. Additionally, roots of Strategy 1 plants have specialized mechanisms for reduction, uptake, and transfer of Fe within the plant. Strategy 2 plants (grasses) produce low molecular weight compounds called phytosiderophores which chelate Fe and take up the chelated Fe with a specific transport system.
Amelioration of Fe chlorosis
Planting sites in calcareous soils should be well drained to provide optimal conditions for root growth and nutrient uptake. Waterlogged and compact soils contain
more carbon dioxide, which reacts with lime to form even more bicarbonate. These conditions, as well as very dry soils, also inhibit microbial activity which aids in
solubilization and chelation of Fe. Prior to planting, soils and water should be tested to determine the pH, lime equivalent, and bicarbonate concentration. Bicarbonate concentrations greater than 3 meq/L in irrigation water increase the hazard of lime accumulation on and around roots. If high bicarbonate water must be used, the pH must be adjusted to 6.0-6.5 to dissolve the bicarbonate and prevent it from negating the effects of soil-based treatments. In microsprinker and drip systems, acidification of irrigation water will also reduce the risk of emitter clogging, a common problem at bicarbonate levels over 2 meq/L. The cost of reducing the pH of irrigation water will more than compensate for the savings incurred from avoiding wasted investment in failed soil- and plant-based remedies. Systems can be set up to continuously and safely inject water with acids such as sulfuric, urea-sulfuric, or phosphoric during irrigations. Specific choice and rate will depend on crop, soil type, other nutrient needs, availability, and cost. Downstream pH meters are available to continuously adjust rate of acid use. Acetic and citric acid can be utilized by organic growers.
Soil based pre-plant treatments to reduce pH include elemental sulfur (S) and acids as mentioned above. It is only necessary to treat a limited area near the root zone to ameliorate symptoms because the tree only needs to take up a small amount of Fe. Material can be shanked in or banded and incorporated in the prospective tree row. One ton of elemental sulfur per treated acre is needed to mitigate three tons of lime, and may need to be re-applied every 3 to 5 years after planting. The addition of organic matter such as well-composted manures will benefit poorly drained or compact soils by increasing aeration for better root growth, fostering chelation of nutrient cations, and reducing pH (depending on source material).
If possible, choose a Fe efficient species or cultivar. In perennial systems, lime-tolerant rootstocks may be the first line of defense in combating Fe deficiency. Some rootstocksmentioned are peach-almond and Krymsk-86 for stone fruit, Gisela 5 for cherry, and Pyrus communis for pear. Ongoing research studies in Europe focus on screening rootstocks of grape and olive for lime tolerance.
Once soil and water quality improvements are made, post-plant management strategies may also be implemented to ameliorate lime-induced Fe chlorosis in the short term. Soil can be acidified as described above. Individual trees can be treated by digging four to six 12-24 inch
holes around the drip line and burying a mixture of sulfur and Fe fertilizer. Historically, two principal methods have been utilized: 1) foliar application of inorganic Fe salts (ie. ferrous sulfate), and 2) soil or foliar application of synthetic chelates. Application of Fe salts to foliage may have mixed results due to limited penetration of Fe into leaves and inadequate mobilization within the plant. Use of Fe chelates may be of benefit; however, they are expensive and pose an environmental concern due to their mobility within the soil profile. Because soil lime interferes with Fe mobility with the plant, repeat application of inorganic Fe salts or Fe chelates may be necessary throughout the growing season.
Choice of nitrogen (N) fertilizer may also influence solubility of rhizosphere Fe. When N is applied in the ammonium form (NH4+), the root releases a proton (H+) to maintain a charge balance, thus reducing rhizosphere pH. Alternately, fertilization with nitrate (NO3-) results in root release of hydroxyl ions (OH-), resulting in an increase in rhizosphere pH. Solubility of Fe3+ increases 1000 fold with each one unit decrease in pH; therefore, fertility-induced rhizosphere pH changes may significantly influence Fe availability.
New methods for amelioration of Fe chlorosis are under investigation. For example, container studies have demonstrated that inter-planting sheep's fescue, a Strategy 2 plant, with a Fe-inefficient grape rootstock may ameliorate Fe chlorosis in grape. In this system, the grass produces a phytosiderophore that enhances Fe availability to the grape. Additionally, soil amendment with Fe3(PO4)2• 8H2O), a synthetic iron(II)-phosphate analogous to the mineral vivianite, has been effective at preventing Fe chlorosis in lemon, pear, olive, kiwi, and peach. Vivianite has a high Fe content (~30%) and serves as a slow release source of Fe in calcareous soils.
Figures below: 1) Shoot dieback in citrus, 2) Interveinal chlorosis in citrus and 3) Various stages of iron chlorosis in avocado.