Phytophthora Root Rot

Healthy, pale avocado rootlet compared with a black feeder root killed by Phytophthora root rot, caused by Phytophthora cinnamomi

Old drooping leaves with necrotic tips on an avocado with Phytophthora root rot, caused by Phytophthora cinnamomic

Symptoms and Signs

Foliar symptoms of Phytophthora root rot include small, pale green or yellowish leaves. Leaves often wilt and have brown, necrotic tips. Foliage is sparse and
new growth is rare. There may be little leaf litter under infected trees. Small branches die back in the tree top, exposing other branches and fruit to sunburn
because of the lack of shading foliage. Fruit production declines, but diseased trees frequently set a heavy crop of small fruit.
Small, fibrous feeder roots are scarce at advanced stages of this disease. Where present, small roots are black, brittle, and dead from infection. Foliage is wilted
even when soil under diseased trees is wet. Affected trees will decline and often die either rapidly or slowly.

Comments on the Disease

Phytophthora root rot is the most serious and important disease of avocado worldwide. The causal agent, Phytophthora cinnamomi, has over 1,000 hosts,
including many species of annual flower crops, berries, deciduous fruit trees, ornamentals, and vegetables.

Root rot thrives in areas of excess soil moisture and poor drainage. Trees of any size and age may be affected. The pathogen is easily spread through movement
of contaminated nursery stock of avocado and other plants, on equipment and shoes, in seed from fruit lying on infested soil, or by any activity by people or
animals that moves moist soil from one place to another. Phytophthora produces four different spore stages that are involved in disease development and
survival: sporangia, zoospores, chlamydospores, and oospores. They spread easily and rapidly in water moving over or through the soil. Entire areas can readily
become infested. Phytophthora species are not true fungi but have many fungal-like attributes.

Management

Inspect roots before planting and if their health appears questionable seek advice from a farm advisor or private consultant before planting trees. Employ
stringent sanitation measures, good cultural practices, and appropriate chemical controls. The most important control of this disease is good irrigation
management. For example, where new trees are interplanted among older trees, separate irrigation lines are needed to ensure appropriate irrigation timing and
amounts for the different aged trees.

Cultural Control

Use cultural practices that promote healthy growth of the tree while discouraging growth of the pathogen. Provide favorable soil conditions. In new plantings,
avoid soils and soil conditions favorable to root rot development, including poorly drained, saline, or pathogen-infested soils. Plant on well-drained soil, or
improve drainage by planting on a soil berm, deep-ripping impervious subsoils, or installing subsurface drains. In established plantings, manage soils carefully so
that excess moisture does not accumulate.

Use certified disease-free nursery stock

Request certified, disease-free plants, especially when planting new areas, because disease is especially damaging to young trees. Nurseries should disinfest
propagation material, such as by immersing seed in water at 120 to 122°F for 30 minutes and then quickly cooling it. Nurseries should also use pasteurized soil
mix, clean irrigation water from deep wells or disinfested surface water, and stringent sanitation to prevent pathogen introduction and spread. Nurseries that
rely only on fungicides for disease prevention can promote fungicide resistance and produce symptomless plants with infections that develop after planting.

Plant resistant rootstocks

Certain rootstock cultivars are more tolerant of root rot, including Dusa, Latas, and others. Newer recommended cultivars such as Uzi and Zentmyer may also be
available. Barr Duke, Duke 7, and Duke 9 can also be good rootstocks but have less Phytophthora-resistance than some newer cultivars. To obtain rootstocks
with maximum resistance to Phytophthora root rot, choose rootstocks produced by a nursery using the clonal method because clones of recommended cultivars
are more resistant than seedlings. Be aware that resistant rootstocks are not immune to root rot; if they are planted or maintained under adverse conditions,
they may be killed by the combination of adverse conditions and the pathogen.

Prevent soil or water movement from infested areas.

 Excluding P. cinnamomi from an uninfested grove is the most economical control method.

