Mandarins, also known as “zipper skins” and “easy peelers” can have very fragile peels/skins/rinds/exocarp that make them easily subject to more damage than most oranges and lemons. Some are a bit tougher skinned than others, but some are so fragile that any rough handling often prevents them from going through conventional packing operations.
These skins were recently put to the test in the recent fires in Ojai. There was a mix of different varieties - ‘Pixie', ‘Gold Nugget', ‘W. Murcott', ‘Yosemite Gold', ‘Tahoe Gold' and others. Some of them were more sensitive than others, some were closer to the fire, all were affected by smoke to some degree. In Matilija Canyon where smoke was present for many more days than in the east of the Ojai Valley and possibly more ash, the trees have started flowering sooner. That might be temperature difference, either cooler or warmer, so it is hard to say how much effect the smoke has had versus, the ash and/or heat. Smoke has many different gasses in it, one of which is ethylene which is a naturally occurring ripening agent. Smoke not only has gasses, but it occludes the sun so less or more or altered light might have an effect on these fruit. It's not a controlled experiment, so some little scientist is going to have to come along and wriggle out these different effects. Whatever. Fire and smoke have an effect on mandarins as we have seen in other crops, such as cherimoya, avocados and other citrus.
Heat damage. Fruit facing the fire.
Ash effects on fruit coloring. Fruit was covered with ash for several days until rain washed it off. Might be a pH effect (ash is alkaline), temperature effect, uneven light radiation, or other…….
Same sort of uneven coloring, that actually looks like an ashy color, but the ash has washed off the cluster by rain
And here's something interesting where fruit facing the fire is much lighter colored than fruit facing away from the fire. Here are two pieces of fruit, one from the side directly facing the fire, and the other from the other side of the tree. The side of that fruit facing the fire was also lighter colored. So, it had an effect through the canopy (small tree). The canopy was otherwise intact, unaffected heat or flames.
Oh yeah, and there is the characteristic fruit drop from either the heat, smoke gases, water stress or ….
And then there's the fruit that looks like it had actual embers on the skin.
If the tree survives and keeps its green leaves, sometimes the fruit is affected in ways that don't appear for a while. The peel may be affected, but in many cases the fruit is just as sweet as it could be. It just looks terrible. That might even be a selling point. "Here have a wonderous piece of history that braved the horror of the Ojai fires."
There have been some complaints about satsuma mandarin fruit having problems. These are prone to a rind/skin/peel breakdown when the fruit is not picked promptly. It's not clear what the cause is - wet winter, warm winter - but it is less of a problem if the fruit is picked when it is mature. A lot of the time in southern California, satsumas will develop good flavor and sweetness, but for lack of cool weather, they don't turn bright orange, a hallmark of the fruit. So growers will leave the fruit on longer, hoping for color, but the fruit becomes over mature, and more susceptible to breakdown. This weakening of the peel then opens it up to infection by fungi, such as Alternaria. In warm winters, the peel matures more rapidly and is more susceptible. Early maturing varieties like ‘Okitsuwase' are especially prone to breakdown later in the season, since their rind matures earlier. They end up being a mess, as can be seen in the photo below.
Navels can have a similar problem in these winters with erratic rainfall. Common wisdom is you don't irrigate in the winter, right? Wrong. With no, low and widely spaced rain events, the tree roots dry out, and rewet with rain. Navels are building their sugar in the winter and they become suction balls for water as the sugar increases. The fruit will continue to grow as the tree takes up water. When the roots run out of water, and then are suddenly rewetted during this period, the fruit can suck up water so rapidly that the skin cant expand fast enough and will split. So this is what happens with uneven irrigation or rainfall this time of year. One of those abiotic problems in citrus.
- Author: Steve Tjosvold and 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 oftensimilar 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 fungican 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 foliage.
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.
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 requirescareful examination of the entire plant specimen. Symptoms on leaves, stems, or other above ground plant parts might lead one to suspect that afoliar pathogen is involved. However, these symptoms could also resultif 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 allowedto 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 primarypathogen of concern because of the presence of these secondary decay organisms. Root samples should be collected carefully as diseased rootsare 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 bepresent 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 onfoliage. 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. Nutrientdeficiencies 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 similarsymptoms. 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. Herbicidesused to control weeds in crops or in non-cropped areas sometimes injureornamental 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. Pesticides 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 resulti n 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 ofthe shiny appearance of a leaf, leaf spotting and necrosis. Pesticidesapplied as soil drenches can cause poor germination, seedling death, or
distorted plant growth.
Check label precautions against use on certain species. Make sure thepesticide 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 plants.
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. Sunscaldis 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 anddiscolored, 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 pollutants.
Sometimes plants or plant shoots exhibit an unusual and sudden changeof 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 thatare 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 onleaf 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.
Boxer P, Sandmann G. 1989. Target sites of herbicide action. Boca Raton, FL: CRC Press.
