A call from a small grower, surprised at the sudden decline of the avocado trees. It must be a disease was the grower's thought. Well driving up to the site, there were numerous trees with canopies indicating drought stress. In fact most of the trees looked like they had had the water turned off. When I got to the orchard, all the trees had a similar look (see photo below). The fringe of the canopy had turned brown/red where the leaves had collapsed rapidly, while the interior leaves were often still green. All the trees had a similar cast. It turns out the water district had required a cutback just when temperatures were going into the 100's. NO water, no cooling effect of transpiration and the outer fringe of leaves collapsed. This is called the “clothesline” effect. It's like a sheet on a clothesline where the margins of the sheet dry first and gradually the body of the sheet dries. The same thing happens in a canopy. The outside leaves are the first to dry out and then the rest of the canopy goes. When you see a whole orchard go down suddenly, that does not fit into a disease pattern. There's usually an epicenter where it starts – where it's colder, wetter, dryer, hotter, more overgrown, etc. and spreads out from there if it is going to spread. It turns out that the automatic irrigation system had gone down and the grower hadn't noticed until too late. When you see reddish tinged leaves, it means the leaves went down fast. When they are brown, it means they slowly went down over weeks or months.
With all the dead points in the tree, it is now open to disease – twig/leaf blight caused by one of the Botryosphaerias. These decay fungi are everywhere in an orchard decaying organic material on the orchard floor. With the dead material in the tree, now the tree becomes a potential feast for the fungi. The dead stuff has to come out, or the fungus will start eating into the tree. I suggested that instead of pruning out all those little points of death, that they cut back the whole canopy to major scaffold branches. In doing so, it would rapidly and cheaply remove the dead material and reduce the water demand.
Drought Induced Problems in Our Orchards
Abiotic disorders are plant problems that are non-infective. They are not caused by an organism, but through their damage, they may bring on damage caused by organisms. Think of a tree hit by lightning or a tractor. The damage breaches the protective bark which allows fungi to start working on the damaged area, eventually leading to a decayed trunk. It was the mechanical damage, though that set the process in motion.
Too much or too little water can also predispose a plant to disease. Think of Phytophthora root rot or even asphyxiation that can come from waterlogging or too frequent irrigations.
Salinity Effects from Lack of Water
Lack of water and especially sufficient rainfall can lead to salinity and specific salts like boron, sodium and chloride accumulating in the root zone. This happens from a lack of leaching that removes native soil salts from the root zone or the salts from the previous salt-laden irrigation from the root zone. These salts cause their own kind of damage, but they can also predispose a tree to disorders, disease and invertebrate (insect and mite) damage.
Lack of water and salt accumulation act in a similar fashion. Soil salt acts in competition with roots for water. The more soil salt, the harder a tree needs to pull on water to get what it needs. The first symptom of lack of water or salt accumulation may be an initial dropping of the leaves. If this condition is more persistent, though we start to see what is called “tip burn” or “salt damage”. Southern California is tremendously dependent on rainfall to clean up irrigation salts, and when rain is lacking, irrigation must be relied on to do the leaching
As the lack of leaching advances (lack of rainfall and sufficient irrigation leaching) the canopy thins from leaf drop, exposing fruit to sunburn and fruit shriveling.
Leaf drop and fruit shriveling in avocado.
In the case of sensitive citrus varieties like mandarins, water stress can lead to a pithy core with darker colored seeds, almost as if the fruit had matured too long on the tree.
Total salinity plays an important factor in plant disorder, but also the specific salts. These salts accumulate in the older leaves, and cause characteristic symptoms that are characteristic in most trees. Boron will appear on older leaves, causing an initial terminal yellowing in the leaf that gradually turns to a tip burn.
Often times it is hard to distinguish between chloride, sodium and total salinity damage. It is somewhat a moot point, since the method to control all of them is the same – increased leaching. There is no amendment or fertilizer that can be applied that will correct this problem. The damage symptoms do not go away until the leaf drops and a new one replaces it. By that time hopefully rain and/or a more efficient irrigation program has been put in place.
The Impact of Drought on Nutrient Deficiencies
Salinity and drought stress can also lead to mineral deficiencies. This is either due to the lack of water movement carrying nutrients or to direct completion for nutrients. A common deficiency for drought stressed plants is nitrogen deficiency from lack of water entraining that nutrient into the plant.
This usually starts out in the older tissue and gradually spreads to the younger tissue in more advanced cases.
