Sweet orange scab (SOS) is a disease caused by the fungus Elsinöe australis. This disease is aptly named as it results in scab-like lesions that develop primarily on the fruit rind and infrequently, on leaves and twigs. Although there is little affect on internal fruit quality, fruit are severely blemished rendering them unsellable in the fresh produce market. Further, the disease can cause premature fruit drop and stunt young nursery trees and new field plantings.
The first detection in the United States occurred on July 23, 2010 in Spring, Texas, near Houston in residential lemon and tangerine trees. A widespread survey was launched immediately. SOS has now been confirmed in Louisiana, Florida, and most recently, Arizona.
The initial scab forms on very young fruit; it is slightly raised and pink to light brown in color. As the lesion expands, it takes on a cracked or warty appearance and may change color to a yellowish brown and eventually to a dark gray. The scabs typically form a pattern on the fruit like water splashes.
On young stems, the lesions resemble an area of dieback that has been scabbed over.
Lesions begin on the underside of leaves as water soaked spots. They typically form along the edge of the leaf or the mid-vein. However, the samples collected recently in the United States had large SOS lesions on the upper side of sweet orange leaves. Infected leaves are often misshapen, puckered and smaller in size.
Susceptible Varieties. Sweet orange and tangerine (including hybrids) varieties are most susceptible. Grapefruit was previously thought to be tolerant of SOS but E. australis was confirmed on grapefruit trees in a backyard in Texas.
Fruits are highly susceptible to E. australis during the 6 to 8 weeks after petal fall. When infected in their early stage of development, fruit tend to be misshapen and drop from the tree prematurely.
Geographical Distribution. SOS is a common disease in South America, mainly Brazil, Argentina and Paraguay. A specific pathotype, E. australis p.v. natsudaidai, occurs only in Korea on Citrus natsudaidai. In the United States, it has been confirmed in the states of Arizona, Florida, Louisiana, and Texas.
Similar species. Elsinöe fawcettii is a closely related species that causes citrus scab, formerly called sour orange scab. This disease is widespread in humid areas. E. australis and E. fawcettii exhibit similar scab symptoms and are very similar in their morphologies (appearance, growth pattern, types of spores produced). It is very important that suspect scab samples are sent to a regulatory laboratory for verification of species.
USDA APHIS SOS – New/Science/Survey