Avocados and Water
Avocados are the most salt and drought sensitive of our fruit tree crops. They are shallow rooted and are not able to exploit large volumes of soil and therefore are not capable of fully using stored rainfall. On the other hand, the avocado is highly dependent on rainfall for leaching accumulated salts resulting from irrigation water. In years with low rainfall, even well irrigated orchards will show salt damage. During flowering there can be extensive leaf drop due to the competition between flowers and leaves when there is salt/drought stress. In order to reduce leaf damage and retain leaves, an excess amount of water is required to leach salts out of the roots zone. The more salts in the water and the less rainfall, the greater leaching fraction.
Drought stress often leads to diseases, such as black streak, bacterial canker, and blight (stem, leaf, and fruit). Leaf blight (Figure 1) is often confused with salt or tip burn (Fig. 2), but is actually a fungal disease that forms an irregular dead pattern on leaves and leads to defoliation. Blight is associated with lack of water, while salt burn is due to poor quality water and poor irrigation habits. Leaf blight often shows up after Santa Ana conditions, when growers get behind on their irrigations and the root zone dries out suddenly. There has been a high incidence of this disease the last two years. In both cases, defoliation leads to sunburned trees and fruit which can be severe economic losses. The only way to prevent these conditions is to keep up with your irrigation schedule.
To get your water to go further, it is important that the system is tuned in order to get the best distribution uniformity (DU). Many of our systems were installed 40 years ago and old age can lead problems, such as clogging, broken emitters, mixed emitters that put out different amounts and leaks. With poor DU, some trees get too much water and others do not get enough. Even fairly new irrigation systems can have poor DU, especially after a harvest. Poor water pressure on our step slopes is probably our main problem. A DU of 80% means 10% of the emitters are putting out more than the average and 10% are putting out less. The irrigator to compensate for the under irrigated 10% will run the system 10% longer to make sure the under irrigated trees get enough and over irrigating 10% of the trees with 20% more water than they need. A call to the local Resource Conservation District office can get a free DU evaluation and recommendations that are usually pretty reasonable to follow.
Aside from improving DU, it is important to know when and how much water to apply. When to apply can be evaluated by the hand or feel method (https://nutrientmanagement.tamu.edu/content/tools/estimatingsoilmoisture.pdf) which is fast and cheap. Or it can be done by tensiometer, Watermark or some of the more expensive electronic sensors. But these tools only tell you when to irrigate, not how much. This can be done by turning the system on (once you have made sure you have a good DU) and over the period of the irrigation insert a piece of rebar into the soil to determine the depth of infiltration. The rod will go down as far as the soil is moist and stop when it hits dry dirt. When you have about two feet of infiltration you will know how long to run the system to get an appropriate amount of water. A typical loam usually takes about 150 gallons per tree to two feet. Another way to get an approximation of the amount to apply is to use the Irrigation Calculator at http://www.avocadosource.com/tools/IrrigationCalculator.asp.
Managing the Tree Canopy
Significantly pruning trees can reduce the amount of water transpired by the tree. Trees that are about 15 feet in height, can be pruned by half and they will use half the water. Massive 30 foot trees would need to have a major pruning to significantly reduce water use. In extreme drought conditions and for the long term welfare of the grove, large trees should be stumped (Figure 3) or scaffolded (Figure 4) and paint white to prevent sunburn. Scaffolding usually produces fruit much sooner than stumping, because retaining a significant part of the trunk and branches the tree does not exert as much energy to regrow and retains buds that have been under apical dominance for less time. When new shoots appear they should be headed back to force lateral branches which is where the flowers will form.
All the prunings should be chipped and left in the field. This will help conserve water and help control Phytophthora root rot. Root rot or crown rot trees should not be pruned until they have been brought to health with one of the phosphorous acid formulations. They all are effective. Pruning a sick redirects the trees energy to fighting off the disease when it starts pushing new growth and then does not have the energy to fight off the disease. Or if you do have areas that are diseased (sunblotch, root rot, crown rot, etc.), windblown, in shallow soils or areas of recurrent frost, you might just remove the trees completely to save water.
White kaolin (Surround) applied to leaves has been shown to reduce leaf temperatures and water loss. This can be used, but under the direction of the packing house, since it if it is applied to fruit, it is very difficult to remove.
These are some steps that a grower can take to improve water management and create a more efficient use of water to help survive this period of not knowing how long this drought will last.
Figure 1. Leaf blight is a disease that occurs with lack of water of any quality.
