This Table is from the UC Master Gardeners' Handbook

Problem Diagnosis for Avocados

Microscopic Disease-Causing Organisms (Primarily Fungi)

What the Problem Looks Like

Probable Cause Plant Part Attacked

Comments

Small, pale green, wilted leaves. Sparse foliage. New growth absent or if it occurs, new leaves small with poor color. Small branches die back at top of tree, allowing other branches to become sunburned due to lack of foliage. Small, fibrous feeder roots absent or, if present, blackened, brittle, dead.

Avocado Root Rot
(Phytophthora cinnamomi)

A fungus with more than 1000 hosts. Small feeder roots attacked.

The most serious avocado disease in California. Fungus thrives in excess soil moisture (over-irrigation and poor drainage). Attacks trees of any size/age. Absence of feeder roots prevents moisture uptake so the soil under diseased trees stays wet even though tree appears wilted. Roots of pencil-size or larger seldom attacked by fungus. Diseased trees may set a heavy crop of small fruit, but diseased trees will decline and die, either rapidly or slowly. Fungus can spread on contaminated nursery stock, in water in contact with infested soil, on shoes, and on cultivation equipment.

Control Measures. Use an integrated approach of prevention, culture, treatment

Prevention
Plant on soil with good internal drainage. Avoid over-watering.
Use clean nursery stock, preferably certified disease-free.
Use resistant rootstocks. (Resistant does not mean immune)
Prevent soil or water movement from infested areas.

Disease Treatment

Fumigate small spots of disease. If the disease is detected early, cut off the tree at ground level and fumigate soil. Check with your local UC Farm Advisor or a licensed Pest Control Advisor for current availability and use of soil fumigants.

Fungicides. Use as part of an integrated control program.

Replant infested soil to immune plants.

Even though Phytophthora cinnamomi has a wide host range, there are many garden plants that are not susceptible, including all varieties of citrus, cherimoya, all types of vegetables, most annual flower crops, and many deciduous fruit trees and berries.

Poor growth, loss of tree vigor. Small, yellowing leaves; premature leaf drop; wilting, collapse. In winter, Armillaria often forms clusters of mushrooms at base of infected trees a few days after a rain. White, fan-shaped fungus mycelium grows under bark of diseased roots.

Armillaria Root Rot
" Oak root fungus" (Armillaria mellea)

Roots attacked.

Visible symptoms may not appear until fungus is well established in the roots. Can destroy entire root system so the tree dies. Once symptoms appear, it is very difficult to save a tree and disease may have spread to roots of adjacent trees. After aerial parts of infected trees are dead and gone, the fungus remains alive in the roots, ready to infect any replanted, susceptible trees, such as citrus, peach, or avocado. Therefore, fumigate before replanting. Let the soil dry out between irrigations.

Poor growth, loss of tree vigor. Chlorotic (yellowing) foliage; poor fruit production, cankers on trunk and branches; leaf blotching, wilting, rapid death of some new growth; often death of entire tree eventually.

Avocado Black Streak

(ABS)

(Causal organism unknown) Trunk, branches attacked.

This disease has been present in CA for more than 60 years but has been observed only on Guatemalan varieties, such as 'Hass' and 'Reed', and only after prolonged stress. Since many symptoms are similar to those attributable to other causes, the cankers on the trunk and branches are diagnostic of this disease. Cankers have a dry, powdery, water-soluble sugar that exudes through tiny cracks in the bark. Cankers vary in size. Shallow, reddish-brown lesions under cankers are revealed when bark is removed. Management of ABS consists of maintaining tree health with good fertilizer and irrigation practices. Remove un-thriving trees; it is safest to fumigate the soil if replanting to avocado.

Leaves suddenly wilt on one part of tree or on the entire tree and then turn brown and die but do not drop off for months. Brown to gray-brown streaks are visible in wood of branches or roots (plugged up xylem tissues)

Verticillium Wilt (VerticiIIium albo-atrum)

Water-conducting stem tissue (xylem) attacked

A soil fungus that enters the roots and moves upward, attacking and plugging up the water-conducting tissues (xylem) and blocking the wood. May kill the tree or only part of it, with the remainder having complete recovery. if disease is severe and recurring, contact a UC Farm Advisor or Pest Control Advisor regarding soil fumigation. Mexican rootstocks are more resistant than Guatemalan rootstocks. Do not plant an avocado tree on soil that has been used for other crops susceptible to Verticillium wilt, such as tomato, eggplant, pepper, many berries, apricot, potato, and several flower crops, or plant any of them by your avocado tree.

