Posts Tagged: disease
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.
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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.
California Avocado Society's
2017 Annual Meeting
This year marks our 102nd Annual Meeting and it will be held
in Ventura County on October 20 - 21, 2017
**Register & pay online or mail or fax the form with payment.
**Late fee of $25 will be added on registrations after
Monday, October 16th.
Friday Field Tour
Registration at Hansen Agricultural Center
(Directions to the fields will be given at Registration)
Best to register before October 20th
Fruit Growers Laboratories: Water, Leaf and soil analysis technologies along with some pathology analysis.
Brokaw Orchard: High Density. Management changes over time.
Dominguez Orchard: High Density. Management and pruning.
Friday Evening President's BBQ Reception
At Limoneira Ranch Headquarters.
It will be a special evening as members and friends network and share industry information.
Saturday Annual Meeting
Held at Four Points by Sheraton Ventura Harbor, CA
1050 Schooner Drive, Ventura, CA, 93001
We have a great line up of speakers planned
Vice-President Fruit Growers Laboratories
Soil and Plant Analysis--Common and Not So Common Problems.
Dr. Raquel Folgado
Cryopreservation Researcher, The Huntington
Cryopreservation of Avocado Germplasm and New Technology Spin-offs for Propagation and Breeding.
Director of International Cooperation, Brokaw Nursery
Gem Variety Management, and New Rootstocks.
Dr. Peggy Mauk
UCR Director of Ag Operations and CE Subtropical Specialist
Rootstock Trial Progress.
Owner and Chief Pilot, HawkEye Imaging
Remote Sensing Technology—Now and in the Future.
M. Cristina Léon
CAS Board Member
Highlights of the CAS tour to Michoacan, Mexico.
Shawn Martin, Organic Grower Carpinteria; Kevin Ball, Ag Land Services; Ed McFadden, Orchard Manager.
Moderated by Susan Estrada, Newsline Editor.
Awards: Oliver Atkins Award and Award of Honor.
You will find the pricing and detailed information for
Saturday's agenda, Friday's tours and President's reception on the registration pamphlet.
For the CAS's special rate rooms at the Four Points by Sheraton
for single/double rate $129
Please call in-house reservations:
(805) 658-1212 ext. 1290 (Mention "California Avocado Society" for the discounted rates.
Cutoff date for the special rates will be September 27, 2017.
Citrus: UC IPM Pest Management Guidelines
|Publication Number: 3441
Copyright Date: Rev. 2017
Length: 234 pp.
Inventory Type: PDF File
|This is a free publication if you access it as a web page or downloadable PDF document.
These official UC-approved guidelines for pest monitoring techniques, pesticide use, and nonpesticide alternatives for agricultural crops are essential tools for anyone making pest management decisions in the field. This 124-page guideline covers citrus fruit.
Updated August 2015.
A hard copy version of these guidelines can be purchased as Publication 3441P.
The PDF version of this publication is best viewed using the free Adobe® Acrobat® Reader. You can download a free copy of the Acrobat Reader from Adobe Systems Incorporated.
Some users have experienced problems using Preview with these documents; we recommend using the Adobe® Acrobat® Reader.
SACRAMENTO — The California Department of Food and Agriculture (CDFA) and the United States Department of Agriculture (USDA) have established a 94-square mile quarantine in portions of Riverside and San Bernardino counties following the detection of the citrus disease huanglongbing (HLB), or citrus greening, in a single citrus tree in the city of Riverside. HLB is a deadly disease of citrus plants and closely related species, and can be transmitted from tree to tree by the Asian citrus psyllid.
The quarantine boundaries are on the north, Interstate 10; on the east, Box Springs Mountain Reserve; on the west, Riverside Municipal Airport; and on the south, East Alessandro Boulevard. HLB quarantine maps for Riverside and San Bernardino counties are available online at: www.cdfa.ca.gov/plant/pe/InteriorExclusion/hlb_quarantine.html. Please check this link for future quarantine expansions in these counties, should they occur. Quarantines are already in place for HLB in portions of Los Angeles and Orange counties.