Figure 2. Salt damage from poor quality water and poor irrigation habits.
Figure 3. Stumped avocados for lack of water.
Figure 4. Scaffolded avocado that should produce fruit sooner than a stumped avocado.
This is a great website to view plant nutrient symptoms by plant or by nutrient. It is of ornamental plants, but hey, once you can see it on one plant you will something very similar on avocado, lemon, apple, almond, lychee or whatever alphabetic fruit you work with. Kudos to University of Florida. Of course once you see what iron deficiency looks like, you'll be able to identify it on most other plants:
Below is Boron toxicity from a grower who got too excited about correcting B deficiency,
- Author: Steve Tjovosvold
- Author: Steve Koike
Diseases, disorders and other plant problems are critical concerns
for the wholesale nursery. These include biotic problems — caused by
living organisms such as pathogens, nematodes, and insects and other
arthropods — as well as abiotic problems — caused by factors such as
temperature and moisture extremes, mechanical damage, chemicals,
nutrient deficiencies or excesses, salt damage and other environmental
factors. Many plant problems, especially biotic problems, if not
recognized and controlled early in their development, can result in
significant economic damage for the producer. Therefore, timely and
accurate diagnoses are required so that appropriate pest and disease
management options and other corrective measures can be implemented.
Definition of Plant Diagnosis and Steps
Diagnosis is the science and art of identifying the agent or cause of
the problem under investigation. When one renders a diagnosis, one has
collected all available information, clues and observations and then
arrives at an informed conclusion as to the causal factor(s). Hence,
plant problem diagnosis is an investigative, problem-solving process
that involves the following steps:
- Ask and answer the appropriate questions to define the problem and
obtain information that is relevant to the case under investigation.
- Conduct a detailed, thorough examination of the plants and production areas.
- Use appropriate field diagnostic kits and lab tests to obtain clinical information on possible causal agents and factors.
- Compile all the collected information and consult additional resources and references.
- Finally, make an informed diagnosis.
Throughout this process compile all notes, observations, maps,
laboratory results, photographs and other information. This compilation
will be the information base for the present diagnosis and can also be a
useful resource for future diagnostic cases. Keep an open mind as the
information is analyzed and do not make unwarranted assumptions.
Distinguishing Abiotic and Biotic Problems
The first step is to determine whether the problem is caused by an
infectious agent, and this can be difficult. Plant symptoms caused by
biotic factors such as infectious diseases and arthropod pests are often
similar to damage caused by other factors. Leaf spots, chlorosis,
blights, deformities, defoliation, wilting, stunting and plant death can
be common symptoms of both biotic and abiotic problems; therefore, the
presence of these symptoms does not necessarily mean the problem is a
disease. Some general guidelines for distinguishing abiotic and biotic
problems follow and are summarized in table 1.
Table 1 DISTINGUISHING ABIOTIC AND BIOTIC PROBLEMS
often affects several species or plants of various ages
often affects one species or cultivar of the same age
Pattern of plant symptoms
often related to environmental or physical factors or cultural practices; may be regular or uniform
often initially observed in random or irregular locations
Rate of symptom development
relatively uniform, extent of damage appears similar among plants
relatively uneven, time of appearance and damage severity varies among affected plants
no evidence of the kinds of pests or pathogens known to cause the current symptoms
presence of insects, mites,
is not infectious, is not progressive, commonly caused by one incident and does not spread
infectious, spreads on host over time if environmental conditions are suitable
possibly previously associated with current or prior environmental conditions or cultural practices
possibly caused by pests that
Adapted from Table 18, ANR Pub 3420
Biotic problems. Identifying
biotic problems is sometimes facilitated if signs of a pathogen,
primarily the growth of a fungus, are present. The most obvious
examples of such signs are the mycelium and spores produced by rusts and
powdery and downy mildews. However, in other cases nonpathogenic fungi
can grow on top of damaged plant tissues and appear to be signs of a
pathogen, resulting in possible misdiagnoses.
Biotic problems often affect one species or cultivar of the same age
and typically are initially observed in random or irregular locations;
symptoms appear at varying times, and severity varies among affected
plants. Biotic problems are infectious, spreading when environmental
conditions are favorable, and may be associated with pests that have
affected the crop. This infectious aspect is important, as biotic
diseases will many times be progressive and continue to affect
additional tissues and more plants.