Bark cankers exude white powder and outer bark cracks and sheds easily. Diseased trees die back and may look Un-thrifty but rarely die.

Dothiorella Canker
(Dothiorella gregaria)

Trunk, branches attacked.

A minor fungal problem. As with most fungi, It is favored by moisture, so keep irrigation water off the tree base. Guatemalan rootstocks or scion tops are much more susceptible than Mexican. Control is not usually needed. Scraping off outer bark will remove some infection and encourage regeneration of vigorous bark.

Trunk cankers at the base of older trees, originating at or below ground level, Canker appears as a dark region with a red, resinous exudate that dries to a white, crystalline deposit. Underneath the superficial canker is an orange-tan to brown lesion, instead of the normal white or cream-colored tissue. Lesion has a fruity odor when exposed. As with its sister species, P. cinnimomi, there may be a gradual decline over years or a sudden tree death

Phytophthora Canker or Collar Rot

(Phytophthora citricola)

Lower tree trunk attacked. Attacks phloem tissue,

(Same fungus can attack fruit --see below)

Collar rot is now widespread in CA, second only to avocado root rot in severity. As with all Phytophthora species, disease is favored by excess soil moisture, such as occurs with over-irrigation or poor drainage. The fungus can be spread on contaminated nursery stock, irrigation water, and cultivation equipment. Use sanitation measures noted for other Phytophthora species. Seedling rootstocks are generally more sensitive than clonal stocks, such as 'Duke 7' & 'Toro Canyon'. Since P. citricola is found increasingly with P. cinnimoma, an integrated approach to control both species is important. Do not allow the lower trunks of trees to stay wet because the high humidity favors infection. Drip emitters should be placed away from tree trunks and mini-sprinklers should be aimed to avoid wetting tree trunks. Also avoid wounding the trunks. If cankers are detected at an early stage, they can sometimes be controlled by cutting out the infected tissue. No chemicals are currently registered for use on this disease.

Fruit hanging at or near the ground have a distinct, rounded black area, usually at the end toward the soil. The rot soon extends internally, sometimes to the seed.

Phytophthora Fruit Rot (Phytophthora citricola)

Fruit attacked. (Same fungus can attack trunk, See above)

Limited to prolonged wet weather in a dry climate like California. Probably caused by disease organisms splashing up from the soil, so a mulch or leaf layer should help. Removing fruit that touches the ground will remove a likely source of disease inoculation since this soil fungus can sporulate easily on them,

Unlike Phytophthora fruit rot (above), symptoms develop after fruit is picked and starts to soften, not while fruit Is still on the tree. Purplish- brown spots appear on fruit surface. Spots can enlarge until they cover entire fruit. Fruit flesh becomes discolored, smells bad.

Dothlorella Fruit Rot (Dothiorella gregaria)

Fruit attacked. (Same fungus can attack trunk. See above.)

Like Phytophthora fruit rot, this disease is rarely important in our dry climate. Fungi usually prefer moisture. When it does develop, it is usually on dead branches, leaves, and leaf margins; therefore, if needed, remove dead material. Do not let dead debris accumulate. Minimize leaf tip burn; avoid saline conditions because the fungus can live on the dead portions of leaves. (See Excess Salts below.) After picking, cool fruit to a minimum of 41F as quickly as possible. Ripen under 60F (cooler than room temperature) to minimize rot.

Brown, scattered, dead areas on leaves, which, if extensive, cause severe leaf drop. Infected fruits develop small, dark spots at lenticels. Like Dothiorella Fruit Rot, major fruit rotting develops only on ripening, after harvest; unlike Dothiorella, the flesh rots are many and smaller.