The quarantine will prohibit the movement of all citrus nursery stock out of the area, while maintaining existing provisions allowing the movement of only commercially cleaned and packed citrus fruit. Any fruit that is not commercially cleaned and packed, including residential citrus, must not be removed from the property on which it is grown, although it may be processed and/or consumed on the premises.
Residents are urged to take several steps to help protect citrus trees:
- Do not move citrus plants, leaves or foliage into or out of the quarantine area, or across state or international borders. Keep it local.
- Cooperate with agricultural crews placing traps, inspecting trees and treating for the pest.
- If you no longer wish to care for your citrus tree, consider removing it so it does not become a host to the pest and disease.
CDFA crews have already removed the infected tree and are in the midst of a treatment program for citrus trees to knock down Asian citrus psyllid infestations within 800 meters of the find site. By taking these steps, a critical reservoir of the disease and its vectors will be removed, which is essential to protect the surrounding citrus from this deadly disease.
HLB is a bacterial disease that attacks the vascular system of plants. It does not pose a threat to humans or animals. The Asian citrus psyllid can spread the bacteria as the pest feeds on citrus trees and other plants. Once a tree is infected, there is no cure; it typically declines and dies within a few years.
CDFA, in partnership with the USDA, local county agricultural commissioners and the citrus industry, continues to pursue a strategy of controlling the spread of the Asian citrus psyllids while researchers work to find a cure for the disease.
—California Department of Food and Agriculture
Over 20 new trees in Southern California have been confirmed HLB-positive. The new finds raise the total number of trees with huanglongbing disease found in California to around 100. All of the trees found in the state have been located in residential areas.
The Citrus Pest and Disease Prevention Program (CPDPP) issued a press release that stated 21 trees in Anaheim, and four trees in Pico Rivera tested HLB-positive. The CPDPP, a program of the California Department of Food and Agriculture (CDFA), stated the current quarantine in Southern California would be slightly expanded in Orange and Los Angeles Counties.
The new detections were found due to intensive surveying that's part of the response program. Highly trained crews sample trees where HLB-positive Asian citrus psyllids have been found. CDFA says their Sacramento facility can process 10,000 samples a month.
All of the detections in California have been residential trees. The CPDPP is currently running an outreach program that involves public service announcements, coordination with officials, and large public events in the quarantine area. The goal is to educate residents on the disease and the insect that spreads it. Go to the CPDPP website to find out more about the disease, insect, and quarantines.
Photo: HLB symptoms
Photo: Sun Blotch Branch
Sun-blotch, a viroid disease affecting the avocado, was observed as early as 1914 in Southern California. Sun-blotch is widespread in California, and likely to be found in all districts where avocados are being grown. The main counties in which the disease is found are San Diego, Ventura, Los Angeles, Orange, Riverside, and Santa Barbara.
Symptom's and signs
Sunblotch causes a wide variety of symptoms or may exhibit no symptoms in some hosts. The rate of spread of sunblotch virus appears to be extremely slow due to improved cultural practices in the nursery and field, and the apparent absence of an insect vector. Symptoms of sunblotch include necrotic, red, yellow, or white discolorations on fruit, often in depressions or scars in the fruit surface. Twigs can develop narrow, necrotic, red or yellow streaks on their surface or in shallow lengthwise indentations along the twig. Leaves may have white or yellowish variegated areas and be deformed, but leaf symptoms are uncommon. Rectangular cracking and checking of the bark, called "alligator bark," often occurs on the trunk and larger branches. Infected trees may be stunted and have a disproportionate amount of horizontal growth or sprawling lateral low limbs. Trees with visible sunblotch symptoms often have reduced yields. Infected trees can also be symptomless, although large reductions in yield of previously vigorous trees may indicate the presence of the viroid in otherwise symptomless carriers.