1. Install water-tight drains to divert surface runoff if a diseased area lies above a healthy grove.
2. Control gophers, as their burrows can provide means of moving the pathogen in water.
3. Do not work in infested groves when the soil surface is wet; Phytophthora is readily spread by activities such as walking or driving on infested wet soil.
4. Bring only clean bins and equipment into groves.
5. Begin harvesting and other activities in healthy areas of the grove; work in diseased areas last to minimize pathogen movement.

Soil solarization

Soil solarization can be effective for treating infested soil following tree removal in warm inland areas of California through a process in which radiant heat from
the sun is trapped under clear polythene sheets laid on the surface of the soil. Solarization is effective when soil temperatures in the top 2 inches of soil reach
between 108° to 131°F.

Establish a barrier

If Phytophthora root rot occurs in only one area of the grove and cannot spread downhill in surface runoff or drainage water, erect a physical barrier and post
warning signs to prevent people and activities from spreading the fungus into protected areas. Establish the barrier around healthy sections of the grove, at least
two tree rows beyond where tests indicate the fungus is present.

Irrigate carefully

Appropriate irrigation is the single most critical practice for improving tree health and managing root rot. Schedule irrigation frequency and amount using
sophisticated methods, such as based on local evapotranspiration or by installing soil moisture monitoring devices, such as tensiometers. Good irrigation
management is especially important where trees are diseased, near the margins of diseased areas of groves, and beneath thick mulch. It may be necessary to
replace irrigation emitters around unhealthy trees by installing lower output sprinklers to avoid saturating the soil. Install valves for irrigation lines for infected
portions of the grove because infected trees do not use water at the same rate as the healthy portion of the grove. Do not water soil that is already wet because
it will become waterlogged and accelerate disease.

Use high-quality irrigation water

Irrigation water with high overall salinity or an excess of boron, chloride, or sodium promotes infection of roots by Phytophthora. Phytophthora can contaminate
irrigation water, such as surface water that is runoff from infested soil. The extra cost of purchasing high quality water can often be justified by reduced disease
and increased crop quality and yield.

Apply gypsum and mulch

1. Create soil conditions that suppress development of Phytophthora root rot.

2. Apply, wood chip mulch and gypsum when the orchard is being established. Consider periodically applying additional mulch. Reapply gypsum as the old
material dissolves from view.
3. Apply gypsum under the canopy of each tree, perhaps 25 lb beneath a medium-size tree. Gypsum supplies calcium, which suppresses the formation
of Phytophthora spores.
4. Apply at least 4 to 6 inches of coarse wood chip mulch onto soil beneath canopies, but keep mulch several inches away from the trunk. Use coarse
organic mulch such as avocado trimmings, composted greenwaste (yard trimmings), or hardwood chips which provides better Phytophthora control than
naturally dropped leaves. Mulching promotes root growth into the mulch, enhances the development of beneficial microorganisms antagonistic to P.
cinnamomi, and reduces the adverse effects of saline soil and water.

Provide appropriate nutrition

Moderate amounts of nitrogen promote good growth that helps avocado better tolerate root rot. Avoid excess amounts of fertilizer, especially avoid large
amounts of animal manures or other products high in ammonia or salts. Avocado roots are sensitive to ammonia and salts.

Rotate crops

Replanting infested soil to resistant crops for at least several years reduces Phytophthora root rot propagules in soil. The fungus has a wide host range, but
plants such as cherimoya, citrus, and persimmon are highly resistant to the Phytophthora sp. causing Phytophthora root rot in avocado.

Chemical Control

Certain phosphonate fungistats (phosphorous acid and phosphonate compounds) can markedly improve trees' ability to tolerate, resist, or recover from
infection by Phytophthora cinnamomi. Good control requires using materials in combination with other recommended practices, such as careful irrigation
practices and applying wood chip mulch. Phosphonates cannot eradicate Phytophthora from the grove and Phytophthora root rot requires ongoing management
throughout the life of the trees. Fumigation is not recommended even if the maximum rate of fumigant is applied. Often P. cinnamomi re-invades fumigated soil
and the Phytophthora root rot becomes worse than before because the soil microbial community and competing microorganisms have been reduced by the
fumigation.

