Sunblotch—Avocado Sunblotch Viroid
Sunblotch of avocado is caused by dozens of variants (strains) of submicroscopic particles of genetic material (viroids) that can alter the development and growth of infected plants. The disease results in a variety of symptoms, and sometimes no visible symptoms at all.
Identification
Symptoms of infection by Avocado sunblotch viroid 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. Leaves on infected plants may remain smaller than normal. Leaves may also develop yellowish or white variegated areas and be deformed, but these leaf symptoms are uncommon.
Deep, rectangular cracking (checking) of the bark called “alligator bark” commonly occurs on the trunk and larger branches of avocados with sunblotch. 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 fruit yields. A large reduction in yield on previously vigorous trees may indicate the presence of the viroid in otherwise symptomless carriers.
Note that crick (crick-side) and mechanical injury can cause symptoms on fruit that resemble those of sunblotch. See this table for photos and a summary of causes of damage to avocado fruit.
Life cycle
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 to nearby avocado trees. Mechanical transmission through wounds caused by contaminated harvest clippers, pruning tools, and injection equipment may be important if infected trees are in the grove. Spread via pollen from an infected tree to the flower ovule of a noninfected avocado, resulting in infected seed, can cause fruit to develop symptoms. But fruit infected via pollen do not spread the disease unless the seed is propagated. There is no evidence of insect transmission.
Damage
Sunblotch is generally a disease of older avocado trees. Careful propagation of commercial nursery stock during recent years has virtually eliminated viroids from new avocado plants.
Sunblotch is easily overlooked in part because some infected trees are symptomless and when symptoms are present, they can be variable. Trees with visible sunblotch symptoms commonly have reduced fruit yield amount and quality.
Solutions
Sunblotch management methods are mostly preventive. Purchase stock from a reputable commercial supplier. Obtain new plants that are certified as disease free if available.
Where a symptomless tree is suspected of being infected by sunblotch viroid, certain plant diagnostic laboratories can perform indexing tests to determine whether they have the viroid. One indexing method involves grafting shoots of a symptomless avocado to young Mexican avocado seedlings. Growth of leaves and twigs are then observed for sunblotch symptoms that develop relatively quickly on infected, grafted avocado plants.
When growing multiple avocado trees, consider disinfecting harvest and pruning tools and any chemical injection equipment before moving to work on another tree. First scrub or wipe the equipment clean then for several minutes soak it in a 10% bleach solution (1 part bleach to 9 parts clean water).
The only direct control of sunblotch is to remove infected trees. If uninfected avocado are growing nearby, removing the infected trees reduces the likelihood that healthy trees will become infected. After cutting down an infected avocado, consider promptly treating the stump with a systemic herbicide registered (labeled) to kill stumps, which includes certain glyphosate formulations such as Roundup Weed and Grass Killer Super Concentrate. This can prevent the spread of the pathogen through natural root grafts between nearby avocado trees.
Adapted from Integrated Pest Management for Avocados and Pest Management Guidelines: Avocados University of California Statewide Integrated Pest Management Program (UC IPM). |