Anthracnose (Colletotrichum gloeosporioides)

Leaves, fruit attacked

This third fruit fungus is also much more severe in wetter, tropical climates, becoming serious in California only with wet, mild winters. As with Dothiorella Fruit Rot, cool fruits quickly after picking and ripen them at below room temperature If possible. Likewise, removing dead material and pruning to open the tree canopy for better aeration are helpful, if needed. The spores germinate and penetrate the fruit pre-harvest, causing brown to black spots, but the disease does not develop further until after harvest. During ripening, the fungus resumes growth.

Active lesions on bark are dark, slightly sunken areas with watery, necrotic pockets under the surface. Bark splits on one side of canker and watery fluid oozes out, and dries, leaving a white, powdery residue at the lesion. Cankers range from 1" to 4" in diameter. Cankers usually appear first at the base of the tree and often spread upward on one side of the trunk or branch.

Bacterial Canker

(Xanthomonas campestris)

Trunk, branches attacked.

A widespread disease but relatively unimportant in CA. Most groves have a few infected trees without noticeable harm. Affected trees often have leaf symptoms of boron deficiency. If the disease is severe, affecting yield, the tree should be removed. Otherwise, mild infections seem to have little effect on the tree, and are too common and spread too little to justify removal.

On twigs --narrow, lengthwise, yellow, red, or necrotic shallow indentations.
On fruit
-- White, yellow, or reddish blotches or streaks that may be depressed.
On bark
--"Alligator bark" -- rectangular cracking of bark on trunk and larger branches.
Tree in general
-- Stunted, sprawling growth

Sunblotch (Avocado Sunblotch Viroid)
(ASBVD)

(A small single-stranded, circular RNA molecule)

All parts of tree attacked

It formerly caused devastation in California avocado groves, but the discovery 20+ years ago that it is the result of a viroid (a smaller "naked" virus) led to effective control. One can purchase "registered" trees, for which scion top and rootstock are "indexed" to be viroid-free. An established, infected tree can contaminate nearby healthy avocados by unseen root-to-root grafting and by human-mediated wound-to-wound cutting tools. Removal is recommended in such cases. Occasional symptomless trees are a complication that can cause infection directly or through symptomless seedlings used in rootstocks. Pruning tools and harvesting clippers should be sterilized between trees.

Mites, Insects, and Garden snails and slugs

Light green or yellow areas on upper leaf surfaces along the midrib, later extending to the smaller veins and entire leaf. Areas of severe feeding later turn brown (bronzing of leaves) and leaves may drop.

Avocado Brown Mite

(Oligonychus punicae)

Leaf upper surface attacked.

Trees are injured in proportion to the amount of green leaf area lost. Tiny, brown-colored mite about the size of a period, the same size as the Persea period. It is a pest of avocados primarily in coastal areas. See Persea mite below for more details.

Light green or yellow areas on undersides of leaves along the midrib and larger veins. Heavy infestations can cause leaf drop

Avocado Mite Leaf undersides attacked.

Formerly known as six-spotted mite. Tiny, yellow to pale green mite about the size of a period. It is a pest of avocados primarily in coastal areas. See Persea mite below for more details.

Small necrotic spots on the undersides of leaves along the midrib and main veins, As population increases, new necrotic spots appear between the veins. Each spot is covered with fine webbing that shines silvery in sunlight. Leaves look like they have "a severe case of the measles". Necrotic spots can coalesce and block transport of carbohydrates from leaf cells to veins. At this point leaves will drop, which is followed by fruit drop if leaf drop is extensive.

Persea Mite

(Oligonychus perseae)

Leaf undersides attacked.

A yellowish mite, about the size of a printed period; thus, it is barely visible. This new mite pest, a native to Mexico, first entered CA in 1990. It spreads rapidly as its webbing protects it from the major predacious mite here. Persea mites feed and lay eggs beneath their webbing and are protected from the predacious mite Amblyselus hibisci, a common biological control agent In CA avocado groves. On severely Infested leaves, the mite population can reach 1000 mites/leaf. Its numbers peak with dry summer heat and decline rapidly in the fall, but enough winter survival occurs (eggs over-winter) to repeat the cycle, allowing buildup of adult populations in spring.

The 'Gwen' is a favorite host, then 'Hass' and 'Reed' and other varieties. Certain new UCR experimental selections are comparatively resistant. Other hosts include citrus fruits (not leaves), deciduous fruits (apricot, peach, nectarine, plum, persimmon), grapes, Sumac and Liquidambar trees, roses, and Acacia.