Application methods

Varying with the product label, phosphonate (phosphite; FRAC Group 33) may be sprayed onto bark or foliage, injected into soil with irrigation water
(chemigation), or injected into trunk vascular tissue. If permitted on the product label, proper trunk injection is generally the most effective application method
when treating severely diseased trees. Proper application timing is critical. Phosphites can move both up and down within plants. To induce phosphites to move
to roots, apply phosphites prior to initiation of new root growth. This effective application time is when about three-fourths of leaf flush is complete or just as
new leaves harden, usually in late spring (May) and summer (August). Optimal application dates vary according to local conditions. If applied during early flush or
when many new leaves are flushing, most of the phosphite will move to leaves and provide little Phytophthora control. If injected when new leaves are
hardening, phosphites will move upward in the xylem stream, then move downward in the phloem where they can encourage healthy new root growth.
Inject trunks using proper equipment, such as spring powered or gas powered (CO 2) injectors. Drill relatively small diameter holes to the depth of the drill bit, at
a slightly downward and sidewise angle so that more of the phosphonate material is deposited in the outer wood. Larger holes do not heal properly and
continuous weeping and bacterial infection in the holes often occurs. Drill holes into smooth sections of the trunk or main limbs, avoiding knots and side
branches. Where feasible, locate holes above any trunk area that is wetted by mini-sprinklers to facilitate injection wound closure.
Application (spraying) directly onto bark is usually not effective for managing Phytophthora root rot. Bark application may be more effective in managing the
trunk canker fungus Phytophthora mengei. Application through the irrigation system is more effective in slowing down the spread of Phytophthora root rot
disease than it is in controlling disease in already infected trees.
Common name Amount per acre REI‡ PHI‡

(Example trade name) (hours) (days)

Pesticide precautions Protect water Calculate VOCs Protect bees

Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective
and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's
properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.

NONBEARING TREES

A. ALUMINUM TRIS PHOSPHONATE

(Aliette WDG) Drench: 5 oz/10 gal 12 NA

(Aliette WDG) Foliar: 5 oz/100 gal 12 NA

COMMENTS: For drench application: apply 1 qt per pot or sleeve of each tree 2 to 3 days before transplanting. For foliar application: begin
application at transplanting or the start of the growing season and continue for up to 4 applications per year at 60-day intervals.

BEARING TREES

A. PHOSPHOROUS ACID

(Agri-fos, Fosphite) Label rates 4 0

MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33)

COMMENTS: Do not apply with copper-based fungicides or fertilizers; allow 10 days before applying copper-based compound after phosphorous
acid treatment or 20 days before applying phosphorous acid after copper treatment. Do not apply to dormant or heat- or moisture-stressed trees.

B. ALUMINUM TRIS PHOSPHONATE

(Aliette WDG) 5 lb 12 0.5 (12 hours)

COMMENTS: Begin application at the start of the growing season and repeat every 60 days. Do not exceed 20 lb/acre per year.

C. MEFENOXAM

(Ridomil Gold SL) Label rates 48 NA – drench;

28 – chemigation

MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4)

COMMENTS: Apply as a drench or by chemigation. Trials indicate this material is less effective on older trees, but is effective for a few years on
young trees that have been replanted into Phytophthora-infested soil.

‡ Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective
clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the
minimum time that must elapse before harvest.

N Not applicable.
A

1
Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group
number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action
group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make
no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.

Signs (visible pathogen structures) include Armillaria mushrooms, and Ganoderma fruiting bodies, and white fungal mycelium growing beneath bark, and
rhizomorphs (not common in CA) (i.e., Armillaria mellea).
Symptoms of diseased plants include:

1. Leaves that are downward-hanging, necrotic-tipped, pale or yellow, or wilted.

2. Premature leaf drop or a sparse canopy of drooping older leaves,
3. New shoots of small pale leaves.
4. Abundant small fruit.
5. Fruit that are blotched, discolored, spotted, streaked, or rotted.
6. Cankered, cracked, discolored, or oozing bark.
7. Black, brittle, or dead roots and relatively few small roots (rootlets).
8. Variegated stems, leaves or fruit indicative of sunblotch
9. White crusty exudates on the main trunk or major branches indicative of bacterial, crown rot, Dothiorella blight or black streak