Abiotic problems. In contrast to biotic
factors, abiotic problems often affect several species or plants of
various ages; typically, damage is relatively uniform, doesn't spread
and is often not progressive. Abiotic problems are not associated with
pests. They are often caused by a single incident and are related to
environmental or physical factors or cultural practices. Once the
responsible factor has dissipated and is no longer affecting the plant,
the plant may grow out of the problem and develop new, normal appearing
Diagnosing Biotic Problems
Infectious diseases. To confirm if
a problem is caused by a pathogenic fungus, bacterium, nematode, or
virus, it is often necessary to have symptomatic tissues analyzed by a
trained horticulturalist or plant pathologist. Such experts will
attempt to microscopically observe the agent and recover it, if
culturable, through isolation procedures. Lab analysis is particularly
important to determine if multiple pathogens are infecting the plant. A
downside is that obtaining a diagnosis from lab analysis is not a fast
process. However, quick test kits (fig. 1A) are available that can be
used to rapidly identify many common diseases in the field. (Editors'
note: See Steve Tjosvold's regional report for more details.)
Fig.1. Diagnosing biotic
problems. Plant pathogens can sometimes be rapidly diagnosed using
commercially available quick tests, such as these test strips for
viruses (A). Arthropod pests such as Cuban laurel thrips (shown here on Ficus) cause feeding damage, which can help in pest identification (B). Photos: S.T. Koike (A), J. K. Clark (B).
It is worthwhile to emphasize that diagnosing plant diseases requires
careful examination of the entire plant specimen. Symptoms on leaves,
stems, or other above ground plant parts might lead one to suspect that a
foliar pathogen is involved. However, these symptoms could also result
if the roots are diseased. Therefore, it is important to conduct a
complete examination of the symptomatic plant.
Because biotic diseases are caused by living microorganisms, the
collecting and handling of samples is particularly critical. Samples
that are stored for too long a time after collecting or that are allowed
to dry out or become hot (if left inside a vehicle, for example) will
sometimes cause the pathogen in the sample to die, making pathogen
recovery and identification impossible. Plants that have been diseased
for a long time and that are in the late stages of disease development
will often be colonized by nonpathogenic saprophytic organisms. If
these tissues are collected, it will be difficult to recover the primary
pathogen of concern because of the presence of these secondary decay
organisms. Root samples should be collected carefully as diseased roots
are sometimes difficult to dig out of the potting mix or soil, are
usually colonized by the pathogen as well as secondary agents, and are
very sensitive to high temperatures and drying conditions.
Arthropod and other invertebrate pests. Insects,
mites, slugs and snails cause damage while feeding on the plant (fig.
1B). Feeding damage is usually associated by the type of feeding
characteristics and mouthparts of the insect or pest. For example,
mites and insects such as whiteflies, aphids and mealybugs have tubular
sucking mouthparts that suck plant fluids, causing buds, leaves, or
flowers to discolor, distort, wilt, or drop. Thrips have rasping
mouthparts that result in dried out, bleached plant tissue.
Caterpillars, weevils, snails and slugs have chewing mouthparts that
make holes and cuts in foliage or flowers. They can also prune plant
parts and sometimes consume entire plants.
If present, these pests are visible with the naked eye, a 10 X hand
lens, or stereomicroscope, all depending upon their size. An
assessment of whether the identified arthropod or invertebrate matches
the plant damage it is associated with must be determined. Sometimes
the identified arthropod or invertebrate may not be the sole problem or
could, in fact, be a beneficial organism or insignificant pest.
Aphids, whiteflies, thrips, leafhoppers and some other insects that
suck plant juices may vector pathogens such as viruses and phytoplasmas
(and to a lesser extent fungi and bacteria). They can feed on infected
plants, acquire the pathogen, feed on healthy host plants and transmit
the pathogen to the new host. The insects do not necessarily have to be
present in large numbers to cause a significant disease outbreak. The
insect vectors are not always present at the same time the disease
symptoms are being expressed.
The excrement and byproducts from these pests can also provide clues
that the pests have been or are actively present. Caterpillars and
other chewing pests produce dark excrement or droppings. Greenhouse
thrips and plant bugs produce dark, watery, or varnish-like droppings on
foliage. Aphids, whiteflies, soft scales, and some other sap-sucking
insects excrete excess plant fluids as honeydew, a sticky sap, which
provides a medium for the growth of sooty mold.
