Citrus black spot (CBS) is a disease caused by the fungus Guignardia citricarpa. It is NOT KNOWN to occur in California at this time but it was first detected in Florida on March 8, 2010. This was the first detection in North America and so far it is limited to two counties in Florida (refer to maps). This fungus causes severe lesions on the rind of fruit significantly decreasing fruit quality and marketability as fresh produce (see photos). Significant trade barriers exist such as the prohibition of marketing Argentine lemons grown in areas where the fungus is endemic in California.
Symptoms. The most obvious symptom is the development of lesions on fruit. There are 3 types of lesions; “hard spot”, “freckle spot” and “virulent spot”. Fruit are susceptible to infection from petal fall through the summer.
- Hard spot is the prevalent symptom observed in Florida. These are lesions 1-5 mm in diameter; a raised dark reddish brown border surrounds a light brown necrotic center that contains black spore producing structures (pycnidia). Lesions are most prevalent on the side of fruit that receives the most exposure to sunlight.
- Freckle spots are smaller, raised superficial lesions and typically appear on green fruit (photo 2).
- Virulent spot has not been observed in Florida; these lesions are larger, more scattered on the fruit and often coalesce.
Leaf symptoms. The lesions produced on leaves are inconspicuous however; they serve as the major source of inoculum for this disease. Leaf lesions are small, round, sunken, necrotic spots with gray centers surrounded by a dark brown ring. As leaves grow old they abscise and drop to the ground. Fungal spores are produced in the lesions of decaying leaves and are discharged at the onset of warm weather, in early spring and summer. Irrigation provides the moisture necessary for spore release.
Host Range. Guignardia citricarpa is known to infect only citrus. Late maturing lemons, oranges, mandarins and grapefruit are the most susceptible varieties.
Geographic Distribution. CBS occurs in subtropical areas with summer rainfall but is NOT KNOWN to occur in Mediterranean climates. It was first reported in Australia in 1895 and is currently found in Argentina, Brazil, Hong Kong, China, Indonesia, Japan, Kenya, Peru, Philippines, Taiwan, Uruguay, Venezuela, Zimbabwe, and parts of South Africa.
Control. Timely application of fungicides can control this disease. Cultural practices such as removing leaf litter can prevent and/or limit disease spread.
University of Florida Citrus Research and Education Center
Black Spot Cycle
USDA APHIS CBS – News/Quarantine/Science
Florida Dept of Ag/Fresh from Florida – News/Updates/Maps