Inspecting Trees

1. If a tree looks unhealthy, examine as many of plant parts as possible.

2. Brush away mulch to examine the appearance of small roots for root rot and the main stem where it enters the soil for crown rot.
3. Remove soil from around the root crown and cut beneath unhealthy looking bark to expose cankers or small patches of white fungal mycelium.
4. Look for discolored or oozing bark on main limbs and trunks and examine beneath damaged bark to discern cankers.
5. Use appropriate tools, such as a chisel or knife, to cut away bark and view deeper cankers. Keep monitoring tools, including a chisel, hatchet, hand lens,
pocket-knife, and shovel, close at hand.

Diagnosing the Cause

1. Inspect several nearby trees, which may show earlier, more characteristic or subtle symptoms. Patterns in symptoms among trees can provide clues to
the cause. Do not rely on a single symptom. Compare your observations to photos of common root and crown diseases . If cankers are present,
distinguish among the causes of cankers, which include:

 Abiotic disorders
 Various pathogens
 Certain vertebrates
 Send samples to a diagnostic laboratory or consult an expert to help diagnose the cause. Keep records of testing results.
 Recording the Location of Diseased Trees
 Record the date and location of problem trees or sites.
 Mark problem spots on a map of the grove or using a hand-held GPS (global positioning system).
 Use florescent spray paint and colored plastic flagging to mark trees.
 Mark maps and trees or both with symbols or color codes keyed to symptoms or the suspected or confirmed cause of disease.
 Repeat monitoring at intervals to document the progression or seasonality of symptoms and to assess whether management practices are effective.

Managing Disease
Improve growing conditions, use good sanitation, and provide appropriate cultural care as the primary means of managing:

 Anthracnose fruit rot

Causes of Fruit Damage

 Avocado thrips  Genetic mutations  Nutritional disorders  Sunblotch

 Anthracnose  Greenhouse thrips  Off-season fruiting  Sunburn
 Caterpillars  Latania scale  Persea mite  Vertebrates
 Branch canker and dieback  Mechanical injury  Phytophthora fruit rot  Weather extremes
 Fruit and stem-end rots  Sooty mold  Wildland fire

Abiotic disorders, invertebrates, pathogens, and vertebrates can injure avocado fruit. Many disorders or pests can produce more than one damage
symptom.
Scarred, scabby, or brown fruit skins
 Avocado thrips Mechanical Mechanical injury
Identification tip: Brownish injury Identification tip: Brownish
scars, often in a webbed Identification scars from wind and twig
pattern. Feeding from tip: Brownish abrasion.
high-populations of scars from
avocado thrips. rubbing,
symptoms that
on the right fruit are called carapace spot.
Carapace spot resembles damage from scab
fungus (Sphaceloma perseae) that occurs
elsewhere, but scab disease is not reported in
California.

Avocado thrips Omnivorous Mechanical

Identification looper injury
tip: Slight Identification Identification
scarring, or light tip: Scattered tip: Brown scars
brown streaks brown scars including
on fruit near the and shallow mechanical
stem. Feeding oval gouges injury to a fruit
damage by a low population of avocado thrips. from caterpillar feeding on fruit skins. that is pale yellow (left) from a lack of light because
it grew in dense leaf litter.

Specked, spotted, fouled, or discolored skins—Top of page

Latania scale Anthracnose Sunburn

Identification Identification Identification tip: The
tip: Grayish tip: Black lesion exposed side of sunburned
armored scale spots and fruit initially develops pale
covers specks on fruit yellowish discoloration,
encrusting fruit that can appear which later can darken in
surface. while fruit are the center.
on the tree or not until later after harvest.

