What is Sunblotch?
Sunblotch on avocados was first reported in California during the late 1920’s, and it has subsequently been reported in avocado growing regions around the world. It was several decades until biologists confirmed that the disease, initially believed to be a genetic disorder, is actually caused by a viroid – an infectious entity smaller than a virus. The viroid is known as avocado sunblotch viroid.
Avocado Sunblotch Symptoms
Sunblotch in avocado damages the fruit and is introduced by grafted wood or from seed. Fruit develops cankers, cracks, and is generally unattractive. The biggest issue is reduced fruit yield on trees that are affected. Identifying sunblotch on avocados is tricky because there is such a variation in symptoms, and some host trees are symptomless carriers that may display no symptoms at all. Keep in mind that symptomless carriers have a higher concentration of viroids than trees that display symptoms, thus spreading the disease rapidly. Typical avocado sunblotch symptoms include:
Stunted growth and reduced yields Yellow, red, or white discolorations or sunken areas and lesions on fruit Small or misshapen fruit Red, pink, white, or yellow streaks on bark or twigs, or in lengthwise indentations Deformed leaves with bleached-looking yellow or white areas Cracking, alligator-like bark Sprawling limbs on lower part of tree
Sunblotch Disease Transmission
Most sunblotch is introduced to the plant in the grafting process when diseased bud wood is joined to a rootstock. Most cuttings and seeds from diseased plants are infected. Viroids are transmitted in pollen and affect the fruit and seeds produced from the fruit. Seedlings from seed may not be affected. Sunblotch in avocado seedlings occurs 8 to 30 percent of the time. Some infection may also occur with mechanical transmission such as cutting implements. It is possible for trees with avocado sunblotch viroid disease to recover and show no symptoms. These trees, however, still carry the viroid and tend to have low fruit production. In fact, transmission rates are higher in plants that carry the viroid but do not exhibit symptoms.
Treatment for Sunblotch in Avocados
The first defense is sanitizing. Avocado sunblotch is easily transmitted by pruning tools, but you can prevent transmission by scrubbing tools thoroughly before soaking them with a bleach solution or a registered disinfectant. Be sure to clean tools between each tree. In the orchard setting, the disease progresses quickly from cuts made with infected pruning instruments. Sanitize in a solution of water and bleach or 1.5 percent sodium hydrochloride. Plant only disease-free seeds, or start with registered disease-free nursery stock. Keep a close eye on young trees and remove any that show signs of avocado sunblotch viroid. Use chemicals to kill the stumps. Prune avocado trees carefully and keep in mind that stress caused by severe pruning of symptomless carriers may cause the viroid to become more active in new growth and previously uninfected trees. If you already have trees with the symptoms; unfortunately, you should remove them to avoid spreading the viroid. Watch young plants carefully at installation and as they establish and take steps to nip the problem in the bud at the first sign of sunblotch disease.