Costello L, Perry E, Matheny N, Henry M, Geisel P. 2003. Abiotic
disorders of landscape plants: A diagnostic guide. Oakland: University
of California Division of Agriculture and Natural Resources Publication
Derr JF, Appleton BL. 1988. Herbicide injury to trees and shrubs: A
pictorial guide to symptom diagnosis. Virginia Beach, VA: Blue Crab
Dreistadt SH. 2001. Integrated pest management for floriculture and
nurseries. Oakland: University of California Division of Agriculture and
Natural Resources Publication 3402.
Eagle, DJ. 1981. Diagnosis of herbicide damage to crops. New York, NY: Chemical Publishing Co.
Grogan RG. 1981. The science and art of plant disease diagnosis. Annual Review of Phytopathology 19:333–351.
Ratzinger EJ, Mallory-Smith C. 1997. Classification of herbicides by
the site of action for weed resistance management strategies. Weed
Schubert TS, Breman LL. 1988. Basic concepts of plant disease and how
to collect a sample for disease diagnosis. Plant Pathology Circular No.
307. Florida Department of Agriculture and Consumer Services, Plant
Pathology Circular No. 307.
Sharma MP. 1986. Recognizing herbicide action and injury. Alberta Environmental Centre, Alberta Agriculture. Agdex 641–647.
Shurtleff MC, Averre CW. 1997. The plant disease clinic and field
diagnosis of abiotic diseases. St. Paul, MN: American Phytopathological
Stewart TM, Galea VJ. 2006. Approaches to training practitioners in
the art and science of plant disease diagnosis. Plant Disease
Tickes B, Cudney D, and Elmore C. 1996. Herbicide injury symptoms.
Tucson, AZ: University of Arizona Cooperative Extension Publication No.
- 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: Shoot dieback in citrus and iron chlorosis in avocado
Avocado harvest time and the growers are in the orchard checking things out a little more closely and to see what is going into the bins…..and they see some unusual shaped fruit. Here's what's been popping up and some possible explanations.
Crick-side - First described by Dr. J. Eliot Coit as kink-neck and later by Horne (1931) as kink-side. Finally, the name crick-side (Horne, 1934) was adopted. It is characterized by a definite depression on one side between the stem end and the larger portion of the fruit causing a distortion. In some cases, the area of depression turns black and the fruit drops. In other cases, the fruit grows and matures but the distortion remains. Crick-side is usually found on trees carrying a heavy load of fruit. It has been suggested that high temperatures or temporary water-stress may relate to the occurrence of crick-side, but no definite determination as to its cause has been made.
Carapace Spot - First described by Horne (1929), the name carapace-spot was chosen because of the resemblance to a turtles' back. This external blemish is corky and usually cracked into somewhat regular, angular divisions. The flesh under the carapace spot is undamaged, but exterior appearance is such that the fruit is reduced in grade. Slight rubbing or brushing of tender young fruit on leaves or stems appears to cause this corky growth to start. Fruit on trees exposed to strong winds are more apt to develop the trouble. Windbreaks should reduce injury in windy areas.
Photo: Avocado thrips damage, carapace damage and greenhouse thrips damage.
Sunblotch - This is a viroid that can affect fruit, leaves, and stems with a yellow or reddish streaking, cause a compacted growth and willowy growth habit. The streaking in the fruit is usually depressed and doesn't extend the length of the body.
Sunburn - Fruit exposed to full sun may be injured by sunburn. This occurs when trees defoliate, or partially defoliate, from any of several causes, leaving the fruit exposed. It is normally most severe on fruit on the south and southwest portion of the tree. Sunburn shows as a pale yellowish area on the exposed side of the fruit. Often the center of this area turns brown to black and may wither.
Ring Neck - This trouble has been observed occasionally, particularly with Hass. The cause is unknown but is believed to be related to soil-plant water deficiency at a critical time. A ring of tissue on the pedicel just above the attachment to the fruit dies, turns black and peels off. If only superficial, the fruit remains on the tree. Growth may be retarded because the restriction impedes movement of nutrients and water outward to the fruit. Most severe in humid coastal areas.
Embossment - Occasionally, and particularly on Fuertes, a section of the surface will be raised slightly or be a darker or lighter color. This is referred to as a sectional chimera or genetic mutation.
Healed over damage - if fruit has mild damage that allows it to heal over (remember avocado fruit expand by cell multiplication not enlargement), then a scar is left, such as this likely amorbia feeding
Cuke - As in cucumber but not a squash. These are seedless fruit that can most often be seen from a fruit set in cooler weather or due to some hormonal stimulus. We don't know the reason, but seems to occur more commonly along the coast.
Double Fruit - In some instances there may be a normal shaped fruit with a single cuke attached ot in some cases there is a double ovary and two fruit are attached.
Woody Avocados - For some unknown reason, avocado fruit will form into a grotesque woody structure hardly resembling an avocado. The cause is genetic and non-transmissible.
Sources: R.G. Platt - California Avocado Society Yearbook 1972-73 and Reuben Hofshi and M.L. Arpaia Yearbook 2002.