Greenhouse Persea mite Sooty mold

thrips Identification Identification tip: Blackish
Identification tip: Necrotic fungal growth on surfaces
tip: Brownish or spots in fruit contaminated with
pale skin, honeydew excreted by
discoloration on uncommon mealybugs, soft scales, or
the skin. Fruit damage by whiteflies.
may be covered with black specks of greenhouse persea mites that usually feed only on leaves and
thrips excrement. commonly cause foliage spotting and premature
leaf drop.

Black or discolored large blotches on skin or decayed fruit—Top of page

Wildland fire Sunburn Fruit rot

Identification Identification Identification
tip: Large tip: Discolored tip: Shriveled dry
circular circular blotch or soft and
necrotic blotch, from heat decayed fruit.
usually on the damage to Skin may be
bottom end of exposed fruit, covered with
fruit. Resembles damage from fruit rots and often beginning near top of fruit. patches of brown to purplish spores from fungi that
sunburn. also commonly cause trunk cankers and limb
dieback.

Phytophthora Stem end rot Crick-side

fruit rot Identification Identification
Identification tip: Fruit decay tip: Black or
tip: Black soft or dark rot that necrotic
fungal decay, develops after indentations in
often in a harvest due to fruit. In coastal
roundish infection by growing areas
blotch, frequently near the bottom of fruit. Usually various various fungi, including those occurs after weather suddenly changes from cool to
develops only after harvest, and then rots fruit causing anthracnose and fruit and stem-end rot. hot.
flesh.

Chewed or gouged fruit—Top of page

 Caterpillars Omnivorous looper Sunblotch
Identification Identification tip: Gouged Identification
tip: Brown to fruit skins from caterpillar tip: Indented or
blackish, circular chewing, often forming discolored
to irregular brownish pits. Nearby streaks in skins
indentations or leaves also are usually from a complex
streaks chewed in fruit by amorbia (western chewed. of viroids.
avocado leafroller), omnivorous looper, or orange Sunblotch can also deform fruit and streaks on
tortrix green branches, have a willowy canopy and/or
alligator bark.

Coyote or dog Roof rat Mechanical injury

Identification Identification Identification tip: Chain saw
tip: Distinctive tip: Deep pruning caused brownish
triangular chewing on a gouges on fruit.
canine tooth ripe, fallen
marks chewed avocado fruit
in fruit. (left) and
shallow chewing on a green fruit picked from a
tree. Opossums, raccoons, tree squirrels, and
certain other vertebrates cause similar damage.

Abnormally shaped or distorted fruit

Cuke Off-season Embossment or sectional

Identification fruiting chimera
tip: Abnormally Identification Identification tip: A raised,
elongate fruit tip: Abnormally dark ridge in fruit skin
lacking a seed round fruit, caused by genetic mutation.
pit. A genetic which can also
mutation called be caused by
‘cuke’ because fruit resemble cucumbers. It’s zinc deficiency. The dark patches are leaf shadows.
more common on varieties like Fuerte.

Sunblotch Crick or crick- Woody avocado

Identification side Identification
tip: Deformed, streaked, or Identification tip: A hard
discolored fruit from tip: A brown, gnarled
pathogenic viroids. depressed, structure
often black, superficially
indentation on resembling an
the stem end of avocado fruit. Cause unknown, avocado fruit. The cause is unknown.
possibly due to high temperatures or water stress.

Symptoms and Signs

Anthracnose symptoms can develop on flowers, fruit, leaves, or twigs. Infected fruit is the most serious concern, but most fruit damage does not develop until
after harvest. External symptoms are difficult to see on ripe 'Hass' fruit because of its dark skin color. Unhealthy or dead leaves are the most obvious symptom
in groves. Spots form on leaves, beginning as yellow, then brown discolorations that coalesce into large dead areas. Necrosis occurs across or between leaf
veins, on leaf margins, and most often at leaf tips. If disease is severe, trees drop many leaves prematurely. New shoots can develop brown or purplish lesions,
and shoots may dieback. Infected flower heads can turn dark and die without producing fruit, or young fruit may form and then drop.

