Update on Sweet Orange Scab in California

Josh McGill California Corner, Diseases

A California researcher recently provided an update on sweet orange scab (SOS). The fungal disease has been detected in the state 18 times since 2013. University of California, Riverside professor and plant pathologist Jim Adaskaveg said most of the detections have been in Southern California, especially in Imperial County. Eleven detections have been in commercial lemon orchards, three in commercial tangelo orchards, three in oranges ­(two residential and one commercial), and one was an unspecified variety in a research orchard.

sweet orange scab
Sweet orange scab

Adaskaveg said the disease is very difficult to detect. The California Department of Food and Agriculture (CDFA) uses molecular detection methods to find SOS. CDFA and the U.S. Department of Agriculture have established SOS quarantine areas in the state.

SOS commonly impacts fruit but rarely harms leaves or stems, Adaskaveg said. Oranges, mostly sweet oranges, have the most severe SOS symptoms. Sour oranges and hybrids are the varieties most tolerant of SOS. Grapefruit, lemons and mandarins have moderate symptoms.

Cultivation techniques for management of SOS include not using overhead irrigation, since fungal spores are carried by water drops and spread from fruit to fruit. Growers are also urged to use only certified propagation material.

The fungicides azoxystrobin, trifloxystrobin, pyraclostrobin and ferbam, as well as copper fungicides, create a barrier on the fruit during its most susceptible growing period, reducing the amount of inoculum. 

Timing is important, since the fungus attacks mostly young fruit. Fruit are susceptible for six to eight weeks after petal fall. Management is based on two fungicide applications: one at two-thirds petal fall and a second application two to three weeks later.

Fruit with symptoms of SOS should not be removed from the orchard. Harvest bins should be tarped. Scab and wind-scarring symptoms should be sorted and removed during packing.

Adaskaveg’s presentation was part of the Citrus Research Board of California’s 2023 Citrus Growers Educational Webinar Series. In addition to addressing sweet orange scab, his presentation also included a fungicide update.

About the Author

Ernie Neff

Senior Correspondent at Large

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