Photo 1. Fruit and leaf symptoms of avocado sunblotch viroid disease. The fruit has distinctive sunken streaks.
Avocado sunblotch viroid. The abbreviation is ASBVd.
Avocado and some other members of the Lauraceae family.
The viroid causes sunken, yellow, irregular streaks on the leaves, stems and fruit (Photos 1&2). Vein-yellowing may also occur on the leaves. However, the appearance of leaf symptoms is uncommon and erratic, and the depressed streaks that occur on the young stems are a bettersymptom of the disease. Typically, rectangular cracking occurs on the bark of older branches and trunks (called alligator bark). Symptoms on the fruit are similar to sunburn, hence the name "sunblotch" of this disease. They should not be confused with streaks of yellow and other colours that are due to genetic causes (Photo 3).
Symptoms depend on the avocado variety, the environment and the viroid strain. Often, trees show no symptoms (symptomless carriers). The seed from these trees may be 100% infected, but the seedlings used as rootstocks may not show symptoms. Symptoms may develop, however, when scions are grafted on them.
Symptomless carriers have a high concentration of the viroid and spread the disease rapidly. By contrast, trees with symptoms may not produce infected seed, and likewise budwood and shoot cuttings from these trees may not contain viroid. Nevertheless, whether with symptoms or not, trees infected with avocado viroid show large yield reductions.
Spread occurs in a number of ways. It can occur by root to root grafts that occur naturally in orchards, and it can also occur via infected budwood or rootstocks, on pruning tools, pollen, and via seed.
A serious disease causing up to 95% loss of yield in some varieties (e.g., symptomless Calinente and Reed). Hass which is a popular variety worldwide is susceptible. Observations reported in Mexico are typical with 75% and 52% reduction weight of fruit in trees of two orchards with symptoms, and 30% reduction on trees without symptoms. Yield reductions of another variety, Mendez, were even lower.
Look for early symptoms, which appear as narrow, white, yellow or pink streaks on the surface of young stems. Look for sunken, white, yellow or pink blotches or streaks on small deformed fruit. PCR is used to detect infection, and there are biological indexing protocols, which generally require long incubation times.
It is important to carefully monitor nurseries and plantations; especially ensure that nursery stock is free from the disease. By doing so, avocado sunblotch will be reduced to a minor disease.
Before planting (in the nursery):
During growth (in the plantation):
Note that pruning mature trees to reduce height may stimulate the virus to become active in new growth.
Chemical control is not appropriate for this disease.
AUTHOR Grahame Jackson
Information from CABI Avocado sunblotch viroid (avocado sun blotch) (2018) Crop Protection Compendium. (www.cabi.org/cpc); and Avocado sunblotch (2016) University of California Agriculture and Natural Resources, Statewide Integrated Pest Management Program. (http://ipm.ucanr.edu/PMG/r8101011.html); and from Saucedo-Carabez JR et al. (2014) Effect of Avocado sunblotch viroid (ASBVd) on avocado yield in Michoacan, Mexico. European Journal of Plant Pathology 138(4):799-805. (https://link.springer.com/article/10.1007/s10658-013-0354-9). Photos 1-3 Diseases of fruit crops in Australia (2009). Editors, Tony Cooke, Denis Persley, Susan House. CSIRO Publishing.
Produced with support from the Australian Centre for International Agricultural Research under project HORT/2016/185: Responding to emerging pest and disease threats to horticulture in the Pacific islands, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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