Before harvest, brown to black lesions less than 0.2 inch (5 mm) in diameter develop around lenticels on infected fruit. These small discolorations can be
overlooked while fruit are still on the tree, and lesions usually do not enlarge until fruit ripens after harvest. Large lesions sometimes occur on avocados on the
tree, usually after infected fruit is injured by insects or mechanical wind rubbing. After harvest, lesions become blacker, larger, and increasingly sunken. Lesions
eventually spread over the entire fruit surface and throughout pulp. When the fruit is cut in half through one of the lesions, rot extending into the flesh often
exhibits a hemispherical pattern. Decayed pulp initially is firm, but becomes soft and putrid as decay advances. Pink spore masses may form on the fruit surface
and, under wet conditions, a slimy mass of pink spores erupts through the fruit skin.

Comments on the Disease

Colletotrichum gloeosporioides is widespread in avocado and citrus groves. It normally is of little importance because unusually large numbers of spores are
required to produce damaging infections. Low humidity and no rain during much of the growing season limit disease development in California. With extended
foggy or rainy conditions and mild winter temperatures, and where many dead leaves and twigs and mummified fruit accumulate in trees, the fungus can
produce enough spores to cause a disease problem. Spores spread in splashing water and can cause infection anytime from fruit set to harvest. Once infected
fruit starts to ripen, temperatures of 75°F and above will accelerate anthracnose development, while temperatures below 59°F retard disease development.

Fuerte, Rincon, and Wurtz scion cultivars are more susceptible to anthracnose than Hass. Healthy trees often recover from foliar infections and defoliation once
conditions become dry. Anthracnose becomes a postharvest problem after the grove has been excessively wet for extended periods. Poor growing practices
and mishandling of fruit during or after harvest greatly increase the potential for significant fruit loss.

Management
  Control anthracnose primarily with good cultural practices in the grove and proper preharvest and postharvest fruit handling.
 Prune out dead limbs and twigs where fungi sporulate. If many dead leaves are entwined in the canopy, knock them out of the tree.
 Prune low limbs to at least 2 feet off the ground to reduce humidity within canopies by improving air circulation.
 Prune and harvest only during dry conditions and minimize fruit contamination and injury.
 Dispose of dead wood and old fruit away from avocado trees before bloom.

Post-harvest treatments should not be needed if fruit is properly handled. Keep fruit dry and cool until sold. Postharvest temperature is especially critical to
anthracnose development. Cool fruit to 41°F as soon as possible after harvest. Delays of longer than 6 hours before cooling and higher pulp (air) temperatures
during these delays will result in increased postharvest fruit decay. Cooling fruit promptly is of increasing importance as the season progresses because fruit
ripens faster as it increases in maturity. Avoid storage temperatures below 41°F because chilling injury may occur. Market fruit rapidly.

Chemical Control

Anthracnose is rarely significant enough in avocado groves to warrant fungicide application. Copper or other fungicides thoroughly sprayed on healthy tissue
can prevent infection.
Common name Amount per acre REI‡ PHI‡

(Example trade name) (hours) (days)

Pesticide precautions Protect water Calculate VOCs Protect bees

Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective
and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's
properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.

A. AZOXYSTROBIN

(Abound) 6–15.5 fl oz 4 0

MODE OF ACTION GROUP NAME (NUMBER1): Single-site, locally systemic QoI (strobilurin) (11)

COMMENTS: Applications should begin prior rot diseases development and continue throughout the season on 10- to 14-day schedule. Do not
apply more than 92.3 fl oz of product/acre per season.

B. COPPER HYDROXIDE

(Champ WG, Kocide 3000) Label rates 48 0

MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)

COMMENTS: Apply as a trunk spray. Make the first application at the start of the growing season and repeat every 60 days. Repeat applications at
60 days are important; a single trunk spray is not sufficient to arrest the disease. Do not exceed 20 lb/acre per year.

‡ Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without
protective clothing. Preharvest interval (PHI) is the number of days from treatment until harvest. In some cases the REI exceeds the PHI. The longer of
these two intervals is the minimum time that must elapse before harvest.

1
Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different
group number are suitable to alternate in a resistance management program. Make no more than one application of fungicides with mode-of-action
group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers,
make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.

