- Author: Ben Faber
It's that time of year to see some drama in avocado orchards. Once healthy-looking trees can suddenly turn brown in a weekend and all the surrounding trees still look fine. And it can be quite common in some years along the coast. The winter weather will have mild, cool even rainy days and then suddenly there's one of those 97 deg days and the tree goes down,
The entire tree or only one or several branches wilt suddenly when affected by Verticillium wilt. Leaves turn brown and die, but the dead leaves usually remain on the tree for several months. Brown to gray-brown streaks are visible in the xylem of the branches or roots when the bark is removed. Sometimes the streaking is visible in the branches, but often it is found at the base of the trunk.
Trees with Verticillium wilt often send out new, vigorous shoots within a few months after the initial wilting. If well cared for, affected trees often recover completely with no reoccurrence of the disease. However, not all trees survive an infection and disease symptoms sometimes reoccur after an apparent recovery.
The fungal pathogen Verticillium dahliae infects many hosts, including various berry and flower crops, cotton, eggplant, olive, pepper, stone fruit trees, strawberry, and tomato. Verticillium wilt is present throughout the state but is less common in avocado than root rot and canker diseases. Verticillium dahliae persists for years as microsclerotia in soil. Microsclerotia spread in infested organic matter and soil that is moved. The fungus infects through feeder roots, and then moves up in the water-conducting xylem system, restricting or preventing water movement to foliage from the roots.
No known methods are effective in curing infected trees. Trees often recover completely and display no further symptoms, even though they are still infected. After dieback ceases and new growth begins, prune off dead branches. Provide optimal irrigation and modest fertilization to promote new growth. If a tree dies from Verticillium, remove it. But give it a chance, there's a good chance it will recover.
In areas where V. dahliae is known to occur, plant Mexican rootstocks instead of the more Verticillium-susceptible Guatemalan rootstocks. Do not plant avocado on land where crops susceptible to Verticillium wilt have previously grown. Do not interplant avocado with other hosts of Verticillium, which are listed in publications such as Plants Resistant or Susceptible to Verticillium Wilt (PDF). Even if they have recovered, do not use trees infected with Verticillium wilt as a source of budwood or seed.

- 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 |
||
Characteristics |
Abiotic |
Biotic |
Hosts |
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 |
Signs |
no evidence of the kinds of pests or pathogens known to cause the current symptoms |
presence of insects, mites, |
Spread |
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 |
Recurrence |
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.

A B
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.

A B
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.

A B
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.
References
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
3420.
Derr JF, Appleton BL. 1988. Herbicide injury to trees and shrubs: A
pictorial guide to symptom diagnosis. Virginia Beach, VA: Blue Crab
Press.
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
Technology 11:384–393.
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
Society Press.
Stewart TM, Galea VJ. 2006. Approaches to training practitioners in
the art and science of plant disease diagnosis. Plant Disease
90:539–547.
Tickes B, Cudney D, and Elmore C. 1996. Herbicide injury symptoms.
Tucson, AZ: University of Arizona Cooperative Extension Publication No.
195021.

- Author: Ben Faber
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.
- Author: Ben Faber
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
Boron toxicity
Nitrogen deficiency
Blossom end rot
Potassium deficiency
Bot gumming in lemon
Black Streak in Avocado
Bacterial Canker
Citrus red mite
Polyphagous Shot Hole Borer damage on avocado
Valencia Rind Stain











- 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 |
||
Characteristics |
Abiotic |
Biotic |
Hosts |
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 |
Signs |
no evidence of the kinds of pests or pathogens known to cause the current symptoms |
presence of insects, mites, |
Spread |
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 |
Recurrence |
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
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. (Editors'
note: See Steve Tjosvold's regional report for more details.)

A B
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.

A B
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
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. 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.

A B
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
pollutants.
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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