To confirm the identity of Persea mite, hold a horizontal white sheet of paper under symptomatic foliage and rap the stem sharply; the mites will be evident on the paper as moving specks. With a hand lens, the 8 distinguishing mite legs will be visible and yellow color should be definitive.

The Persea mite is gradually coming under good biological control. The population of a predacious mite native to CA, Galendromus annectens, which can penetrate the Persea mite webbing, is building up and beginning to control the Persea mite problem. Another predacious mite, imported to CA by UC scientists for the purpose of controlling Persea mite, (Galendromus helveolus), also holds promise. In the meantime, small (and few) trees can be helped by water-jet washing, which is more effective if insecticidal soap is added. To minimize initial infection, avoid drought and other stress. You can contact your local UC Farm Advisor or pest control expert for up-to-date recommendations for control methods approved for home growers.

Scarring on young fruit that starts near the stem end and spreads over entire surface. Feeding on fruit stems causes fruit drop. Pest will also feed on leaves, but defoliation is not primary problem. Darkened, leathery patches on upper leaf surface and random feeding lines on lower leaf surface. Unlike mites, thrips leave small black fecal pellets.

Avocado Thrips

(Scirtothrips perseae)

Similar to
Citrus Thrips
(Scirtothrips citri)
A tiny, oval, yellow insect about 1 mm long.
Very active
Fruit, leaves attacked.

A new, exotic avocado pest first noticed in July 1996 in Ventura County which has spread to many avocado groves statewide. Believed native to Central America. A tiny, very active, oval, yellow insect about 1 mm long that looks similar to the thrips species that attacks citrus. Feeds on young fruit and leaves. Scarring can be severe, leading to "alligator skin". Damage Is mainly cosmetic, usually does not injure flesh. Edible fruit unaffected. Sanitary precautions recommended, do NOT spray trees because insecticides disrupt beneficial insects (biological control). Thrips can fly but are also spread by wind, contaminated clothing, and equipment. UC entomologists are working on introducing new biocontrols.

Fruit and leaves covered with honeydew and sooty mold. Mealybugs present.

Mealybugs

(Pseudococcus /Planococcus spp.)

Foliage, fruit attacked

Mealybugs are soft, oval, segmented insects, usually whitish, under 1/4 inch long, covered with a mealy wax. They suck plant juices, leading to stunting and, rarely death. Natural enemies usually control mealybugs, but ants protect mealybugs from their natural enemies. Control the ants and natural predators -- beneficial insects, such as ladybird beetles -- will control mealybugs. For small mealybug infestations, you can hand pick them, or daub with rubbing alcohol. For larger infestations, hose off mealybugs with water, or apply soap or oil sprays.

Ants present.
Ants do not feed on avocado trees but drive away the natural enemies of insect pests of avocados. Argentine worker ants travel in distinct, narrow trails.

Argentine ant

(Iridomyrmex humilis)

Southern fire ant

(Solenopsis xyloni)

Ants feed on honeydew excreted by scales, mealybugs, and other insect pests and can interrupt biological control of these pests. Control ants by denying access to the tree: Apply a band of sticky material around the base of the trunk of mature trees that mechanically blocks ants; prune trees about 2ft above the ground so ants cannot get into trees without climbing the trunk. Any ant activity is a danger sign. Insecticide or poison baits can reduce ant numbers.

Holes in leaves and fruit; slimy trails Brown Garden Snail is about 1" in diameter. The Gray Garden Slug is a snail relative that lacks shell

Brown Garden Snail

(Helix aspera)

Gray Garden Slug

(Agriolimax retiiculatus)

Fruit, leaves attacked

Most active at night and early morning when ground is damp. Home gardener can hand pick; best hunting is after 10p.m. Or place short, wide boards with cleats at either end to keep the boards about 1 inch off the ground; these will be daytime hiding places. Pests can be squashed -- which attracts more -- or killed with a solution of 1/2 household ammonia and 1/2 water in a spray bottle. Keep ammonia off leaves since it damages plants. Other methods: chemical baits; predatory Decollate snails; drowning snails in fermented liquid, such as beer; copper barriers around trunk. Keep snails out of trees by pruning branches up off the ground.