Armillaria Root Rot

Soil and bark removed to show white mycelium of the Armillaria root rot pathogen,Armillaria mellea, infecting
a young avocado root crow.

Symptoms and Signs

Armillaria is a soil-borne fungus that causes a root and trunk rot of avocado. The fungus can become well established in roots and the root crown before any
symptoms become visible above ground. Infected trees usually die prematurely, and young trees often die quickly after infection. Mature trees may die quickly
or slowly, or may recover at least temporarily if conditions become good for tree growth and poor for disease development.

Wilted, downward-hanging foliage is often the first obvious symptom of Armillaria root rot. Other symptoms include foliage yellowing, leaf drop, and dieback
of upper limbs. During the rainy fall and winter, groups of short-lived mushrooms often grow around the base of Armillaria-infected trees.

The most reliable sign of Armillaria root rot is fungal growth in cambial tissue. If trees exhibit aboveground symptoms of infection, cut off bark at the base of
the tree and crown to diagnose the presence of Armillaria mycelium. Fungal mycelia are whitish and have a strong mushroom odor. Growth typically occurs in
patches in the cambium and inner bark. Large roots can be infected throughout their diameter.

Comments on the Disease

Armillaria root rot infects many crops and native and ornamental plants. Common hosts include avocado, cherimoya, citrus, and oaks. The fungus persists in
infested roots, stumps, and wood in soil, infecting new plantings and spreading to infect nearby plants.

Armillaria mycelia persist for years under the bark of diseased roots or the root crown. Armillaria spreads by any activity that moves soil containing infested
wood fragments, such as during cultivation. Most infections are thought to occur when a healthy root grows into or near an infested dead root piece and the
fungus moves over. In some situations, the fungus can also spread from tree to tree by cordlike rhizomorphs, which resemble small dark roots. In contrast,
healthy avocado roots are lighter-colored, usually light brown to whitish. When pulled apart, rhizomorphs have a cottony interior, while the center of a healthy
root is solid and woody. Rhizomorphs grow on buried wood, the surface of diseased roots and root crowns, and short distances on or through soil. Infection
occurs when rhizomorphs contact and directly penetrate the healthy roots of adjacent trees which is rare in CA.

Long after the aerial parts of a tree are gone, Armillaria can remain alive in roots and stumps. When avocado trees are planted, new roots grow into contact
with Armillaria-infected roots or infested wood pieces, and the new tree becomes infected. Armillaria can also be introduced on infected nursery stock.

Management

Look for diseases and disease-promoting conditions regularly throughout the grove, see MONITORING DISEASES AND DISEASE-PROMOTING CONDITIONS.
Provide a good growing environment and proper cultural practices and use good sanitation to manage Armillaria root rot. Providing good drainage and avoiding
excess irrigation are important. Armillaria fungus is very susceptible to drying. Excavating soil around the trunk to temporarily air-dry the root crown can
prolong the life of citrus trees and may also be effective on avocado, but apparently has not been tested on avocado. Shade any exposed root crowns from
sunburn. Once trees die, remove them and any immediately adjacent trees that may also be infected. Remove the stumps and as many root pieces from the
soil as possible. Thoroughly clean all soil from equipment and leave soil on-site before removing equipment. Consider replanting only with crops not susceptible
to Armillaria.

Avocado black streak

 Bacterial canker
 Branch canker and dieback
 Phytophthora root rot
 Phytophthora trunk canker and crown rot
 Sunblotch

Fruit should be picked by clipping rather than snapping the pedicles. Clippers should be frequently sterilized using a dilution (e.g., 1:10) of household bleach.

Identifying Disease Signs and Symptoms

Conditions that favor pathogen infection and disease development

Near the top On the bottom of Poor drainage and water

of slopes, slopes, trees are puddling beneath trees
trees can be subject to water-logged increases root and crown
stressed due soil and pathogen- diseases and disorders such as
to less contaminated runoff root asphyxiation due to
efficient from above. Armillaria insufficient oxygen in soil.
irrigation, root rot, Phytophthora
shallower root rot, and disorders such as root asphyxiation can be
(eroded) problems here.
soils, and
more
exposure to drying winds. Avocado
black streak and other drought
stress-related maladies often occur
here.