The salts in the root zone can also lead to competition for uptake of other nutrients like calcium and potassium. Apples and tomatoes are famous for blossom end rot when calcium uptake is low, but we have also seen it in citrus. Low calcium in avocado, and many other fruits, leads to lower shelf life. Sodium and boron accumulation in the root zone can lead to induced calcium deficiencies and increased sodium can also further lead to potassium deficiencies. Leaching can help remove these competitive elements.
Drought Effect on Tree Disease
Drought and salt stress can also lead to disease, but in many cases once the problem has been dealt with the disease symptoms slowly disappear. They are secondary pathogens and unless it is a young tree (under three years of age) or one blighted with a more aggressive disease, the disease condition is not fatal. Often times, in the best of years, on hilly ground these diseases might be seen where water pressure is lowest or there are broken or clogged emitters. The symptoms are many – leaf blights, cankers, dieback, gummosis – but they are all caused by decomposing fungi that are found in the decaying material found in orchards, especially in the naturally occurring avocado mulch or artificially mulched orchards. Many of these fungi are related Botryosphaerias, but we once lumped then all under the fungus Dothiorella. These decay fungi will go to all manner of plant species, from citrus to roses to Brazilian pepper.
Another secondary pathogen that clears up as soon as the stress is relieved is bacterial canker in avocado. These ugly cankers form white crusted circles that ooze sap, but when the tree is healthy again, the cankers dry up with a little bark flap where the canker had been.
Drought Effect on Pests
Water/salt stress also makes trees more susceptible to insect and mite attack. Mites are often predated by predacious mites, and when there are dusty situations, they can't do their jobs efficiently and mites can get out of hand. Mite damage on leaves is often noted in well irrigated orchards along dusty picking rows
Many borers are attracted to water stressed trees and it is possible that the Polyphagous and Kuroshio Shot Hole Borers are more attracted to those trees.
And then we have conditions like Valencia rind stain that also appears in other citrus varieties. We know it will show up in water stressed trees, but we aren't sure what the mechanism that causes this rind breakdown just at color break. Could it be from thrips attracted to the stressed tree or a nutrient imbalance, it's not clear?
Water and salt stress can have all manner of effects on tree growth. It should lead to smaller trees, smaller crops and smaller fruit. The only way to manage this condition is through irrigation management. Using all the tools available, such as CIMIS, soil probes, soil sensors, your eyes, etc. and good quality available water are the way to improve management of the orchard to avoid these problems.
Scroll down for Images
Tip Burn, notice sun burn bottom right hand fruit
Endoxerosis with dried out core
Blossom end rot
Bot gumming in lemon
Black Streak in Avocado
Citrus red mite
Polyphagous Shot Hole Borer damage on avocado
Valencia Rind Stain
Years of drought, and a stressed tree are a perfect set up for navel oranges and fruit splitting.
The days have turned cooler and suddenly out of nowhere there is rain. That wonderful stuff comes down and all seems right with the world, but then you notice the navel fruit are splitting. Rats! No, a dehydrated fruit that has taken on more water than its skin can take in and the fruit splits. This is called an abiotic disease. Not really a disease but a problem brought on by environmental conditions.
Fruit splitting is a long-standing problem in most areas where navel oranges are grown. In some years, the number of split fruit is high; in other years it is low. Splitting in navel oranges usually occurs on green fruit between September and November. In some years, splitting may also occur in Valencia oranges but it is less of a problem than in navel oranges.
Several factors contribute to fruit splitting. Studies indicate that changes in weather including temperature, relative humidity and wind may have more effect on fruit splitting than anything else. The amount of water in a citrus tree changes due to weather conditions and this causes the fruit to shrink and swell as water is lost or gained. If the water content changes too much or too rapidly the rind may split. In navel oranges the split usually occurs near the navel, which is a weak point in the rind.
Proper irrigation and other cultural practices can help reduce fruit splitting. Maintaining adequate but not excessive soil moisture is very important. A large area of soil around a tree should be watered since roots normally grow somewhat beyond the edge of the canopy. Wet the soil to a depth of at least 2 feet then allow it to become somewhat dry in the top few inches before irrigating again. Applying a layer of coarse organic mulch under a tree beginning at least a foot from the trunk can help conserve soil moisture and encourage feeder roots to grow closer to the surface.
If trees are fertilized, apply the correct amount of plant food and water thoroughly after it is applied. If the soil is dry, first irrigate, then apply fertilizer and irrigate again.
- 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
Problems, Chapter 11. In Newman, J. (ed) Container Nursery Production
and Business Management. Univ. of Calif. Agric. and Nat. Resources.
Publication 3540. Richmond, CA.
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- 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 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.