Beware of Sweet Orange Scab disease in citrus

Published 12:00 am Saturday, August 20, 2011

Within the last week I have identified two cases of residential citrus trees in the LaPlace area that are affected with the Sweet Orange Scab Disease. Although the disease is identified as the Sweet Orange Scab, or SOS, disease, this disease may also affect other varieties of citrus besides sweet oranges. One case I inspected had navel oranges, lemons and possibly satsumas showing symptoms of the disease. The symptoms included scarring, drying and hardened, pitted sections of the fruit, sometimes affecting almost half of the surface of the fruit. Some of the fruit even had lesions or scabs about the size of a nickel on the surface. About 80-90 percent of the fruit on the trees was infected. The portions of the fruit that are not affected are still safe to eat, but they would not be suitable for sale in the commercial market.

It is important to get a professional opinion on the problem with your citrus fruit because SOS disease can also be mistaken for the Citrus (sour orange) Scab disease, a very similar disease, as well as for other diseases of citrus such as melanose and citrus canker and possibly even bird damage.

SOS disease was first reported in Paraguay in 1882. On July 23, 2010, it was confirmed in the U.S. on residential citrus in Harris County, Texas. It was then confirmed on residential citrus in Orleans Parish on Aug. 10, 2010. On Oct. 22, 2010, it was confirmed on citrus in Mississippi. On Dec. 22, 2010, a quarantine was established for SOS by the US Department of Agriculture’s Animal and Plant Health Inspection Service. The quarantine was established to prevent the spread of SOS to other citrus-growing states. Citrus produced in Texas, Louisiana and Mississippi cannot be transported outside of these states.

SOS disease is spread by spores, which are dispersed by splashing water and wind-driven rain. The spores are produced when scab lesions are wet for at least one to two hours, the humidity is very high and the temperatures are 68-82 degrees Fahrenheit. The scab lesions develop on infected fruit, leaves and twigs and develop spores when the conditions are right. Then the spores are spread by the splashing water and wind-driven rain. The fruit is susceptible to SOS disease when it is first developing, from petal fall (fruit set) until it is about two months old. At around two months of age or around marble size, the surface of the fruit hardens and the spores no longer affect the fruit. It is important if you have citrus trees with SOS disease or if you have trees nearby that you spray your trees with a copper fungicide from petal fall until the fruit is two months old. This should be done in three sprays – the first spray should be applied when the petals fall (fruit set), the second spray 3-4 weeks later and the third spray 3-4 weeks after the second spray. The removal of infected fruit, leaves and plant parts is also recommended to help control the disease. With proper spraying of a copper fungicide and with proper removal of plant parts, SOS disease can be controlled and possibly eliminated.

If you have damaged fruit and you suspect your fruit may be infected with SOS disease, contact your local county agent for proper identification of your problem. If you have further questions concerning the diseases of citrus and/or other questions about citrus, you may want to purchase Publication 1234 – Louisiana Home Citrus Production. The cost is $5. You may purchase the publication from the LSU Ag Center’s website at, or you may visit your local LSU Ag Center Office in your parish. You may also view on the LSU Ag center’s website Publication 3169 – Louisiana Plant Pathology Series – Disease Identification and Management – Scab Diseases of Citrus.

If you have any questions, you may contact David Pichon, LSU Ag Center county agent by phone at 985-497-3261 or you may email him at The LSU Ag Center is available to help you with your problems and usually the help is free.

If you have any questions, please contact your local LSU Ag Center County Agent. You can contact David Pichon, County Agent-St. John Parish at 985-497-3261 or by email at