Photo: Sunblotch fruit
Sunblotch is caused by dozens of variants of submicroscopic particles of genetic material (viroids, smaller than a virus with no protein coat and only nucleic acid) that alter development and growth of infected plants. Sunblotch viroid can move systemically within avocado, and it persists in host tissues. Trees that do not show symptoms even though the viroid is present are known as "symptomless carriers." Nearly all cuttings and seed from symptomless carriers will be infected with viroid. However, seedlings from symptomless carriers do not show symptoms of sunblotch when they are used as rootstocks, but the disease often appears on scions grafted to them. Conversely, most seed from trees with symptoms are not infected, and budwood and shoot cuttings from sunblotch leaf symptomatic trees often do not contain viroid. [BF1] The viroid is transmitted in pollen, but pollen only infects the fruit and seed produced from it. Unless a tree is infected by grafting or some way other than through pollen, there will be no viroid in budwood, root grafts, and shoot cuttings from that tree. [BF2] Transmission of the viroid most often occurs during grafting by using infected budwood or rootstock seedlings from infected trees with or without symptoms. Natural root-to-root grafts are important in transmitting sunblotch in groves. Mechanical transmission through wounds caused by contaminated harvest clippers, pruning tools, and injection equipment can also be important if infected trees are in the grove. [BF3] Spread via pollen from an infected tree to the flower ovule of a noninfected avocado, resulting in infected seed, can cause fruit to be culled, but does not further spread the disease unless seed is propagated. There is no evidence of insect transmission.
Careful propagation of nursery stock to eliminate viroid has greatly reduced sunblotch to a relatively minor disease. However, ongoing monitoring and management is required in nurseries and established groves. Sunblotch can be easily overlooked, and there are many ways that trees can become infected. Look for diseases and disease-promoting conditions regularly throughout the grove. In the nursery, carefully select disease-free scions and seed sources. Use stringent sanitation and frequent disinfection to avoid spreading pathogens. Periodically confirm that propagation sources are disease-free (indexing) by grafting propagative source material to young Mexican seedlings and observing leaves and twigs for sunblotch symptoms, or by performing a genetic test. Plant only indexed nursery stock that is registered as disease-free.
Promptly remove symptomatic trees from the grove and chemically kill the stumps. Do not retain infected, symptomless trees just because yield does not seem to be affected; symptomless carriers are a highly infective source that can dramatically reduce yield on other trees. If only fruit and seed are infected (from infected pollen), it may not be necessary to remove that tree if indexing indicates the rest of the tree is not infected. However, trees with only fruit and seed infection indicate that other infected (possibly symptomless) trees nearby need to be indexed or removed. The danger of spreading viroid increases in established orchards where mature trees are pruned to reduce tree size and restimulate or maintain fruit production. Severe pruning of symptomless carriers, and perhaps other severe causes of tree stress, are suspected of causing viroid to become active in the new growth, inducing previously symptomless trees to exhibit symptoms. Disinfect pruning tools, harvest clippers, and injection equipment before beginning work on a new tree. Scrubbing tools clean and then soaking them in a 1.5% sodium hypochlorite solution is effective. Growers must use a registered disinfectant and follow label directions.
Photo: Sunblotch leaf
As sunblotch is systematic in the avocado tree, it can exist in the roots as well as the above-ground parts. Thus, it might be transmitted through natural root graftage between a healthy and diseased tree. It is known that tree roots will sometimes become forced together in some manner until finally a natural root graft has taken place. The natural grafting between roots of trees has been attributed to a number of causes, some of which are as follows: (1) Roots growing in a shallow soil underlaid by a hardpan when crowded for growing space will sometimes become grafted together. (2) Two roots growing between two rocks or other barriers when wedged together tightly may form a graft. (3) Roots from mature avocado trees which are planted close together in a grove may, if crowded for growing space, form a graft. It is believed, through evidence existing in certain avocado orchards, that sun-blotch disease has infected healthy trees through natural root graftage.