Put On The Spot
June 1, 2010 
In late March, a new fungal disease, black spot, was found on citrus in a grove near Immokalee. Though it is a problem in Southeast Asia, Africa, South America, and Australia, this discovery marks the first confirmation of the disease in the U.S.
Fresh Fear Factor
After recent wins with canker shipping rules, fresh fruit growers are concerned about black spot, because it causes necrotic lesions that result in elimination at the packinghouse. It also can lead to premature fruit drop, reducing yield in both fresh and processed fruit.
All cultivars of citrus are vulnerable to black spot, but lemons, grapefruit, limes, and mandarins are most susceptible. Late maturing varieties suffer most severely due to premature fruit drop. Florida’s warm, wet climate is conducive to the spread of black spot. The causal agent Guignardia citricarpa is spread by ascospores, which are ejected onto the fruit by rain or irrigation.
What To Look For
The primary symptom of the disease is black spot lesions on the rind of the fruit that start out as small orange or red spots with black margins. Later, the spots become necrotic lesions. Fruit is susceptible for four to five months after petal fall. Symptoms may not appear on fruit until it matures.
Control Of Black Spot
Fungicides will be required to control black spot if it becomes endemic in the state. Protective treatments using strobilurin, copper, and mancozeb must be properly timed and applied up to five times during the window of susceptibility.
Other preventive measures like removal of dead leaves in groves help to reduce inoculum potential. Black spot can be spread over distances by nursery stock, so care should be taken to avoid bringing propagating materials from infected areas.
At presstime, no new cases of black spot had been confirmed, but the Florida Department of Agriculture has formed a task force of researchers, growers, and other stakeholders to search for other cases and investigate new control measures.
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