Directly wetting trunks promotes Phytophthora trunk canker

and crown rot. Watering too often or with excess amounts
increases avocado root rot. Mite outbreaks, nutritional
disorders, salt toxicity, sunburn, and reduced fruit quality and
yield are more prevalent where irrigation is inappropriate.

Root and crown diseases

 Armillaria root Armillaria root rot Armillaria root rot
rot Identification tip: Identification
Identification White, cottony fungal mycelia tip: Short-lived
tip: Commonl growing in cambial tissue, revealed mushrooms growing
y causes by cutting under bark. around the base of
unhealthy trees during the rainy
looking, fall and winter.
wilted,
downward-
hanging
foliage, leaf
drop, and dieback of upper limbs.

Phytophthora root rot Phytophthora root rot

Identification tip: Shoot Identification
dieback and a paucity tip: Unhealthy small
of chlorotic or pale rootlets turn black,
older leaves. are brittle and
readily break when
bent. Healthy
Phytophthora root rot
Identification tip: Symptoms include rootlets flex without breaking and are pale beneath
unusually abundant small fruit, a their outer layer.
sparse canopy of drooping leaves
with brown tips, and new shoots of
small pale leaves.

Cankers on limbs and trunks caused by pathogens

Avocado black Avocado black streak Bacterial canker

streak Identification tip: Infected Identification tip: Liquid or
Identification cambium exposed by cutting pale, dry ooze, often occurring
tip: Dark under discolored bark in small patches in a row on
staining, bark.
cankers on
bark, and
powdery
white
exudate,
typically
occurring on trees stressed by
insufficient irrigation and excess salts
in water.

Branch canker Branch canker and Branch canker and

and dieback dieback dieback
Identification Identification Identification
tip: Exudate tip: Cankered graft tip: Exudate and
and dark union from young tree dark discolored or
discoloring killed in the field by oozing bark on
on infected fungal infection caused older trees
green bark of by contaminated nursery stock. (typically occurs on bark at least several feet
young trunk. aboveground), unhealthy foliage, and declining or dead
limbs.

Phytophthora trunk canker and crown Phytophthora trunk canker and

rot crown rot
Identification tip: Reddish brown Identification tip: Rough,
resin and white powder on bark. cracked, dark stained flaking
Symptoms usually appear relatively bark from fungal infection in
low on the trunk. cambium near the root crown.
Phytophthora trunk canker and crown
rot
Identification tip: black, brownish,
orange, or reddish discolored
cambium and wood beneath
unhealthy bark.
 Sunblotch Verticillium wilt Ganoderma sp.
Identification Identification tip: Discolored, Identification
tip: Symptom brown, grayish, or reddish streaks tip: Conks (fungal
s include in wood or other tissue beneath fruiting bodies) on
discolored bark indicate an unhealthy vascular crowns, trunks, or
indentations system. limbs indicate wood
on fruit, decay.
rectangular
bark cracking
or checking
on limb or
trunk, called alligator bark, and
stunted tree growth.

Cankers on limbs and trunks, caused by abiotic disorders

Mechanical Mechanical injury Mechanical injury

injury Identification tip: Hatchet cuts Identification tip: Old
Identification made before spraying chemical wound closing as
tip: Old onto trunk bark. cambial tissue grows.
wound from
chemical
injection into
trunk.

Mechanical Cracked, discolored bark on trunk Sunburn

injury from killed by wildland fire. Identification tip: Cracked and
too tightly roughened bark from heat
staking and damage to formerly shaded
tying a trunk. limb exposed to direct light
after extensive pruning.

Cankers on limbs and trunks, caused by vertebrates

Rabbits Ground squirrels

Identification tip: Bark gnawed by Identification tip: Girdled bark and soil burrowing from ground squirrels.
rabbits, similar to chewing by ground
squirrels and voles.