Diagnosing Abiotic Problems
Nutrient deficiencies and toxicities. Nutrient
deficiencies and toxicities reduce shoot growth and leaf size, cause
leaf chlorosis (fig.2A), necrosis and dieback of plant parts. However,
nutrient deficiencies cannot be reliably diagnosed on the basis of
symptoms alone because numerous other plant problems can produce similar
symptoms. There are general symptoms that can be expressed by
deficiencies of nutrients but usually leaf and/or soil samples are
needed to confirm the problem.
Fig. 2. Examples of abiotic problems. Iron deficiency on sweet gum (Liquidambar styracifolia) showing interveinal chlorosis (A). Chorotic spots on Hedera caused by a miticide application at a higher dosage rate than specified on the pesticide label (B). Photos: E. Martin (A), S. A. Tjosvold (B).
Herbicide, insecticide and fungicide phytotoxicity. Herbicides
used to control weeds in crops or in non-cropped areas sometimes injure
ornamental crops when they are not used in accordance with label
instructions. Examples include when an herbicide is used in or around
sensitive non-target crops, when an herbicide rate is increased above
tolerable limits, or when an applicator makes a careless application.
By understanding the mode of action of the herbicide, one can determine
if the symptom fits an herbicide application. Herbicide detection in
affected plants is possible with the help of a specialized laboratory
but the analysis can be expensive. To minimize the cost of testing, the
laboratory will need to know the suspected herbicide or its chemical
group to narrow the analysis.
Insecticides and fungicides occasionally cause obvious plant damage.
Symptoms can vary widely. Generally, flower petals are more
susceptible to damage from pesticide applications than are leaves. The
younger and more tender the leaves the more susceptible they are to
pesticide applications. Hot weather can exacerbate the damage the
chemicals cause. Pesticides that have systemic action can have a more
profound effect. Some active ingredients can adversely affect the
photosynthetic mechanism or other physiological processes and can result
in a general leaf chlorosis, interveinal chlorosis, leaf curling and
stunting. Emulsifiable concentrate (EC) formulations, soaps and oils
can adversely affect the waxy surface layer that protects the leaf from
desiccation. Applications with these products can result in the loss of
the shiny appearance of a leaf, leaf spotting and necrosis. Pesticides
applied as soil drenches can cause poor germination, seedling death, or
distorted plant growth.
Check label precautions against use on certain species. Make sure the
pesticide is not applied more frequently or at a higher rate (fig. 2B)
than recommended, or that the pesticide is not mixed with incompatible
pesticides. When in doubt as to whether the plant species is sensitive
to the pesticide, spray a few plants and observe them for several days
to a week for any signs of damage before spraying any more of the
Physiological and Genetic Disorders
There are numerous disorders that can occur because of environmental
extremes — too much or too little of an environmental element such as
light, temperature, water, or wind. Sunburn is damage to foliage and
other herbaceous plant parts caused by a combination of too much light
and heat and insufficient moisture. A yellow or brown area develops on
foliage, which then dies beginning in areas between the veins. Sunscald
is damage to bark caused by excessive light or heat. Damaged bark
becomes cracked and sunken. Frost damage causes shoots, buds and
flowers to curl, turn brown or black and die. Hailstones injure leaves,
twigs, and in serious cases even the bark. Chilling damage in
sensitive plants can cause wilting of foliage and flowers and
development of dark water-soaked spots on leaves that can eventually
turn light brown or bleached, and die. Physical and mechanical injury
can occur when plants are mishandled during transport or routine
cultural practices. Wounds might serve as entry sites for plant
pathogens and can attract boring insects to woody stems.
In closed environments such as greenhouses and nursery storage areas,
plants can be exposed to toxic levels of ethylene gas. Sources of
ethylene include improperly functioning or unvented greenhouse heaters;
exhaust from engines of forklifts and vehicles; cigarette smoke;
damaged, decaying, or dying plants; and ripe or decaying fruit. Toxic
levels of ethylene gas can cause premature abscission of flower buds,
petals (fig. 3) and leaves. Other symptoms include wilted flowers,
chlorosis, twisted growth or downward bending of stems and leaves and
undersized or narrow leaves.
Fig. 3. Poor air quality can
lead to physiological disorders. Shattering (petal drop) on geranium was
caused by plant exposure to low levels of ethylene in the greenhouse or
during postharvest storage (A). Yellowish and brownish patches on
Japanese maple leaves are damage caused by ozone (B), an outdoor air
pollutant. Photos: J. K. Clark.