New leaves have holes and are webbed and rolled together. Caterpillars also feed on developing fruit, often rolling and webbing fruit and leaves together and scarring fruit. Caterpillars make shelters for themselves by webbing two leaves together or a leaf and a fruit. Caterpillar pupates inside fruit. Adult, night-flying brownish moth emerges. Leaf damage on terminal shoot growth is especially evident for omnivorous looper.

Avocado-worms or Leaf-rollers

Amorbia Moth

(Amorbia essigana)

Omnivorous Looper

(Sabulodes aegrotata)

Orange Tortrix

(Argyrotaenia citrana)

Leaves, fruit attacked

Different types of leaf roller pests are often called avocado-worms.

Omnivorous looper. It eats holes in avocado leaves, skeletonizing them, so that only the midrib and larger veins remain. It also feeds on avocado fruit and causes scarring. Crawls with a looping motion. Usually found near damaged leaves. It can spin a silken thread and hang suspended from it when disturbed. it may vary in color from pale green to pink or yellow with stripes or other markings. Grows to 1-1/2 to 2" long.

Amorbia moth caterpillars are yellowish-green, about 1" long. Different parasitic small wasps and flies usually keep the avocado-worm population low. Certain fungi and viruses are also natural biological controls. The home gardener can destroy avocado-worms by picking them out of their shelters or squashing them in place. Rare, severe outbreaks can be sprayed with Bacillus thuringiensis or a chemical insecticide as a last resort

Other Pathological Conditions

Excess chloride --
Tip and marginal burn of older leaves, premature defoliation, and sometimes a progressive mottled yellowing behind the burn

Excess sodium --
interveinal leaf burn and twig dieback.

Excess salts

(Na and Cl)

Only rarely are other elements in harmful excess

Leaves affected

Salt accumulations are often confused with nutritional deficiencies. Avocados are particularly sensitive to salts; they accumulate chlorides and sodium more readily than do most other tree crops. Rapid burn at the base or leaf tip, followed by defoliation suggests either an excessive fertilizer application or inadequate irrigation. Extra root-zone leaching during the summer is indicated.

Pale green to yellow, small leaves w/yellow veins; lack of vegetative growth; lower yields; premature defoliation

Nitrogen (N) deficiency

Leaves, fruit yield affected

Apply during the first irrigation of each month for the 8 mo. from March-Oct. to avoid forgetting. Young trees need N applications at different rates from older, mature trees.

Light yellow (chlorotic) areas between veins, starting at leaf margins, extending to midrib and base: small, narrow leaves; pear-shaped fruit become oval to round, smaller than normal; terminal growth looks like feather duster; twig dieback; defoliation

Zinc deficiency

Leaves, twigs, fruit yield affected.

Zinc deficiency can be controlled by applying it as a spray to foliage or to the soil. Foliar applications are most effective in June and July. Methods of soil application vary and their effectiveness can last longer than foliar sprays.

interveinal yellowing on leaves;

tip and marginal leaf burn; defoliation; twig dieback; reduced yields.

Iron deficiency

Leaves, fruit yield affected

Can occur in high-pH soils containing lime (calcium carbonate) but not common in CA. Deficiency accentuated by excess soil moisture. Mexican race rootstocks are less sensitive.

Leaves, twigs look water soaked, then wither, darken. Branches die back and bark splits in severe cases. Leaves may drop quickly or persist on tree. When fruit freezes, flesh dries out and brownish pits called ice marks may form on skin. Xylem (water-conducting elements) in the fruit turn black.

Frost Damage

Leaves, fruits attacked first; progressively larger wood with harder frost.

Allow the tree time to recover before removing frost-killed wood. After new growth appears in early spring, wait for any dieback, then cut back to live wood (identified by a green layer just under the bark). Pruning cuts will heal naturally, so there's no need to paint them.

Larger, small branches blacken, die. Wood peels off in patches. Fruit skin develops tough, brownish spots and fruit may dry out.

Sunburn

Trunk, branches, fruits affected.

A problem in hot-sun areas. Wrap the trunk in white cardboard or use whitewash or flat white latex paint. Adequate nitrogen and water for good foliage.