What Is Sunblotch: Treatment For Sunblotch In Avocado Plants

By Bonnie L. Grant

Sunblotch disease occurs on tropical and subtropical plants. Avocados seem particularly susceptible, and there is no treatment for sunblotch since it arrives with the plant. The best recourse is prevention through careful stock selection and resistant plants. So what is sunblotch? Read on to learn more.

What is Sunblotch?

Sunblotch in avocado is caused by a genetic plant disease. The disease 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.

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Sunblotch is caused by viroids, which are genetic markers that change plant development and behavior. It is introduced to the plant in the rootstock at grafting time, or will be present in the seed.

Symptoms vary dependent on the host. Fruit may bear red, yellow, or brown necrotic streaks that may spread to stems and twigs. Alligator bark is another characteristic of sunblotch disease, which shows as cracking and scaling of bark. The avocado sunblotch viroid causes cracking and fissures in the skin which hardens at each damaged area.

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 eight 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

The first defense is sanitizing. 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.

When purchasing plants, make sure they are certified virus free or are from resistant rootstocks.

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.

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