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Publication #PP134

Blue Mold1

G.E. Brown2


Blue mold is caused by the fungus Penicillium italicum, and is a much minor decay in Florida than green mold. Blue mold is most prevalent in cold storage fruit where it is able to develop slowly at cold temperatures. At room temperature, green mold invades fruit much more rapidly than blue mold, and predominates in mixed infections. Spores of P. italicum are airborne and large numbers are produced by the fungus on the surface of infected fruit. These spores will contaminate the packinghouse, equipment, water used in drenchers and soak tanks, storage rooms, transit containers, and even the retail market area.

Blue mold. 

The fungus survives in the field on soil debris and produces spores that infect split and injured fruit in the tree and on the ground. At cooler fall and winter temperatures that favor fungal development, large numbers of spores are produced and carried by wind currents to surfaces of fruit in the tree canopy. The spores germinate and infect fruit when nutrients and moisture are released at injuries that are formed during harvesting and handling, and even injuries that involve only a few oil glands are sufficient to induce infection. The fungus can also invade fruit through certain physiologically induced injuries, such as injuries associated with chilling injury and stem-end rind breakdown. P. italicum can spread from infected to healthy fruit in packed containers causing a nest of decay. A condition known as soilage occurs when masses of spores produced on infected fruit contaminate surfaces of healthy fruit in the carton. The soiled fruit must be cleaned before retail sale. The infection and sporulation cycle can be repeated many times in a packinghouse and in storage rooms during extended storage. This prolific spore production ability of P. italicum enables it to eventually develop strains with resistance to chemical fungicide treatments.


Initial symptoms of blue mold are similar to those of green mold and sour rot. The small decayed area appears as a soft watery spot that is more firm than comparable stages of sour rot. When lesions enlarge to 1 to 2 inches in diameter, white mycelium is formed in the center, and blue spores are soon produced. The sporulating area is surrounded by a narrow band of white mycelium, which is encompassed by a definite band of water-soaked rind. Occasionally, P. italicum will sporulate in the flesh of the fruit. The infected fruit can be covered entirely by a mass of blue spores, which are easily dispersed by any physical movement or by air currents.

sporulating area. 


Careful harvesting and handling is needed to minimize injuries to the rind and the risk of blue mold. High populations of spores must not be allowed to accumulate in the packinghouse or storage rooms. Stringent sanitary practices must be enforced to limit airborne spore populations. Infected fruit should be removed promptly, exhaust fans can be used at the dump to remove spores from the packinghouse atmosphere, and dump areas where high spore populations are most likely to occur should be designed so air currents do not transfer spores to the packing area. Repacking, because of high blue mold infestations, should not be conducted in the packinghouse or storage facilities, but should be performed in a remote area to prevent contamination of newly packed crops. The pallets, packinghouse, and packingline, including washer brushes, should be sanitized daily to eradicate inoculum. Aqueous solutions in drenchers and soak tanks should be treated continuously with a sanitizer, such as chlorine, to prevent the accumulation of blue mold spores. Blue mold can be controlled with the same treatments used to control green mold (see Circular 359-A, Postharvest Decay Control Recommendations for Florida Citrus Fruit).

Penicillium italicum can develop resistance to postharvest fungicides. Resistance problems can be minimized with the use of thorough sanitation procedures, and treatments with two or more chemically unrelated fungicides. The packinghouse should be checked periodically for the presence of fungicide resistant strains of blue mold, particularly during the cool part of the packing season when blue mold is more prevalent.

Storage temperatures of 40°F or less will suppress development of blue mold, but not completely inhibit its growth. For this reason, blue mold is most commonly observed in Florida citrus when fruit are placed in cold storage.



This document is PP134, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date June 1994. Reviewed April 2003. Reviewed February 2011 by Mark Ritenour. Visit the EDIS website at


G.E. Brown, courtesy professor, Department of Plant Pathology, Citrus Research and Education Center, Lake Alfred, Florida, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.

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U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.