Outdoors, exposure of nursery plants to air pollutant gases such as
ozone (fig. 3), carbon monoxide, nitrous oxides and sulfur dioxide can
cause damage. Typical symptoms vary widely, but include slow growth and
discolored, dying, or prematurely dropping foliage. Damage is often
found where plants are located near sources of polluted air such as near
freeways or industries or where weather and topography concentrate the
Sometimes plants or plant shoots exhibit an unusual and sudden change
of color producing discrete markings of variegation. For example, a
plant with entirely green leaves suddenly produces a shoot that has
leaves with edges lacking green pigment, stripes, or blotches. A new
shoot such as this is probably a chimera (fig. 4). It is produced when a
genetic mutation occurs in a specific region of the growing tip
resulting in a section with genetically different cells. The ostensible
result of the genetic change is dependent on the arrangement of the
genetically different cells in the shoot tip and their expression. This
can lead to sometimes bizarre variegation forms or sometimes forms that
are quite desirable. Sometimes variegation can be caused by viruses.
Viruses usually cause non-uniform chlorosis, such as mosaics, while
chimeras usually produce patterned forms such as variegation of color on
leaf margins, stripes, or complete loss of pigment. Some viroids may
also cause bleaching of pigments in leaves; such symptoms, however, are
generally produced throughout the plant and are not restricted to a
single shoot. Some nutrient disorders can cause variegation but these
disorders usually do not arise from a specific shoot as with chimeras.
Fig. 4. Genetic disorder.
Growing points with variegated leaves can sometimes arise spontaneously
from some species such as this Origanum. Genetic variants such as this are sometimes confused with plants with virus disease or nutrient deficiency symptoms. Photo: S. A. Tjosvold.
Steve Tjosvold is Environmental Horticulture Advisor and
Steve Koike is Plant Pathology Farm Advisor, UC Cooperative Extension,
Santa Cruz and Monterey counties.
This article was condensed from: Diagnosing Plant
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When you see dieback in an avocado it could be due to several reasons, and here are three very common causes of dieback, one of which has been especially common when there is little rain. The first major cause of dieback is an overgrown tree where there is no light that penetrates into the canopy. Branches with leaves in a darkened interior will naturally shut down and dieback, leaving these twiggy dried out branches. This is a natural process whereby the tree just gets rid of leaves that are not performing.
Another cause of dieback is our old friend Avocado Root Rot, Phytophthora cinnamomi. This causes dieback, also called “stag horning” because of the dead branches standing out from the surrounding canopy. This is normally accompanied by a thinning canopy with smaller, yellow leaves and a lack of leaf litter because of lack of energy. It's also hard to find roots and if they are found, they are black at the tips and brittle.
The third major cause of dieback is a result of water stress. This shows up with low water pressure, at the top of the hill where the most wind occurs, where a sprinkler gets clogged, when the irrigation schedule is not meeting tree's needs or when there is not adequate rainfall to get sufficient leaching. And after four years of drought, this is very common. This appears as dead spots in the canopy, a branch here and there where the leaves have died and are still hanging. It's been called “salt and pepper” syndrome, because it can have a few branches here and there that have died back while the rest of the canopy is normal, the leaves are normally sized and green. In young trees, in severe cases, the fungus that causes this blight can work its way down to the graft union and kill the tree. In mature trees, it just causes an unthrifty look to the tree. Although we have always seen this problem in avocado orchards, this has become a very common affliction in orchards these last few years
1) Lack of light dieback
2) Phytophthora cinnamomi dieback
3) Stem and Leaf Blight
From Peru This Week, Hillary Ojeda:
Unfavorable climate conditions around the world have increased demand from Peru.
In February, the Ministry of Irrigation and Agriculture confirmed that Peru is the second largest exporter of avocados. This will prove crucial in the coming months, as production around the world is at a shortage.
“Prior to the season a higher production was expected in Peru for 2015 compared to 2014, but now it turns out that the total Peruvian harvest is a bit lower,” Ine Potting (Mission Produce) told Fresh Plaza.
The produce news portal reports that the avocado market is currently experiencing a shortage due to primary exporters struggling to produce up to expectations.
“Due to hail in Mexico, which affected a lot of the Flor Loca harvest blossom, and the drought in California, the harvests there will also be less than satisfactory. This means that there will be more demand from the United States for avocados from Peru,” says Ine.
Considering these circumstances, the price is increasing (of green varieties as well in addition to Hass avocados) and less shipments will make it Europe this season.
Peru's avocado industry made US$ 308 million in 2014 and exported a record amount of 177,800 tons of the and creamy Hass avocado variety. These results secured its place as second largest exporter of the large berry, after Mexico.