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Publication #PP130

Green Mold1

G.E. Brown2

CAUSAL ORGANISM AND DISEASE CYCLE

Green mold is caused by the fungus Penicillium digitatum which is ubiquitous to all citrus growing regions. Spores of this organism are airborne and large numbers are produced by the fungus on the surface of infected fruit. These spores will contaminate the packinghouse and its equipment, storage rooms, transit containers, and even the retail marketplace. Spores will also accumulate in water used in drenchers and soak tanks.

Green 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. 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. Fruit decaying with green mold produce relatively large amounts of ethylene gas which is a natural plant hormone that promotes respiration, senescence, and premature color development. P. digitatum does not usually spread from infected to healthy fruit in packed cartons, but it can cause a condition known as soilage when masses of spores produced on infected fruit are disseminated to surfaces of healthy fruit. Such 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. digitatum enables it to eventually develop strains with resistance to chemical fungicide treatments.

SYMPTOMATOLOGY

Initial symptoms of green mold are similar to those of sour rot and blue mold. The small decayed area appears as a soft watery spot that is more firm than comparable stages of sour rot. White mycelium is produced on the lesion surface, and when the lesion enlarges to approximately one inch in diameter, olive green spores are produced in the center. A broad zone of white mycelium surrounds the sporulating area, and the outer region of the lesion is composed of softened rind. The entire fruit is soon encompassed by a mass of olive green spores, which are easily dispersed by any physical motion or air currents.

CONTROL

Careful harvesting and handling are needed to minimize injuries to the rind and the risk of green 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 green 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 the 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 green mold inoculum. Chemical sanitizers and treatments available for the control of green mold are discussed in Circular 359-A, Postharvest Decay Control Recommendations for Florida Citrus Fruit .

Penicillium digitatum 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, particularly during the cooler part of the packing season when green mold is predominant, for the presence of fungicide resistant strains of green mold.

A temperature of 40°F will suppress development of green mold, but decay develops rapidly after the infected fruit are returned to ambient temperatures, when the decay can involve the entire fruit in 2 to 3 days.

Footnotes

1.

This document is PP130, 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 http://edis.ifas.ufl.edu.

2.

G.E. Brown, courtesy professor, Department of Plant Pathology, Citrus Research and Education Center, Lake Alfred, Florida, University of Florida.

The use of trade names in this publication is solely for the purpose of providing specific information. It is not a guarantee or warranty of the products named, and does not signify that they are approved to the exclusion of others of suitable composition.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

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.