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Avocado

Identifying Disease Signs and Symptoms

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Conditions that favor pathogen infection and disease development

Avocado conditions favoring disease
Near the top of slopes, trees can be stressed due to less efficient irrigation, shallower (eroded) soils, and more exposure to drying winds. Avocado black streak and other drought stress-related maladies often occur here.

Avocado conditions favoring disease
On the bottom of slopes, trees are subject to water-logged soil and pathogen-contaminated runoff from above. Armillaria root rot, Phytophthora root rot, and disorders such as root asphyxiation can be problems here.

Avocado conditions favoring disease
Poor drainage and water puddling beneath trees increases root and crown diseases and disorders such as root asphyxiation due to insufficient oxygen in soil.

Avocado conditions favoring disease
Directly wetting trunks promotes Phytophthora 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.

Signs and symptoms of disease

Phytophthora root rot
Shoot dieback and a paucity of leaves are usually caused by unhealthy crowns, trunks, or roots. Cause: Phytophthora root rot.

Dothiorella canker or Dothiorella leaf and stem blight
Limb dieback can be due to crown, trunk, or root disease. Mechanical injury to, or direct pathogen infection of, aerial parts also cause limb dieback. Cause: Dothiorella canker or Dothiorella leaf and stem blight.

Armillaria root rot
Wilted, downward-hanging, foliage has many causes, including Armillaria and Phytophthora root rots, improper irrigation, and poor drainage. Cause: Armillaria root rot.

Avocado root rot
A sparse canopy of drooping older leaves, new shoots of small pale leaves, and unusually abundant small fruit indicate advanced disease. Cause: Avocado root rot.

Phytophthora root rot
Chlorotic or pale, wilted leaves, with brown necrotic tips are caused by adverse soil conditions and anything that makes roots unhealthy. Cause: Phytophthora root rot.

Phytophthora fruit rot
Blotched, discolored, or decayed fruit has many causes, including various fruit rot fungi and disorders such as sunburn and wildland fire. Cause: Phytophthora fruit rot.

Citricola canker or Phytophthora canker and crown rot
Reddish brown resin and white powder on bark. Causes include avocado black streak, bacterial canker, Dothiorella canker, and mechanical injury. Cause: Citricola canker or Phytophthora canker and crown rot.

Dothiorella canker
Bark, limbs, or trunks can discolor, exude liquid, or develop whitish powder well above ground or near the soil. Cause: Dothiorella canker.

Phytophthora canker
Black, brownish, orange, or reddish discolored cambium and wood often develop beneath unhealthy bark due to avocado black streak, bacterial canker, Dothiorella canker, or mechanical injury. Cause: Phytophthora canker.

Verticillium wilt
Discolored, brown, grayish, or reddish streaks in wood or other tissue beneath bark indicate an unhealthy vascular system. Cause: Verticillium wilt.

Phytophthora canker and crown rot
Rough, dark stained bark near soil due to diseased cambium and other unhealthy tissues. Cause: Phytophthora canker and crown rot.

Sunblotch
Rectangular cracking or checking on limb or trunk bark, called alligator bark. Cause: Sunblotch.

Armillaria root rot
Short-lived mushrooms growing around the base of trees during the rainy fall and winter. Cause: Armillaria root rot.

Ganoderma sp.
Conks (fungal fruiting bodies) on crowns, trunks, or limbs indicate wood decay. Cause: Ganoderma sp.

Armillaria root rot
White, cottony fungal mycelia growing in cambial tissue, revealed by cutting under bark. Unhealthy aboveground symptoms are often due to infected crowns or roots. Cause: Armillaria root rot.

Avocado root rot
Unhealthy small rootlets turn black, are brittle and readily break when bent. Healthy rootlets flex without breaking and are pale beneath their outer layer. Cause: Avocado root rot.

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